text stringlengths 26 3.6k | page_title stringlengths 1 71 | source stringclasses 1 value | token_count int64 10 512 | id stringlengths 2 8 | url stringlengths 31 117 | topic stringclasses 4 values | section stringlengths 4 49 ⌀ | sublist stringclasses 9 values |
|---|---|---|---|---|---|---|---|---|
Although red Cellulose Acetate Butyrate (CAB) (generally known trade names are Cellidor, Tenite and Tenex) scaled Swiss Army knives are most common, there are many colors and alternative materials like more resilient nylon and aluminum for the scales available. Many textures, colors and shapes now appear in the Swiss Army Knife. Since 2006 the scales on some knife models can have textured rubber non-slip inlays incorporated, intended for sufficient grip with moist or wet hands. The rubber also provides some impact protection for such edged scales. Modifications have been made, including professionally produced custom models combining novel materials, colors, finishes and occasionally new tools such as firesteels or tool 'blades' mounting replaceable surgical scalpel blades to replacement of standard scales (handles) with new versions in natural materials such as buffalo horn. In addition to 'limited edition' productions runs, numerous examples from basic to professional-level customizations of standard knives—such as retrofitting pocket clips, one-off scales created using 3D printing techniques, decoration using anodization and new scale materials—can be found by searching for "SAK mods".
Assembly
During assembly, all components are placed on several brass rivets. The first components are generally an aluminium separator and a flat steel spring. Once a layer of tools is installed, another separator and spring are placed for the next layer of tools. This process is repeated until all the desired tool layers and the finishing separator are installed. Once the knife is built, the metal parts are fastened by adding brass flanged bushings to the rivets. The excess length of the rivets is then cut off to make them flush with the bushings. Finally, the remaining length of the rivets is flattened into the flanged bushings.
After the assembly of the metal parts, the blades on smaller knives are sharpened to a 15° angle, resulting in a 30° V-shaped steel cutting edge. From sized knives the blades are sharpened to a 20° angle, resulting in a 40° V-shaped steel cutting edge. Chisel ground blades are sharpened to a 24° angle, resulting in a 24° asymmetric-shaped steel cutting edge where only one side is ground and the other is deburred and remains flat. The blades are then checked with a laser reflecting goniometer to verify the angle of the cutting edges. | Swiss Army knife | Wikipedia | 496 | 313833 | https://en.wikipedia.org/wiki/Swiss%20Army%20knife | Technology | Knives | null |
Finally, scales are applied. Slightly undersized holes incorporated into the inner surface enclose the bushings, which have truncated cone cross-section and are slightly undercut, forming a one-way interference fit when pressed into the generally softer and more elastic scale material. The result is a tight adhesive-free connection that nonetheless permits new identical-pattern scales to be quickly and easily applied.
Sizes
Victorinox models are available in , , , , , , and lengths when closed. The thickness of the knives varies depending on the number of tool layers included. The models offer the most variety in tool configurations in the Victorinox model line with as many as 15 layers.
Wenger models are available in , , , , and lengths when closed. Thickness varies depending on the number of tool layers included. The models offer the most variety in tool configurations in the Wenger model line, with as many as 10 layers.
Knives issued by the Swiss Armed Forces
Since the first issue as personal equipment in 1891 the Soldatenmesser (Soldier Knives) issued by the Swiss Armed Forces have been revised several times. There are five different main Modelle (models). Their model numbers refer to the year of introduction in the military supply chain. Several main models have been revised over time and therefore exist in different Ausführungen (executions), also denoted by the year of introduction. The issued models of the Swiss Armed Forces are:
Modell 1890
Modell 1890 Ausführung 1901
Modell 1908
Modell 1951
Modell 1951 Ausführung 1954
Modell 1951 Ausführung 1957
Modell 1961
Modell 1961 Ausführung 1965
Modell 1961 Ausführung 1978
Modell 1961 Ausführung 1994
Soldatenmesser 08 (Soldier Knife 08)
Soldier Knives are issued to every recruit or member of the Swiss Armed Forces and the knives issued to officers have never differed from those issued to non-commissioned officers and privates. A model incorporating a corkscrew and scissors was produced as an officer's tool, but was deemed not "essential for survival". Officers were free to purchase it individually on their own account.
Soldier knife model 1890 | Swiss Army knife | Wikipedia | 439 | 313833 | https://en.wikipedia.org/wiki/Swiss%20Army%20knife | Technology | Knives | null |
The Soldier Knife model 1890 had a spear point blade, reamer, can-opener, screwdriver and grips made out of oak wood scales (handles) that were treated with rapeseed oil for greater toughness and water-repellency, which made them black in color. The wooden grips of the Modell 1890 tended to crack and chip so in 1901 these were changed to a hard reddish-brown fiber similar in appearance to wood. The knife was long, thick and weighed .
Soldier knife model 1908
The Soldier Knife model 1908 had a clip point blade rather than the 1890s spear point blade, still with the fiber scales, carbon steel tools, nickel-silver bolster, liners, and divider. The knife was long, thick and weighed . The contract with the Swiss Army split production equally between the Victorinox and Wenger companies.
Soldier knife model 1951
The soldier Knife model 1951 had fiber scales, nickel-silver bolsters, liners, and divider, and a spear point blade. This was the first Swiss Armed Forces issue model where the tools were made of stainless steel. The screwdriver now had a scraper arc on one edge. The knife was long, thick and weighed .
Soldier knife model 1961
The Soldier Knife model 1961 has a long knurled alox handle with the Swiss crest, a drop point blade, a reamer, a blade combining bottle opener, screwdriver, and wire stripper, and a combined can-opener and small screwdriver. The knife was thick and weighed
The 1961 model also contains a brass spacer, which allows the knife, with the screwdriver and the reamer extended simultaneously, to be used to assemble the SIG 550 and SIG 510 assault rifles: the knife serves as a restraint to the firing pin during assembly of the lock. The Soldier Knife model 1961 was manufactured only by Victorinox and Wenger and was the first issued knife bearing the Swiss Coat of Arms on the handle.
Soldier knife 08 | Swiss Army knife | Wikipedia | 407 | 313833 | https://en.wikipedia.org/wiki/Swiss%20Army%20knife | Technology | Knives | null |
In 2007 the Swiss Government made a request for new updated soldier knives for the Swiss military for distribution in late 2008. The evaluation phase of the new soldier knife began in February 2008, when Armasuisse issued an invitation to tender. A total of seven suppliers from Switzerland and other countries were invited to participate in the evaluation process. Functional models submitted by suppliers underwent practical testing by military personnel in July 2008, while laboratory tests were used to assess compliance with technical requirements. A cost-benefit analysis was conducted and the model with the best price/performance ratio was awarded the contract. The order for 75,000 soldier knives plus cases was worth . This equates to a purchase price of , , in October 2009 per knife plus case.
Victorinox won the contest with a knife based on the One-Hand German Army Knife as issued by the German Bundeswehr and released in the civilian model lineup with the addition of a toothpick and tweezers stored in the nylon grip scales (side cover plates) as the One-Hand Trekker/Trailmaster model. Mass production of the new Soldatenmesser 08 (Soldier Knife 08) for the Swiss Armed Forces was started in December 2008, and first issued to the Swiss Armed Forces beginning with the first basic training sessions of 2009.
The Soldier Knife 08 has an long ergonomic dual density handle with TPU rubbery thermoplastic elastomer non-slip inlays incorporated in the green Polyamide 6 grip shells and a double liner locking system, one-hand long locking partly wavy serrated chisel ground (optimized for right-handed use) drop point blade sharpened to a 24° angle, wood saw, can opener with small slotted screwdriver, locking bottle opener with large slotted screwdriver and wire stripper/bender, reamer sharpened to a 48° angle, Phillips (PH2) screwdriver and diameter split keyring. The Soldier Knife 08 width is , thickness is , overall length opened is and it weighs . The Soldier Knife 08 was not manufactured by Wenger.
Knives issued by other militaries
The armed forces of more than 20 different nations have issued or approved the use of various versions of Swiss army knives made by Victorinox, among them the forces of Germany, France, the Netherlands, Norway, Malaysia and the United States (NSN 1095-01-653-1166 Knife, Combat). | Swiss Army knife | Wikipedia | 496 | 313833 | https://en.wikipedia.org/wiki/Swiss%20Army%20knife | Technology | Knives | null |
Space program
The Swiss Army knife has been present in space missions carried out by NASA since the late 1970s. In 1978, NASA sent a letter of confirmation to Victorinox regarding a purchase of 50 knives known as the Master Craftsman model. In 1985, Edward M. Payton, brother of astronaut Gary E. Payton, sent a letter to Victorinox, asking about getting a Master Craftsman knife after seeing the one his brother used in space. There are other stories of repairs conducted in space using a Swiss Army knife.
Cultural impact
The Swiss Army knife has been added to the collection of the Museum of Modern Art in New York and Munich's State Museum of Applied Art for its design. The term "Swiss Army" currently is a registered trademark owned by Victorinox AG and its subsidiary, Wenger SA.
In both the original television series MacGyver as well as its 2016 reboot, character Angus MacGyver frequently uses different Swiss Army knives in various episodes to solve problems and construct simple objects.
The term "Swiss Army knife" has entered popular culture as a metaphor for usefulness and adaptability. The multi-purpose nature of the tool has also inspired a number of other gadgets.
A particularly large Wenger knife model, Wenger 16999, has inspired a large number of humorous reviews on Amazon. This model was recognized by Guinness World Records as 'The World's Most Multifunctional Penknife'.
When U.S. District Court for the Southern District of California Roger Benitez overturned California's 30-year-old ban on assault weapons in Miller v. Bonta, he compared the Swiss Army knife to the AR-15 rifle in the first sentence of his opinion, "Like the Swiss Army Knife, the popular AR-15 rifle is a perfect combination of home defense weapon and homeland defense equipment." In response, California Governor Gavin Newsom stated that the comparison "completely undermines the credibility of this decision".
Gallery | Swiss Army knife | Wikipedia | 397 | 313833 | https://en.wikipedia.org/wiki/Swiss%20Army%20knife | Technology | Knives | null |
Ichthyostega (from , 'fish' and , 'roof') is an extinct genus of limbed tetrapodomorphs from the Late Devonian of what is now Greenland. It was among the earliest four-limbed vertebrates ever in the fossil record and was one of the first with weight-bearing adaptations for terrestrial locomotion. Ichthyostega possessed lungs and limbs that helped it navigate through shallow water in swamps. Although Ichthyostega is often labelled a 'tetrapod' because of its limbs and fingers, it evolved long before true crown group tetrapods and could more accurately be referred to as a stegocephalian or stem tetrapod. Likewise, while undoubtedly of amphibian build and habit, it is not a true member of the group in the narrow sense, as the first modern amphibians (members of the group Lissamphibia) appeared in the Triassic Period. Until finds of other early stegocephalians and closely related fishes in the late 20th century, Ichthyostega stood alone as a transitional fossil between fish and tetrapods, combining fish and tetrapod features. Newer research has shown that it had an unusual anatomy, functioning more akin to a seal than a salamander, as previously assumed.
History
In 1932 Gunnar Säve-Söderbergh described four Ichthyostega species from the Late Devonian of East Greenland and one species belonging to the genus Ichthyostegopsis, I. wimani. These species could be synonymous (in which case only I. stensioei would remain), because their morphological differences are not very pronounced. The species differ in skull proportions, skull punctuation and skull bone patterns. The comparisons were done on 14 specimens collected in 1931 by the Danish East Greenland Expedition. Additional specimens were collected between 1933 and 1955.
Description
Ichthyostega was a fairly large animal for its time, as it was broadly built and about 1.5 m (4.9 ft) long. The skull was low, with dorsally placed eyes and large labyrinthodont teeth. The posterior margin of the skull formed an operculum covering the gills. The spiracle was situated in an otic notch behind each eye. Computed tomography has revealed that Ichthyostega had a specialized ear, including a stapes with a unique morphology compared to other tetrapods or to any fish hyomandibula. | Ichthyostega | Wikipedia | 499 | 313925 | https://en.wikipedia.org/wiki/Ichthyostega | Biology and health sciences | Prehistoric amphibians | Animals |
Postcranial skeleton
The legs were large compared to contemporary relatives. It had seven digits on each hind leg, along with a peculiar, poorly ossified mass which lies anteriorly adjacent to the digits. The exact number of digits on the forelimb is not yet known, since fossils of the hand have not been found. While in water, the foot would have functioned like a fleshy paddle more than a fin.The vertebral column and ribcage of Ichthyostega was unusual and highly specialized relative to both its contemporaries and later tetrapods. The thoracic vertebrae at the front of the trunk and the short neck have tall neural spines that lean backwards. They attach to pointed ribs which increase in size and acquire prominent overlapping flanges. Past the sixth or seventh flanged rib, the ribs abruptly decrease in size and lose their flanges. The lumbar vertebrae at the back of the trunk have strong muscle scars and neural spines which are bent forwards and decrease in size towards the hips. The sacral vertebrae above the hips have fan-shaped neural spines that transition from forward-leaning to backward-leaning as they approach the tail. The vertebrae right behind the hips have unusually large ribs similar to those of the thoracic region. The caudal vertebrae have slender spines that lean backwards. The tail of Ichthyostega retained a low fin supported by bony lepidotrichia (fin rays). The tail fin was not as deep as in Acanthostega, and would have been less useful for swimming.
Ichthyostega is related to Acanthostega gunnari, which is also from what is now East Greenland. Ichthyostega'''s skull seems more fish-like than that of Acanthostega, but had apelvic girdle morphology that seems stronger and better adapted to life on land. Ichthyostega also had more supportive ribs and stronger vertebrae with more developed zygapophyses. Whether or not these traits were independently evolved in Ichthyostega is debated. It does, however, show that Ichthyostega may have ventured onto land on occasions, unlike contemporaneous limbed vertebrates, such as Elginerpeton and Obruchevichthys.
Classification | Ichthyostega | Wikipedia | 469 | 313925 | https://en.wikipedia.org/wiki/Ichthyostega | Biology and health sciences | Prehistoric amphibians | Animals |
Traditionally, Ichthyostega was considered part of an order named for it, the "Ichthyostegalia". However, this group represents a paraphyletic grade of primitive stem-tetrapods and is not used by many modern researchers. Phylogenetic analysis has shown Ichthyostega is intermediate between other primitive stegocephalian stem-tetrapods. The evolutionary tree of early stegocephalians below follows the results of one such analysis performed by Swartz in 2012.
Paleobiology
Early limbed vertebrates like Ichthyostega and Acanthostega differed from earlier tetrapodomorphs such as Eusthenopteron or Panderichthys in their increased adaptations for life on land. Though tetrapodomorphs possessed lungs, they used gills as their primary means of discharging carbon dioxide. Tetrapodomorphs used their bodies and tails for locomotion and their fins for steering and braking; Ichthyostega may have used its forelimbs for locomotion on land and its tail for swimming.Its massive ribcage was made up of overlapping ribs and the animal possessed a stronger skeletal structure, a largely fishlike spine, and forelimbs apparently powerful enough to pull the body from the water. These anatomical modifications may have been a result of selection to overcome the lack of buoyancy experienced on land. The hindlimbs were smaller than the forelimbs and unlikely to have borne full weight in an adult, while the broad, overlapping ribs would have inhibited side-to-side movements. The forelimbs had the required range of movement to push the body up and forward, probably allowing the animal to drag itself across flat land by synchronous (rather than alternate) "crutching" movements, much like that of a mudskipper or a seal. It was incapable of typical quadrupedal gaits as the forelimbs lacked the necessary rotary motion range. | Ichthyostega | Wikipedia | 407 | 313925 | https://en.wikipedia.org/wiki/Ichthyostega | Biology and health sciences | Prehistoric amphibians | Animals |
Ichthyosauria is an order of large extinct marine reptiles sometimes referred to as "ichthyosaurs", although the term is also used for wider clades in which the order resides.
Ichthyosaurians thrived during much of the Mesozoic era; based on fossil evidence, they first appeared around 250 million years ago (Ma) and at least one species survived until about 90 million years ago, into the Late Cretaceous. During the Early Triassic epoch, ichthyosaurs and other ichthyosauromorphs evolved from a group of unidentified land reptiles that returned to the sea, in a development similar to how the mammalian land-dwelling ancestors of modern-day dolphins and whales returned to the sea millions of years later, which they gradually came to resemble in a case of convergent evolution. Ichthyosaurians were particularly abundant in the Late Triassic and Early Jurassic periods, until they were replaced as the top aquatic predators by another marine reptilian group, the Plesiosauria, in the later Jurassic and Early Cretaceous, though previous views of ichthyosaur decline during this period are probably overstated. Ichthyosaurians diversity declined due to environmental volatility caused by climatic upheavals in the early Late Cretaceous, becoming extinct around the Cenomanian-Turonian boundary approximately 90 million years ago.
Scientists became aware of the existence of ichthyosaurians during the early 19th century, when the first complete skeletons were found in England. In 1834, the order Ichthyosauria was named. Later that century, many finely preserved ichthyosaurian fossils were discovered in Germany, including soft-tissue remains. Since the late 20th century, there has been a revived interest in the group, leading to an increased number of named ichthyosaurs from all continents, with over fifty genera known. | Ichthyosauria | Wikipedia | 376 | 314101 | https://en.wikipedia.org/wiki/Ichthyosauria | Biology and health sciences | Prehistoric marine reptiles | Animals |
Ichthyosaurian species varied from in length. Ichthyosaurians resembled both modern fish and dolphins. Their limbs had been fully transformed into flippers, which sometimes contained a very large number of digits and phalanges. At least some species possessed a dorsal fin. Their heads were pointed, and the jaws often were equipped with conical teeth to catch smaller prey. Some species had larger, bladed teeth to attack large animals. The eyes were very large, for deep diving. The neck was short, and later species had a rather stiff trunk. These also had a more vertical tail fin, used for a powerful propulsive stroke. The vertebral column, made of simplified disc-like vertebrae, continued into the lower lobe of the tail fin. Ichthyosaurians were air-breathing, warm-blooded, and bore live young. Many, if not all, species had a layer of blubber for insulation.
Like other ancient marine reptiles, such as those in the clades Mosasauria and Plesiosauria, the genera in Ichthyosauria are not part of the clade Dinosauria.
History of discoveries
Early finds
The first known illustrations of ichthyosaur bones, vertebrae, and limb elements were published by the Welshman Edward Lhuyd in his of 1699. Lhuyd thought that they represented fish remains. In 1708, the Swiss naturalist Johann Jakob Scheuchzer described two ichthyosaur vertebrae assuming they belonged to a man drowned in the Universal Deluge. In 1766, an ichthyosaur jaw with teeth was found at Weston near Bath. In 1783, this piece was exhibited by the Society for Promoting Natural History as those of a crocodilian. In 1779, ichthyosaur bones were illustrated in John Walcott's Descriptions and Figures of Petrifications. Towards the end of the eighteenth century, British fossil collections quickly increased in size. Those of the naturalists Ashton Lever and John Hunter were acquired in their totality by museums; later, it was established that they contained dozens of ichthyosaur bones and teeth. The bones had typically been labelled as belonging to fish, dolphins, or crocodiles; the teeth had been seen as those of sea lions. | Ichthyosauria | Wikipedia | 453 | 314101 | https://en.wikipedia.org/wiki/Ichthyosauria | Biology and health sciences | Prehistoric marine reptiles | Animals |
The demand by collectors led to more intense commercial digging activities. In the early nineteenth century, this resulted in the discovery of more complete skeletons. In 1804, Edward Donovan at St Donats uncovered a ichthyosaur specimen containing a jaw, vertebrae, ribs, and a shoulder girdle. It was considered to be a giant lizard. In October 1805, a newspaper article reported the find of two additional skeletons, one discovered at Weston by Jacob Wilkinson, the other, at the same village, by Reverend Peter Hawker. In 1807, the last specimen was described by the latter's cousin, Joseph Hawker. This specimen thus gained some fame among geologists as 'Hawker's Crocodile'. In 1810, near Stratford-upon-Avon, an ichthyosaur jaw was found that was combined with plesiosaur bones to obtain a more complete specimen, indicating that the distinctive nature of ichthyosaurs was not yet understood, awaiting the discovery of far better fossils.
The first complete skeletons
In 1811, in Lyme Regis, along the Jurassic Coast of Dorset, the first complete ichthyosaur skull was found by Joseph Anning, the brother of Mary Anning, who in 1812 while still a young girl, secured the torso of the same specimen. Their mother, Molly Anning, sold the combined piece to squire Henry Henley for £23. Henley lent the fossil to the London Museum of Natural History of William Bullock. When this museum was closed, the British Museum bought the fossil for a price of £47/5s; it still belongs to the collection of the independent Natural History Museum and has the inventory number NHMUK PV R1158 (formerly BMNH R.1158). It has been identified as a specimen of Temnodontosaurus platyodon.
In 1814, the Annings' specimen was described by Professor Everard Home, in the first scientific publication dedicated to an ichthyosaur. Intrigued by the strange animal, Home tried to locate additional specimens in existing collections. In 1816, he described ichthyosaur fossils owned by William Buckland and James Johnson. In 1818, Home published data obtained by corresponding with naturalists all over Britain. In 1819, he wrote two articles about specimens found by Henry Thomas De la Beche and Thomas James Birch. A last publication of 1820 was dedicated to a discovery by Birch at Lyme Regis. The series of articles by Home covered the entire anatomy of ichthyosaurs, but highlighted details only; a systematic description was still lacking. | Ichthyosauria | Wikipedia | 511 | 314101 | https://en.wikipedia.org/wiki/Ichthyosauria | Biology and health sciences | Prehistoric marine reptiles | Animals |
Home was very uncertain how the animal should be classified. Though most individual skeletal elements looked very reptilian, the anatomy as a whole resembled that of a fish, so he initially assigned the creature to the fishes, as seemed to be confirmed by the flat shape of the vertebrae. At the same time, he considered it a transitional form between fishes and crocodiles, not in an evolutionary sense, but as regarded its place in the , the "Chain of Being" hierarchically connecting all living creatures. In 1818, Home noted some coincidental similarities between the coracoid of ichthyosaurians and the sternum of the platypus. This induced him to emphasize its status as a transitional form, combining, like the platypus, traits of several larger groups. In 1819, he considered it a form between newts, like the olm, and lizards; he then gave a formal generic name: Proteo-Saurus. However, in 1817, Karl Dietrich Eberhard Koenig had already referred to the animal as Ichthyosaurus, "fish saurian" from Greek , , "fish". This name at the time was an invalid and was only published by Koenig in 1825, but was adopted by De la Beche in 1819 in a lecture where he named three Ichthyosaurus species. This text would only be published in 1822, just after De la Beche's friend William Conybeare published a description of these species, together with a fourth one. The type species was Ichthyosaurus communis, based on a lost skeleton. Conybeare considered that Ichthyosaurus had priority relative to Proteosaurus. Although this is incorrect by modern standards, the latter name became a "forgotten" . In 1821, De la Beche and Conybeare provided the first systematic description of ichthyosaurs, comparing them to another newly identified marine reptile group, the Plesiosauria. Much of this description reflected the insights of their friend, the anatomist Joseph Pentland.
In 1835, the order Ichthyosauria was named by Henri Marie Ducrotay de Blainville. In 1840, Richard Owen named an order Ichthyopterygia as an alternative concept.
Popularisation during the 19th century | Ichthyosauria | Wikipedia | 464 | 314101 | https://en.wikipedia.org/wiki/Ichthyosauria | Biology and health sciences | Prehistoric marine reptiles | Animals |
The discovery of a hitherto unsuspected extinct group of large marine reptiles generated much publicity, capturing the imagination of both scientists and the public at large. People were fascinated by the strange build of the animals, especially the large scleral rings in the eye sockets, of which it was sometimes erroneously assumed these would have been visible on the living animal. Their bizarre form induced a feeling of alienation, allowing people to realise the immense span of time passed since the era in which the ichthyosaur swam the oceans. Not all were convinced that ichthyosaurs had gone extinct: Reverend George Young found a skeleton in 1819 at Whitby; in his 1821 description, he expressed the hope that living specimens could still be found. Geologist Charles Lyell, to the contrary, assumed that the Earth was eternal so that in the course of time the ichthyosaur might likely reappear, a possibility lampooned in a famous caricature by De la Beche.
Public awareness was increased by the works of the eccentric collector Thomas Hawkins, a pre-Adamite believing that ichthyosaurs were monstrous creations by the devil: Memoirs of Ichthyosauri and Plesiosauri of 1834 and The Book of the Great Sea-Dragons of 1840. The first work was illustrated by mezzotints by John Samuelson Templeton. These publications also contained scientific descriptions and represented the first textbooks of the subject. In the summer of 1834, Hawkins, after a taxation by William Buckland and Gideon Mantell, sold his extensive collection, then the largest of its kind in the world, to the British Museum. However, curator Koenig quickly discovered that the fossils had been heavily restored with plaster, applied by an Italian artist from Lucca; of the most attractive piece, an Ichthyosaurus specimen, almost the entire tail was fake. It turned out that Professor Buckland had been aware of this beforehand, and the museum was forced to reach a settlement with Hawkins, and gave the fake parts a lighter colour to differentiate them from the authentic skeletal elements. | Ichthyosauria | Wikipedia | 420 | 314101 | https://en.wikipedia.org/wiki/Ichthyosauria | Biology and health sciences | Prehistoric marine reptiles | Animals |
Ichthyosaurs became even more popular in 1854 by the rebuilding at Sydenham Hill of the Crystal Palace, originally erected at the world exhibition of 1851. In the surrounding park, life-sized, painted, concrete statues of extinct animals were placed, which were designed by Benjamin Waterhouse Hawkins under the direction of Richard Owen. Among them were three models of an ichthyosaur. Although it was known that ichthyosaurs had been animals of the open seas, they were shown basking on the shore, a convention followed by many nineteenth century illustrations with the aim, as Conybeare once explained, of better exposing their build. This led to the misunderstanding that they really had an amphibious lifestyle. The pools in the park were at the time subjected to tidal changes, so that fluctuations in the water level at intervals submerged the ichthyosaur statues, adding a certain realism. Remarkably, internal skeletal structures, such as the scleral rings and the many phalanges of the flippers, were shown at the outside.
Later 19th-century finds
During the nineteenth century, the number of described ichthyosaur genera gradually increased. New finds allowed for a better understanding of their anatomy. Owen had noticed that many fossils showed a downward bend in the rear tail. At first, he explained this as a post mortem effect, a tendon pulling the tail end downwards after death. However, after an article on the subject by Philip Grey Egerton, Owen considered the possibility that the oblique section could have supported the lower lobe of a tail fin. This hypothesis was confirmed by new finds from Germany. In the Posidonia Shale at Holzmaden, dating from the early Jurassic, already in the early nineteenth century, the first ichthyosaur skeletons had been found. During the latter half of the century, the rate of discovery increased to a few hundred each year. Ultimately, over four thousand were uncovered, forming the bulk of ichthyosaur specimens displayed. The sites were also a , meaning not only the quantity, but also the quality was exceptional. The skeletons were very complete and often preserved soft tissues, including tail and dorsal fins. Additionally, female individuals were discovered with embryos.
20th century | Ichthyosauria | Wikipedia | 450 | 314101 | https://en.wikipedia.org/wiki/Ichthyosauria | Biology and health sciences | Prehistoric marine reptiles | Animals |
In the early twentieth century, ichthyosaur research was dominated by the German paleontologist Friedrich von Huene, who wrote an extensive series of articles, taking advantage of an easy access to the many specimens found in his country. The amount of anatomical data was hereby vastly increased. Von Huene also travelled widely abroad, describing many fossils from locations outside of Europe. During the 20th century, North America became an important source of new fossils. In 1905, the Saurian Expedition led by John Campbell Merriam and financed by Annie Montague Alexander, found twenty-five specimens in central Nevada, which were under a shallow ocean during the Triassic. Several of these are in the collection of the University of California Museum of Paleontology.
After a slack during the middle of the century, with no new genera being named between the 1930s and the 1970s, the rate of discoveries picked up towards its end. Other specimens are embedded in the rock and visible at Berlin–Ichthyosaur State Park in Nye County. In 1977 the Triassic ichthyosaur Shonisaurus became the state fossil of Nevada. About half of the ichthyosaur genera determined to be valid were described after 1990. In 1992 Canadian paleontologist Elizabeth Nicholls uncovered the largest known specimen, a Shastasaurus. The new finds have allowed a gradual improvement in knowledge about the anatomy and physiology of what had already been seen as rather advanced "Mesozoic dolphins". Christopher McGowan published a larger number of articles and also brought the group to the attention of the general public. The new method of cladistics provided a means to exactly calculate the relationships between groups of animals, and in 1999, Ryosuke Motani published the first extensive study on ichthyosaur phylogenetics.
21st century
In 2003, McGowan and Motani published the first modern textbook on the Ichthyosauria and their closest relatives. Two jawbones of gigantic ichthyosaur were discovered in 2016 and 2020 in Lilstock and Somerset respectively, UK. Simple scaling would suggest that this ichthyosaur has an estimated total length of up to 26 meters (82 feet), the largest known to date marine reptile. The fossils of this individual were dated to be 202 million-year-old.
Evolutionary history
Origin | Ichthyosauria | Wikipedia | 461 | 314101 | https://en.wikipedia.org/wiki/Ichthyosauria | Biology and health sciences | Prehistoric marine reptiles | Animals |
The origin of the ichthyosaurs is contentious. Until recently, clear transitional forms with land-dwelling vertebrate groups had not yet been found, the earliest known species of the ichthyosaur lineage being already fully aquatic. In 2014, a small basal ichthyosauriform from the upper Lower Triassic was described that had been discovered in China with characteristics suggesting an amphibious lifestyle. In 1937, Friedrich von Huene even hypothesised that ichthyosaurs were not reptiles, but instead represented a lineage separately developed from amphibians. Today, this notion has been discarded and a consensus exists that ichthyosaurs are amniote tetrapods, having descended from terrestrial egg-laying amniotes during the late Permian or the earliest Triassic. However, establishing their position within the amniote evolutionary tree has proven difficult, due to their heavily derived morphology obscuring their ancestry. Several conflicting hypotheses have been posited on the subject. In the second half of the 20th century, ichthyosaurs were usually assumed to be of the Anapsida, seen as an early branch of "primitive" reptiles. This would explain the early appearance of ichthyosaurs in the fossil record, and also their lack of clear affinities with other reptile groups, as anapsids were supposed to be little specialised. This hypothesis has become unpopular for being inherently vague because Anapsida is an unnatural, paraphyletic group. Modern exact quantitative cladistic analyses consistently indicate that ichthyosaurs are members of the clade Diapsida. Some studies showed a basal, or low, position in the diapsid tree. More analyses result in their being Neodiapsida, a derived diapsid subgroup. | Ichthyosauria | Wikipedia | 353 | 314101 | https://en.wikipedia.org/wiki/Ichthyosauria | Biology and health sciences | Prehistoric marine reptiles | Animals |
Since the 1980s, a close relationship was assumed between the Ichthyosauria and the Sauropterygia, another marine reptile group, within an overarching Euryapsida, with one such study in 1997 by John Merck showing them to be monophyletic archosauromorph euryapsids. This has been contested over the years, with the Euryapsida being seen as an unnatural polyphyletic assemblage of reptiles that happen to share some adaptations to a swimming lifestyle. However, more recent studies have shown further support for a monophyletic clade between Ichthyosauromorpha, Sauropterygia, and Thalattosauria as a massive marine clade of aquatic archosauromorphs originating in the Late Permian and diversifying in the Early Triassic.
Affinity with the Hupehsuchia
Since 1959, a second enigmatic group of ancient sea reptiles is known, the Hupehsuchia. Like the Ichthyopterygia, the Hupehsuchia have pointed snouts and show polydactyly, the possession of more than five fingers or toes. Their limbs more resemble those of land animals, making them appear as a transitional form between these and ichthyosaurs. Initially, this possibility was largely neglected because the Hupehsuchia have a fundamentally different form of propulsion, with an extremely stiffened trunk. The similarities were explained as a case of convergent evolution. Furthermore, the descent of the Hupehsuchia is no less obscure, meaning a possible close relationship would hardly clarify the general evolutionary position of the ichthyosaurs.
In 2014, Cartorhynchus was announced, a small species with a short snout, large flippers, and a stiff trunk. Its lifestyle might have been amphibious. Motani found it to be more basal than the Ichthyopterygia and named an encompassing clade Ichthyosauriformes. The latter group was combined with the Hupesuchia into the Ichthyosauromorpha. The ichthyosauromorphs were found to be diapsids.
The proposed relationships are shown by this cladogram: | Ichthyosauria | Wikipedia | 447 | 314101 | https://en.wikipedia.org/wiki/Ichthyosauria | Biology and health sciences | Prehistoric marine reptiles | Animals |
Early Ichthyopterygia
The earliest ichthyosaurs are known from the Early and Early-Middle (Olenekian and Anisian) Triassic strata of Canada, China, Japan, and Spitsbergen in Norway, being up to 246 million years old. These first forms included the genera Chaohusaurus, Grippia, and Utatsusaurus. Even older fossils show they were around 250 million years ago, just two million years after the Permian mass extinction. This early diversity suggests an even earlier origin, possibly late Permian. They more resembled finned lizards than the fishes or dolphins to which the later, more familiar species were similar. Their bodies were elongated and they probably used an anguilliform locomotion, swimming by undulations of the entire trunk. Like land animals, their pectoral girdles and pelves were robustly built, and their vertebrae still possessed the usual interlocking processes to support the body against the force of gravity. However, they were already rather advanced in having limbs that had been completely transformed into flippers. They also were probably warm-blooded and viviparous.
These very early "proto-ichthyosaurs" had such a distinctive build compared to "ichthyosaurs proper" that Motani excluded them from the Ichthyosauria and placed them in a basal position in a larger clade, the Ichthyopterygia. However, this solution was not adopted by all researchers.
Later Triassic forms | Ichthyosauria | Wikipedia | 299 | 314101 | https://en.wikipedia.org/wiki/Ichthyosauria | Biology and health sciences | Prehistoric marine reptiles | Animals |
The basal forms quickly gave rise to ichthyosaurs in the narrow sense sometime around the boundary between the Early Triassic and Middle Triassic; the earliest Ichthyosauria in the sense Motani gave to the concept, appear about 245 million years ago. These later diversified into a variety of forms, including the still sea serpent-like Cymbospondylus, a problematic form which reached ten metres in length, and smaller, more typical forms like Mixosaurus. The Mixosauria were already very fish-like with a pointed skull, a shorter trunk, a more vertical tail fin, a dorsal fin, and short flippers containing many phalanges. The sister group of the Mixosauria were the more advanced Merriamosauria. By the Late Triassic, merriamosaurs consisted of both the large, classic Shastasauria and more advanced, "dolphin-like" Euichthyosauria. Experts disagree over whether these represent an evolutionary continuum, with the less specialised shastosaurs a paraphyletic grade that was evolving into the more advanced forms, or whether the two were separate clades that evolved from a common ancestor earlier on. Euichthyosauria possessed more narrow front flippers, with a reduced number of fingers. Basal euichthyosaurs were Californosaurus and Toretocnemus. A more derived branch were the Parvipelvia, with a reduced pelvis, basal forms of which are Hudsonelpidia and Macgowania.
During the Carnian and Norian, Shastosauria reached huge sizes. Shonisaurus popularis, known from a number of specimens from the Carnian of Nevada, was long. Norian Shonisauridae are known from both sides of the Pacific. Himalayasaurus tibetensis and Tibetosaurus (probably a synonym) have been found in Tibet. These large (10- to 15-m-long) ichthyosaurs have by some been placed into the genus Shonisaurus.
The gigantic Shonisaurus sikanniensis (considered as a shastasaurus between 2011 and 2013) whose remains were found in the Pardonet Formation of British Columbia, has been estimated to be as much as in length. Ichthyotitan, found in Somerset, has been estimated to be as much as 26 m long—if correct, the largest marine reptile known to date. | Ichthyosauria | Wikipedia | 480 | 314101 | https://en.wikipedia.org/wiki/Ichthyosauria | Biology and health sciences | Prehistoric marine reptiles | Animals |
In the Late Triassic, ichthyosaurs attained the peak of their size and diversity. They occupied many ecological niches. Some were apex predators; others were hunters of small prey. Several species perhaps specialised in suction feeding or were ram feeders; also, durophagous forms are known. Towards the end of the Late Triassic, a decline of variability seems to have occurred. The giant species seemed to have disappeared at the end of the Norian. Rhaetian (latest Triassic) ichthyosaurs are known from England, and these are very similar to those of the Early Jurassic. A possible explanation is an increased competition by sharks, Teleostei, and the first Plesiosauria. Like the dinosaurs, the ichthyosaurs and their contemporaries, the plesiosaurs, survived the Triassic–Jurassic extinction event, and quickly diversified again to fill the vacant ecological niches of the early Jurassic.
Jurassic
During the Early Jurassic, the ichthyosaurs still showed a large variety of species, ranging from in length. Many well-preserved specimens from England and Germany date to this time and well-known genera include Eurhinosaurus, Ichthyosaurus, Leptonectes, Stenopterygius, and the large predator Temnodontosaurus. More basal parvipelvians like Suevoleviathan were also present. The general morphological variability had been strongly reduced, however. Giant forms, suction feeders and durophagous species were absent. Many of these genera possessed streamlined, dolphin-like thunniform bodies, although more basal clades like Eurhinosauria, which include Leptonectes and Eurhinosaurus, had longer bodies and long snouts.
Few ichthyosaur fossils are known from the Middle Jurassic. This might be a result of the poor fossil record in general of this epoch. The strata of the Late Jurassic seem to indicate that a further decrease in diversity had taken place. From the Middle Jurassic onwards, almost all ichthyosaurs belonged to the thunnosaurian clade Ophthalmosauridae. Represented by the Ophthalmosaurus and related genera, they were very similar in general build to Ichthyosaurus. The eyes of Ophthalmosaurus were huge, and these animals likely hunted in dim and deep water. However, new finds from the Cretaceous indicate that ichthyosaur diversity in the Late Jurassic must have been underestimated.
Cretaceous | Ichthyosauria | Wikipedia | 506 | 314101 | https://en.wikipedia.org/wiki/Ichthyosauria | Biology and health sciences | Prehistoric marine reptiles | Animals |
Traditionally, ichthyosaurs were seen as decreasing in diversity even further with the Cretaceous, though they had a worldwide distribution. All fossils from this period were referred to a single genus: Platypterygius. This last ichthyosaur genus was thought to have become extinct early in the late Cretaceous, during the Cenomanian about 95 million years ago, much earlier than other large Mesozoic reptile groups that survived until the very end of the Cretaceous. Two major explanations have been proposed for this extinction including either chance or competition from other large marine predators such as plesiosaurs. The overspecialisation of ichthyosaurs may be a contributing factor to their extinction, possibly being unable to 'keep up' with fast teleost fish, which had become dominant at this time, against which the sit-and-wait ambush strategies of the mosasauroids proved superior. This model thus emphasised evolutionary stagnation, the only innovation shown by Platypterygius being its ten fingers.
Recent studies, however, show that ichthyosaurs were actually far more diverse in the Cretaceous than previously thought. Fragments previously referred to "Platypterygius" have been found to be from several different taxa. As of 2012, at least eight lineages are known to have spanned the Jurassic-Cretaceous boundary including Acamptonectes, Sveltonectes, Caypullisaurus, and Maiaspondylus. In 2013, a Cretaceous basal thunnosaurian was revealed: Malawania. Indeed, likely a radiation during the Early Cretaceous occurred due to an increase of coastlines when the continents further broke up. | Ichthyosauria | Wikipedia | 342 | 314101 | https://en.wikipedia.org/wiki/Ichthyosauria | Biology and health sciences | Prehistoric marine reptiles | Animals |
The demise of the ichthyosaurs has been described as a two-step process. A first extinction event in the beginning of the Cenomanian eliminated two of the three ichthyosaur feeding guilds still present: the 'soft-prey specialists' and the 'generalists', leaving only an unspecialized apex predator group. The second extinction event took place during the Cenomanian-Turonian boundary event, a marine 'anoxic event', after which just a single lineage survived, Platypterygius hercynicus, which then disappeared about 93 million years ago. Ichthyosaur extinction was thus a pair of abrupt events rather than a long decline, probably related to the environmental upheavals and climatic changes in the Cenomanian and Turonian. Competition with early mosasaurs is unlikely to have been a contributing factor since large mosasaurs did not appear until 3 million years after the ichthyosaur extinction, filling the resulting ecological void left by the extinction of ichthyosaurs. Plesiosaurian polycoltylids perhaps also filled some of the niches previously occupied by ichthyosaurs, although they had coexisted for 19 million years. The extinction was most likely the result of ecological change and volatility that caused changes in migration, food availability, and birthing grounds. This part of the Cretaceous was one in which many other marine extinctions occurred, including those of some types of microplankton, ammonites, belemnites, and reef-building bivalves. | Ichthyosauria | Wikipedia | 330 | 314101 | https://en.wikipedia.org/wiki/Ichthyosauria | Biology and health sciences | Prehistoric marine reptiles | Animals |
Phylogeny
In modern phylogeny, clades are defined that contain all species forming a certain branch of the evolutionary tree. This also allows one to clearly indicate all relationships between the several subgroups in a cladogram. In 1999, a node clade Ichthyopterygia was defined by Motani as the group consisting of the last common ancestor of Ichthyosaurus communis, Utatsusaurus hataii and Parvinatator wapitiensis; and all its descendants. Within Motani's phylogeny, the Ichthyopterygia were the larger parent clade of a smaller stem clade Ichthyosauria that was defined as the group consisting of Ichthyosaurus communis and all species more closely related to Ichthyosaurus than to Grippia longirostris. Motani's concept of the Ichthyosauria was thus more limited than the traditional one that also contained basal forms, such as Grippia, Utatsusaurus, and Parvinatator.
The following cladogram is based on Motani (1999):
An alternative terminology was proposed by Maisch & Matzke in 2000, trying to preserve the traditional, more encompassing content of the concept Ichthyosauria. They defined a node clade Ichthyosauria as the group consisting of the last common ancestor of Thaisaurus chonglakmanii, Utatsusaurus hataii, and Ophthalmosaurus icenicus, and all its descendants. Ichthyosauria sensu Motani might materially be identical to a clade that Maisch & Matzke in 2000 called Hueneosauria, depending on the actual relationships.
Cladogram based on Maisch and Matzke (2000) and Maisch and Matzke (2003) with clade names following Maisch (2010):
Description | Ichthyosauria | Wikipedia | 370 | 314101 | https://en.wikipedia.org/wiki/Ichthyosauria | Biology and health sciences | Prehistoric marine reptiles | Animals |
Size
Ichthyosaurs averaged about in length. Some individual specimens were as short as ; some species were much larger: the Triassic Shonisaurus popularis was about long and in 2004 Shonisaurus sikanniensis (classified as a shastasaurus between 2011 and 2013) was estimated to have been in length. Fragmentary finds suggest the presence of a form in the early Jurassic. In 2018, lower jaw fragments from England were reported indicating a length of between 20 and 25 m (66 to 82 ft), which have been recently described as Ichthyotitan severnensis. According to weight estimates by Ryosuke Motani a Stenopterygius weighed around , whilst a Ophthalmosaurus icenicus weighed .
General build
While the earliest known members of the ichthyosaur lineage were more eel-like in build, later ichthyosaurs resembled more typical fishes or dolphins, having a dolphin-like head with a short neck and a long snout. Ichthyosaur fore and hind limbs had been fully transformed into flippers. Some species had a dorsal fin on their backs and a more or less vertical caudal fluke at the rear of a rather short tail. Although ichthyosaurs looked like fish, they were not.
Evolutionary biologist Stephen Jay Gould said that the ichthyosaur was his favourite example of convergent evolution, where similarities of structure are analogous, not homologous, thus not caused by a common descent, but by a similar adaptation to an identical environment:This sea-going reptile with terrestrial ancestors converged so strongly on fishes that it actually evolved a dorsal fin and tail in just the right place and with just the right hydrological design. These structures are all the more remarkable because they evolved from nothing—the ancestral terrestrial reptile had no hump on its back or blade on its tail to serve as a precursor. | Ichthyosauria | Wikipedia | 381 | 314101 | https://en.wikipedia.org/wiki/Ichthyosauria | Biology and health sciences | Prehistoric marine reptiles | Animals |
Diagnostic traits
Derived ichthyosaurs in the narrow sense, as defined by Motani in 1999, differ from their closest basal ichthyopterygian relatives in certain traits. Motani listed a number of these. The external nostril is located on the side of the skull, and is hardly visible from above. The upper rim of the eye socket consists of a bone bar formed by the prefrontal and the postfrontal bones. The postorbital in side view is excluded from the supratemporal fenestra. The opening for the parietal eye is located on the border of the parietal and the frontal bone. The lateral wing of the pterygoid is incompletely and variably ossified. The ulna lacks the part behind the original shaft axis. The rear dorsal vertebrae are disc-shaped.
Skeleton
Skull
Basal Ichthyopterygia already had elongated, triangular skulls. With ichthyosaurs in the narrow sense, their snouts became very pointy. The snout is formed by the premaxilla. The maxilla behind it is usually shorter and sometimes excluded from the external nostril by the rear branch of the premaxilla. Accordingly, the number of premaxillary teeth is high, while the maxillary teeth are fewer in number or even completely absent. The rear top of the snout is formed by the nasal bones. Derived species have a foramen internasale, a midline opening separating the rear of the nasal bones. The nasal bone usually forms the top and front rim of the bony nostril, itself often placed just in front of the eye socket. However, with some Triassic species, the premaxilla is so strongly extended at its back that it even excludes the nasal from the nostril. | Ichthyosauria | Wikipedia | 366 | 314101 | https://en.wikipedia.org/wiki/Ichthyosauria | Biology and health sciences | Prehistoric marine reptiles | Animals |
The rear of the skull is dominated by a large eye socket, often covering the major part of the rear side surface. In the socket, a large scleral ring is present; this is a circular structure of small, overlapping bone segments protecting the eye against the water pressure. Both in the relative and absolute senses, ichthyosaurs have the largest eye sockets of all known vertebrates. The other rear skull elements are typically so compressed and fused that they are difficult to identify. The top rear element of the skull was usually assumed to be the supratemporal bone, while the squamosal and quadratojugal were sometimes fused. However, in 1968, Alfred Sherwood Romer stated that the presumed supratemporal was in fact the squamosal, an interpretation which was supported by McGowan in 1973. In 1990, though, John Steve Massare convinced most researchers that the original identification had been the correct one after all. The supratemporal forms the rear rim of the supratemporal opening; a lower temporal opening at the side is lacking. The front rim of the supratemporal opening is typically formed by the postfrontal; only with the very basal Utatsusaurus the postorbital and the squamosal still reach the edge. Between the paired supratemporal openings, the skull roof is narrow; some species have a longitudinal crest on it as an attachment for the jaw muscles. Basal Ichthyopterygia have a parietal eye opening between the paired parietal bones. With ichthyosaurs proper, this opening moves to the front, first to the border between the parietals and the frontals and ultimately between the frontals, a condition shown by derived species. Postparietal and tabular bones are lacking. Often, the bones of the back of the skull and the palate are incompletely ossified, apparently having partly remained cartilage. The occipital condyle is typically very convex. The stapes, the bone transmitting sound waves from the eardrum to the middle ear, is elongated and not pierced by a foramen. Pterygoid teeth are typically lacking. | Ichthyosauria | Wikipedia | 461 | 314101 | https://en.wikipedia.org/wiki/Ichthyosauria | Biology and health sciences | Prehistoric marine reptiles | Animals |
Lower jaws
Like the snout, the lower jaws are elongated. However, in some species, such as Eurhinosaurus and Excalibosaurus, the front of the snout far protrudes beyond the lower jaws. While the front of the lower jaw is typically low, its rear depth is very variable. The greater part of the lower jaw is formed by the front dentary, the tooth-bearing bone. At its inner side the dentary is covered by a splenial that extends forwards until the symphysis, the common contact surface where both lower jaws are grown together. The jaw joints do not allow a horizontal chewing movement: they function as simple hinges to vertically open or close the jaws.
Teeth
Ichthyosaur teeth are typically conical. Fish-eating species have long and slender tooth crowns that are slightly recurved. Forms specialised in catching larger prey have shorter, broader, and straighter teeth; sometimes, cutting edges are present. Thalattoarchon, an apex predator, had larger teeth formed like flattened blades. Durophagous species that ate shellfish have low, convex teeth that are closely packed. Many ichthyosaur dentitions are heterodont, combining several tooth shapes, e.g. small teeth in the front and larger teeth at the rear. The teeth are usually placed in tooth sockets; derived species possess a common tooth groove, a condition known as aulacodonty. In the latter case, adult individuals sometimes become toothless. Teeth in tooth sockets sometimes fuse with the jawbone. With ichthyosaur teeth, the dentine shows prominent vertical wrinkles. Durophagous forms have teeth with deep vertical grooves and wrinkles in the enamel.
Postcrania
Vertebral column | Ichthyosauria | Wikipedia | 359 | 314101 | https://en.wikipedia.org/wiki/Ichthyosauria | Biology and health sciences | Prehistoric marine reptiles | Animals |
Basal Ichthyopterygia, like their land-dwelling ancestors, still had vertebrae that possessed a full set of processes that allowed them to interlock and articulate, forming a vertebral column supporting the weight of the body. As ichthyosaurs were fully aquatic, their bodies were supported by the Archimedes force exerted by the water; in other words, they were buoyant. Therefore, the vertebral processes had lost much of their function. Early ichthyosaurs proper had rear dorsal vertebrae that had become disc-shaped, like those of typical fishes. With more derived species, the front dorsals also became discs. Gradually, most processes were lost, including those for rib attachment. The vertebral bodies became much shorter. The front and rear sides of the discs were hollowed out, resulting in a so-called amphicoelous condition. A transverse cross-section of such a vertebra has an hourglass shape. This morphology is unique within the Amniota and makes discerning ichthyosaur vertebrae from those of other marine reptiles easy. The only process that kept its function was the spine at the top, serving as an attachment for the dorsal muscles. However, even the spine became a simple structure. The neural arch, of which it was an outgrowth, typically no longer fused to the vertebral centre.
The neck is short, and derived species show a reduction in the number of cervical vertebrae. The short neck positions the skull close to the trunk, usually in a slight oblique elevation to it. Derived species usually also have a reduced number of dorsals, the total of presacral vertebrae totalling about forty to fifty. The vertebral column is little differentiated. Basal Ichthyopterygia still have two sacral vertebrae, but these are not fused. Early Triassic forms have a transversely flattened tail base with high spines for an undulating tail movement. Derived forms have a shorter tail with the characteristic kink at the end; a section of wedge-shaped vertebrae, itself supporting the fleshy upper tail fin lobe, forced the tail end into the lower fin lobe. | Ichthyosauria | Wikipedia | 440 | 314101 | https://en.wikipedia.org/wiki/Ichthyosauria | Biology and health sciences | Prehistoric marine reptiles | Animals |
As derived species no longer have transversal processes on their vertebrae—again a condition unique in the Amniota—the parapophyseal and diapophysael rib joints have been reduced to flat facets, at least one of which is located on the vertebral body. The number of facets can be one or two; their profile can be circular or oval. Their shape often differs according to the position of the vertebra within the column. The presence of two facets per side does not imply that the rib itself is double-headed: often, even in that case, it has a single head. The ribs typically are very thin and possess a longitudinal groove on both the inner and the outer sides. The lower side of the chest is formed by gastralia. These belly ribs have a single centre segment and one or two outer segments per side. They are not fused into a real plastron. Usually two gastralia are present per dorsal rib.
Appendicular skeleton
The shoulder girdle of ichthyosaurs is not much modified from its original condition. Some basal forms show a hatchet- or crescent-shaped shoulder blade or scapula; derived forms have an elongated blade positioned on a broader base. The scapula is not fused with the coracoid into a scapulocoracoid, indicating that the forces exerted on the shoulder girdle were moderate. The shoulder joint is positioned on the border between the scapula and the coracoid. Both coracoids are fused on their common midline. The coracoid shape is very variable, but usually it is rather low. The upper part of the shoulder girdle is formed by two long and slender clavicles, crowned by a central interclavicular bone that is large and triangular with basal forms, small and T-shaped in Jurassic species. Breast bones or sterna are absent. | Ichthyosauria | Wikipedia | 394 | 314101 | https://en.wikipedia.org/wiki/Ichthyosauria | Biology and health sciences | Prehistoric marine reptiles | Animals |
Basal forms have a forelimb that is still functionally differentiated, in some details resembling the arm of their land-dwelling forebears; the ulna and radius are elongated and somewhat separated; the carpals are rounded, allowing the wrist to rotate; the number of phalanges is within the range shown by land animals. Ichthyosaurs proper, to the contrary, have a forelimb that is fully adapted to its function as a flipper. However, the adaptations are very variable. Triassic species typically have a very derived humerus, changed into a disc. Jurassic species tend to have a more elongated humeral form with a rounded head, narrow shaft, and expanded lower end. The radius and ulna are always strongly flattened, but can be circular, with or without notch, or have a waist. Notches can be homologous to the original shafts, but also be newly formed. Jurassic forms no longer have a space, the spatium interosseum, between the radius and ulna. Often, the latter bones gradually merge into lower, disc-shaped elements - the up to four carpals which again differ little in form from the up to five metacarpals. | Ichthyosauria | Wikipedia | 244 | 314101 | https://en.wikipedia.org/wiki/Ichthyosauria | Biology and health sciences | Prehistoric marine reptiles | Animals |
A strongly derived condition show the phalanges, small, disc-shaped elements positioned in long rows. Sometimes, the number of fingers is reduced, to as low as two. This is a rather common phenomenon within the Tetrapoda. Unique, however, for derived tetrapods, is the fact that some species show nonpathological polydactyly, the number of fingers being higher than five. Some species had ten fingers per hand (eg, Caypullisaurus). These fingers, again, can have an increased number of phalanges, up to thirty, a phenomenon called hyperphalangy, also known from the Plesiosauria, mosasaurs, and the Cetacea. The high number of elements allows the flipper to be shaped as a hydrofoil. When a high number of fingers is present, their identity is difficult to determine. It is usually assumed that fingers were added at both the front and at the rear, perhaps to a core of four original fingers. If fingers are added, often the number of metacarpals and carpals is also increased; sometimes even an extra lower arm element is present. Earlier, ichthyosaurs were commonly divided into "longipinnate" and "latipinnate" forms, according to the long or wide shape of the front flippers, but recent research has shown that these are not natural groups; ichthyosaur clades often contain species with and without elongated forelimbs. | Ichthyosauria | Wikipedia | 308 | 314101 | https://en.wikipedia.org/wiki/Ichthyosauria | Biology and health sciences | Prehistoric marine reptiles | Animals |
The ichthyosaur pelvis is typically rather reduced. The three pelvic bones: the ilium, the ischium, and the pubic bone, are not fused and often do not even touch each other. Also, the left and right pelvic sides no longer touch; only basal forms still have sacral ribs connecting the ilia to the vertebral column. The hip joint is not closed on the inside. The pubic bone typically does not connect to the ischium behind it; the space in between is by some workers identified as the fenestra thyreoidea; other researchers deny that the term is applicable given the general loose structure of the pelvis. Some later species have a connected pubic bone and ischium, but in this case, the femoral head no longer articulates with the hip joint. Triassic species have plate-like pubic bones and ischia; in later species these elements become elongated with a narrow shaft and can form a single rod.
Typically, the hindlimbs are shorter than the forelimbs, possessing a lesser number of elements. Often, the rear flipper is only half the length of the front flipper. The thighbone is short and broad, often with a narrow waist and an expanded lower end. The tibia, fibula and metatarsals are merged into a mosaic of bone discs supporting the hydrofoil. Three to six toes are present. The toe phalanges also show hyperphalangy; exceptionally, Ophthalmosaurus shows a reduced number of phalanges.
Soft tissue
The earliest reconstructions of ichthyosaurs all omitted dorsal fins and caudal (tail) flukes, which were not supported by any hard skeletal structure, so were not preserved in many fossils. Only the lower tail lobe is supported by the vertebral column. In the early 1880s, the first body outlines of ichthyosaurs were discovered. In 1881, Richard Owen reported ichthyosaur body outlines showing tail flukes from Lower Jurassic rocks in Barrow-upon-Soar, England. Other well-preserved specimens have since shown that in some more primitive ichthyosaurs, like a specimen of Chaohusaurus geishanensis, the tail fluke was weakly developed and only had a dorsal tail lobe, making the tail more paddle-like. Over the years, the visibility of the tail lobe has faded away in this specimen. | Ichthyosauria | Wikipedia | 499 | 314101 | https://en.wikipedia.org/wiki/Ichthyosauria | Biology and health sciences | Prehistoric marine reptiles | Animals |
The presence of dorsal fins in ichthyosaurs has been controversial. Finely preserved specimens from the Holzmaden Lagerstätten in Germany found in the late 19th century revealed additional traces, usually preserved in black, of the outline of the entire body, including the first evidence of dorsal fins in ichthyosaurs. Unique conditions permitted the preservation of these outlines, which probably consist of bacterial mats, not the remains of the original tissues themselves. In 1987, David Martill argued that, given the indirect method of conservation by bacteria, these outlines were unlikely to have been reliably preserved in any fine detail. He concluded that no authentic dorsal fins had been discovered. After displaced skins flaps from the body would have initially been misinterpreted as fins, fossil preparators later came to expect such fins to be present, and would have identified any discolouration in the appropriate position as a dorsal fin or even have falsified such structures. The lack of a dorsal fin would also explain why ichthyosaurs, contrary to porpoises, retained hind flippers, as these were needed for stability. Other researchers noted that, while the outlines might have been sharpened and smoothed by preparators because fossil bacterial mats usually have indistinct edges, many of the preserved dorsal fins were probably authentic and at least somewhat close to the true body outline. At least one specimen, R158 (in the collections of the Paleontologiska Museet, Uppsala University), shows the expected faded edges of a bacterial mat, so it has not been altered by preparators, yet still preserves a generally tuna-like body outline including a dorsal fin. In 1993, Martill admitted that at least some dorsal fin specimens are authentic.
The fossil specimens that preserved dorsal fins also showed that the flippers were pointy and often far wider than the underlying bones would suggest. The fins were supported by fibrous tissue. In some specimens, four layers of collagen are visible, the fibres of the covering layers crossing those of the collagen below. | Ichthyosauria | Wikipedia | 420 | 314101 | https://en.wikipedia.org/wiki/Ichthyosauria | Biology and health sciences | Prehistoric marine reptiles | Animals |
In 2017, from the German Posidonia Shale the discovery was reported of 182.7-million-year-old vertebrae of Stenopterygius in a carbonate nodule, still containing collagen fibers, cholesterol, platelets, and red and white blood cells. The structures would not have been petrified, but represent the original organic tissues of which the biomolecules could be identified. The exceptional preservation was explained by the protective environment offered by the nodule. The red blood cells found, were one-fourth to one fifth the size of those of modern mammals. This would have been an adaptation for an improved oxygen absorption, also in view of the low oxygen levels during the Toarcian. The cholesterol had a high-carbon-13 isotope component which might indicate a higher position in the food chain and a diet of fish and cephalopods.
In 2018, evidence of blubber was discovered with Stenopterygius.
Skin and colouration
Typically, fossils that preserve it suggest that the skin of ichthyosaurs was smooth and elastic, lacking scales. However, these remains are not impressions per se, but outlines formed from bacterial growth. In one case, a true impression of the skin was reported from a specimen of Aegirosaurus found in the Solnhofen Plattenkalk, rocks which were capable of preserving even the finest detail. Minuscule scales seemed to be visible in this specimen. | Ichthyosauria | Wikipedia | 298 | 314101 | https://en.wikipedia.org/wiki/Ichthyosauria | Biology and health sciences | Prehistoric marine reptiles | Animals |
The colouration of ichthyosaurs is difficult to determine. In 1956, Mary Whitear reported finding melanocytes, pigment cells in which reddish-brown pigment granules would still be present, in a skin specimen of a British fossil, R 509. Ichthyosaurs are traditionally assumed to have employed countershading (dark on top, light at the bottom) like sharks, penguins, and other modern animals, serving as camouflage during hunting. This was contradicted in 2014 by the discovery of melanosomes, black melanin-bearing structures, in the skin of ichthyosaur specimen YORYM 1993.338 by Johan Lindgren of Lund University. It was concluded that ichthyosaurs were likely uniformly dark coloured for thermoregulation and to camouflage them in deep water while hunting. This is in contrast to mosasaurids and prehistoric leatherback turtles, which were found to be countershaded. However, a 2015 study doubted Lindgren and colleagues' interpretation. This study noted that a basal layer of melanosomes in the skin is ubiquitous in reptile coloration, but does not necessarily correspond to a dark appearance. Other chromatophore structures (such as iridiophores, xanthophores, and erythrophores) affect coloration in extant reptiles but are rarely preserved or identified in fossils. Thus, due to the unknown presence of these chromatophores, YORYM 1993.338, could have been countershaded, green, or various other colors or patterns. In 2018, Lindgren and his colleagues also supported that ichthyosaurs would have been countershaded, on the basis of distributional
variation of melanophores that contain eumelanin found on the specimen of Stenopterygius.
Gastroliths
Gastroliths, stomach stones that might have assisted digestion or regulated buoyancy, have only on a few occasions been found associated with ichthyosaur skeletons, once with a specimen of Nannopterygius and a second time in a Panjiangsaurus fossil. Ichthyosaur coproliths, petrified faeces, are very common, though, already being sold by Mary Anning.
Paleobiology | Ichthyosauria | Wikipedia | 466 | 314101 | https://en.wikipedia.org/wiki/Ichthyosauria | Biology and health sciences | Prehistoric marine reptiles | Animals |
Ecology
Apart from the obvious similarities to fish, ichthyosaurs also shared parallel developmental features with dolphins, lamnid sharks, and tuna. This gave them a broadly similar appearance, possibly implied similar activity levels (including thermoregulation), and presumably placed them broadly in a similar ecological niche. Ichthyosaurs were not primarily coastal animals; they also inhabited the open ocean. They have been found in all Mesozoic oceans. This is even true of the earliest Ichthyopterygia, making identification of a certain area as their place of origin impossible.
Feeding
Ichthyosaurs were carnivorous; they ranged so widely in size, and survived for so long, that they are likely to have had a wide range of prey. Species with pointed snouts were adapted to grab smaller animals. McGowan speculated that forms with protruding upper jaws, in the Eurhinosauria, would have used their pointy snouts to slash prey, as has been assumed for swordfish. The most commonly preserved gut contents in ichthyosaurs are the remains of cephalopods. Less commonly, they fed on fish and other vertebrates, including smaller ichthyosaurs. The large Triassic form Thalattoarchon had large, bladed teeth and was probably a macropredator, capable of killing prey its own size, and Himalayasaurus and several species of Temnodontosaurus also shared adaptations for killing very large prey. These food preferences have been confirmed by coproliths which indeed contain the remains of fishes and cephalopods. Another confirmation is provided by fossilised stomach contents. Buckland in 1835 described the presence in a specimen of a large mass of partly digested fishes, recognisable by their scales. Subsequent research in 1968 determined that these belonged to the fish genus Pholidophorus, but also that cephalopod beaks and sucker hooks were present. Such hard food particles apparently were retained by the stomach and regularly regurgitated. Carcasses of drowned animals were eaten as well: in 2003 a specimen of Platypterygius longmani was reported having, besides fishes and a turtle, the bones of a land bird in its stomach. | Ichthyosauria | Wikipedia | 451 | 314101 | https://en.wikipedia.org/wiki/Ichthyosauria | Biology and health sciences | Prehistoric marine reptiles | Animals |
Some early ichthyosaurs were durophagous and had flat convex teeth adapted for crushing shellfish. They thus ate benthos from the floor of shallow seas. Other species were perhaps suction feeders, sucking animals into their mouths by quickly opening their relatively short jaws. This was first assumed for Shonisaurus, which giant by this means might have secured a constant food supply for its huge body, and in 2011 for the short-snouted Guanlingsaurus liangae. However, in 2013 a study concluded that the hyoid bone of ichthyosaurs, at the tongue base, was insufficiently ossified to support a suction feeding movement and suggested the alternative that such species were ram feeders, gathering food by constantly swimming forwards with a wide-open mouth.
Typical ichthyosaurs had very large eyes, protected within a bony ring, suggesting that they may have hunted at night or at great depths; the only extant animals with similarly large eyes are the giant and colossal squids. Sight thus seems to have been one of the main senses employed while hunting. Hearing might have been poor, given the very robust form of the stapes. Grooves in the palate however, suggest that smell might have been acute or even that electro-sensory organs might have been present.
Ichthyosaurs themselves served as food for other animals. During the Triassic their natural predators mainly consisted of sharks and other ichthyosaurs; in the Jurassic these were joined by large Plesiosauria and Thalattosuchia. This is again confirmed by stomach contents: in 2009 e.g., a plesiosaur specimen was reported with an ichthyosaur embryo in its gut.
Locomotion
In ichthyosaurs, the main propulsion was provided by a lateral movement of the body. Early forms employed an anguilliform or eel-like movement, with undulations of the entire trunk and tail. This is usually considered rather inefficient. Later forms, like the Parvipelvia, has a shorter trunk and tail and probably used a more efficient carangiform or even thunniform movement, in which the last third of the body, respectively, the tail end, is flexed only. The trunk in such species is rather stiff. | Ichthyosauria | Wikipedia | 465 | 314101 | https://en.wikipedia.org/wiki/Ichthyosauria | Biology and health sciences | Prehistoric marine reptiles | Animals |
The tail was bi-lobed, with the lower lobe being supported by the caudal vertebral column, which was "kinked" ventrally to follow the contours of the ventral lobe. Basal species had a rather asymmetric or "heterocercal" tail fin. The asymmetry differed from that of sharks in that the lower lobe was largest, instead of the upper lobe. More derived forms had a nearly vertical symmetric tail fin. Sharks use their asymmetric tail fin to compensate for the fact that they are negatively buoyant, heavier than water, by making the downward pressure exerted by the tail force the body as a whole in an ascending angle. This way, swimming forwards will generate enough lift to equal the sinking force caused by their weight. In 1973, McGowan concluded that, because ichthyosaurs have a reversed tail fin asymmetry compared to sharks, they were apparently positively buoyant, lighter than water, which would be confirmed by their lack of gastroliths and of pachyostosis or dense bone. The tail would have served to keep the body in a descending angle. The front flippers would be used to push the front of the body further downwards and control pitch. In 1987 however, Michael A. Taylor suggested an alternative hypothesis: as ichthyosaurs could vary their lung content, contrary to sharks (which lack a swimming bladder), they could also regulate their buoyancy. The tail thus mainly served for a neutral propulsion, while small variations in buoyancy were stabilised by slight changes in the flipper angles. In 1992, McGowan accepted this view, pointing out that shark tails are not a good analogy of derived ichthyosaur tails that have more narrow lobes, and are more vertical and symmetric. Derived ichthyosaur tail fins are more like those of tuna fish and indicate a comparable capacity to sustain a high cruising speed. A comparative study by Motani in 2002 concluded that, in extant animals, small tail fin lobes positively correlate with a high beat frequency. Modern researchers generally concur that ichthyosaurs were negatively buoyant. | Ichthyosauria | Wikipedia | 440 | 314101 | https://en.wikipedia.org/wiki/Ichthyosauria | Biology and health sciences | Prehistoric marine reptiles | Animals |
In 1994, Judy Massare concluded that ichthyosaurs had been the fastest marine reptiles. Their length/depth ratio was between three and five, the optimal number to minimise water resistance or drag. Their smooth skin and streamlined bodies prevented excessive turbulence. Their hydrodynamic efficiency, the degree to which energy is converted into a forward movement, would approach that of dolphins and measure about 0.8. Ichthyosaurs would be a fifth faster than plesiosaurs, though half of the difference was explained by assuming a 30% higher metabolism for ichthyosaurs. Together, within Massare's model these effects resulted in a cruising speed of slightly less than five kilometres per hour. However, in 2002, Motani corrected certain mistakes in Massare's formulae and revised the estimated cruising speed to less than two kilometres per hour, somewhat below that of modern Cetacea. However, as the speeds estimated for plesiosaurs and mosasaurids were also revised downwards, ichthyosaurs maintained their relative position. | Ichthyosauria | Wikipedia | 213 | 314101 | https://en.wikipedia.org/wiki/Ichthyosauria | Biology and health sciences | Prehistoric marine reptiles | Animals |
Ichthyosaurs had fin-like limbs of varying relative length. The standard interpretation is that these, together with the dorsal fin and tail fin, were used as control surfaces for directional stability, controlling yaw, and for stabilising pitch and roll, rather than propulsion. However, during the 1980s the German paleontologist Jürgen Riess proposed an alternative model. After having studied the flying movement made by the forelimbs of plesiosaurs, he suggested that at least those ichthyosaurs that had long flippers used them for a powerful propulsive stroke, moving them up and down. This would explain the non-degenerated shoulder girdle and the evolution of the hand bones, whose perfect hydrofoil profile would have been useless if it was not functionally employed. He thought to have discovered modern analogues in the Queensland lungfish and the Amazon river dolphin, which he presumed also used their long fins for propulsion. Riess expounded upon this hypothesis in a series of articles. This alternative interpretation was generally not adopted by other workers. In 1998, Darren Naish pointed out that the lungfish and the river dolphin actually do not use their fins in this way and that e.g. the modern humpback whale has very long front flippers, supported by a mosaic of bones, but that these nevertheless mainly serve as rudders. In 2013, a study concluded that broad ichthyosaur flippers, like those of Platyptergygius, were not used for propulsion but as a control surface.
Diving
Many extant lung-breathing marine vertebrates are capable of deep diving. There are some indications about the diving capacity of ichthyosaurs. Quickly ascending from a greater depth can cause decompression sickness. The resulting bone necrosis has been well documented with Jurassic and Cretaceous ichthyosaurs, where it is present in 15% and 18% of specimens, respectively, but is rare in Triassic species. This could be a sign that basal forms did not dive as deeply, but might also be explained by a greater predation pressure during the later epochs, more often necessitating a fast flight to the surface. However, this last possibility is contradicted by the fact that, with modern animals, damage is not caused by a limited number of rapid ascension incidents, but by a gradual accumulation of non-invalidating degeneration during normal diving behaviour. | Ichthyosauria | Wikipedia | 490 | 314101 | https://en.wikipedia.org/wiki/Ichthyosauria | Biology and health sciences | Prehistoric marine reptiles | Animals |
Additional evidence is provided by the eyes of ichthyosaurs that among vertebrates are both relatively and absolutely the largest known. Modern leopard seals can dive to up to hunting on sight. Motani suggested that ichthyosaurs, with their relatively much larger eye sockets, should have been able to reach even greater depths. Temnodontosaurus, with eyes that had a diameter of twenty-five centimetres, could probably still see at a depth of 1,600 metres. At these depths, such eyes would have been especially useful to see large objects. Later species, such as Ophthalmosaurus, had relatively larger eyes, again an indication that diving capacity was better in late Jurassic and Cretaceous forms.
Metabolism
Similar to modern cetaceans, such as whales and dolphins, ichthyosaurs were air-breathing. Whales and dolphins are mammals and warm-blooded. Of ichthyosaurs it was traditionally assumed that they were cold-blooded, being reptiles. However, since the 1970s many dominant reptile groups of the Mesozoic, such as theropod dinosaurs, pterosaurs and plesiosaurs, have been considered warm-blooded, as this offers an elegant explanation of their dominance. Some direct evidence is available that ichthyosaurs too might have been endothermic. In 1990, Vivian de Buffrénil published a histological study, indicating that ichthyosaurs possessed a fibrolamellar bone structure, as with warm-blooded animals in general, typified by fast growth and a strong vascularisation. Early Triassic species already show these traits. In 2012, it was reported that even the very basal form Utatsusaurus had this bone type, indicating that the ancestors of ichthyosaurs were already warm-blooded. Additional direct proof for a high metabolism is the isotopes of oxygen ratio in the teeth, which indicates a body temperature of between 35 and 39 °C, about 20° higher than the surrounding seawater. Blubber is consistent with warm-bloodedness as the insulating qualities require the animal to generate its own heat. | Ichthyosauria | Wikipedia | 426 | 314101 | https://en.wikipedia.org/wiki/Ichthyosauria | Biology and health sciences | Prehistoric marine reptiles | Animals |
Indirect evidence for endothermy is provided by the body shape of derived ichthyosaurs, which with its short tail and vertical tail fin seems optimised for a high cruising speed that can only be sustained by a high metabolism: all extant animals swimming this way are either fully warm-blooded or, like sharks and tuna, maintain a high temperature in their body core. This argument does not cover basal forms with a more eel-like body and undulating swimming movement. In 1996, Richard Cowen, while accepting endothermy for the group, presumed that ichthyosaurs would have been subject to Carrier's constraint, a limitation to reptilian respiration pointed out in 1987 by David Carrier: their undulated locomotion forces the air out of the lungs and thus prevents them from taking breath while moving. Cowen hypothesised that ichthyosaurs would have overcome this problem by porpoising: constantly jumping out of the water would have allowed them to take a gulp of fresh air during each jump. Other researchers have tended to assume that for at least derived ichthyosaurs Carrier's constraint did not apply, because of their stiff bodies, which seems to be confirmed by their good diving capacity, implying an effective respiration and oxygen storage system. For these species porpoising was not a necessity. Nevertheless, ichthyosaurs would have often surfaced to breathe, probably tilting their heads slightly to take in air, because of the lower position of the nostrils compared to that of dolphins.
Reproduction | Ichthyosauria | Wikipedia | 307 | 314101 | https://en.wikipedia.org/wiki/Ichthyosauria | Biology and health sciences | Prehistoric marine reptiles | Animals |
Ichthyosaurs were viviparous, i.e. bore live young instead of laying eggs. Although they were reptiles and descended from egg-laying, oviparous, ancestors, viviparity is not as unexpected as it first appears. Air-breathing marine creatures must either come ashore to lay eggs, like turtles and some sea snakes, or else give birth to live young in surface waters, like whales and dolphins. Given their streamlined and transversely flattened bodies, heavily adapted for fast swimming, it would have been difficult, if not impossible, for ichthyosaurs to move far enough on land to lay eggs. This was confirmed as early as 9 December 1845 when naturalist Joseph Chaning Pearce reported a small embryo in a fossil of Ichthyosaurus communis. The embryo, with a length of eleven centimetres, was positioned in the birth canal of its two-and-a-half metre long mother, with its head pointed to the rear. Pearce concluded from the fossil that ichthyosaurs had to have been viviparous.
Later, from the Holzmaden deposits numerous adult fossils were found containing fetuses. In 1880, Harry Govier Seeley, heading a special British paleontological committee studying the problem of ichthyosaur reproduction, concluded that birth was given in the water and that fossils containing fetuses in the birth canal probably represented cases of premature death of the juvenile, causing the demise of the mother animal as well. A comparison has been made with dolphins and whales, whose young need to be born tail-first to prevent drowning; if the juvenile is born head-first, it dies and the mother with it if the corpse gets stuck in the birth canal. However, an alternative explanation is that such fossils actually represent females that had died for other reasons while pregnant, after which the decomposition gasses drove out the fetuses head-first. In 2014, a study reported the find of a fossilized Chaohusaurus female that had died while giving birth to three neonates. Two had already been expelled while a third was present in the birth canal. The fossil also documented that early ichthyosaurs were also born head first, perhaps opposed to later genera. As Chaohusaurus is a very basal ichthyopterygian—previously, the most basal genus of which fetuses were known, had been Mixosaurus—this discovery suggests that the earliest land-dwelling ancestors of ichthyosaurs had already been viviparous. | Ichthyosauria | Wikipedia | 507 | 314101 | https://en.wikipedia.org/wiki/Ichthyosauria | Biology and health sciences | Prehistoric marine reptiles | Animals |
A comprehensive multi-author study published in 2023 examined the evolution of fetal orientation of ichthyosaurs based on known specimens of gravid female ichthyosaurs. Specimens of basal ichthyosaurs, Chaohusaurus and Cymbospondylus, showed evidence of head-first birth, while Mixosaurus had evidence of both head-first and tail-first birth based on three specimens. More derived ichthyosaurus including Stenopterygius, Besanosaurus, Qianichthyosaurus and Platypterygius showed evidence of tail-first birth. This indicates that while basal ichthyosaurs were born with head-first, merriamosaurian ichthyosaurs had preference of tail-first birth over head-first birth. The authors asserted that the derived ichthyosaurs' preference of tail-first birth may have been because it was easy for the female to push on the cranium rather than the pelvis when giving birth, or because it could reduce maternal energy expenditure on trim control. They disagreed with the "increased asphyxiation risk" hypothesis for tail-first birth preference, given that Mixosaurus showed evidence of both fetal orientation of head-first and tail-first birth; if this was indeed the reason, there should have been a higher preference for tail-first births caused by strong stabilizing selection for this trait much earlier in the evolutionary history of every aquatic, viviparous tetrapod clades, which isn't the case.
Compared with placental mammals or plesiosaurs, ichthyosaur fetuses tend to be very small and their number per litter is often high. In one female of Stenopterygius seven have been identified, in another eleven. The fetuses have at most a quarter of the length of the mother animal. The juveniles have about the same body proportions as adult individuals. The main ontogenetical changes during growth consist in the fusion and greater robustness of the skeletal elements. At least one neonate I. communis individual has been identified, with preserved stomach contents indicating feeding on cephalopods and fish. This is unlike other similar species, such as Stenopterygius, where feeding niches shift from small fish to larger cephalopods through ontogeny. | Ichthyosauria | Wikipedia | 472 | 314101 | https://en.wikipedia.org/wiki/Ichthyosauria | Biology and health sciences | Prehistoric marine reptiles | Animals |
Crocodiles, most sea turtles and some lizards determine the sex of their offspring by manipulating the temperature of the developing eggs' environment; i.e. they do not have distinct sex chromosomes. Live-bearing reptiles do not regulate sex through incubation temperature. A study in 2009, which examined 94 living species of reptiles, birds and mammals, found that the genetic control of sex appears to be crucial to live birth. It was concluded that with marine reptiles such control predated viviparity and was an adaptation to the stable sea-climate in coastal regions. Genetics likely controlled sex in ichthyosaurs, mosasaurs and plesiosaurs.
Social behaviour and intelligence
Ichthyosaurs are often assumed to have lived in herds or hunting groups. Little evidence is available about the nature of ichthyosaur social behaviour. Some indications exist that a level of sexual dimorphism was present. Skeletons of Eurhinosaurus and Shastasaurus show two morphotypes. Individuals with a longer snout, larger eyes, a longer trunk, a shorter tail, and longer flippers with additional phalanges, could have represented the females; the longer trunk may have provided room for the embryos.
Generally, the brain shows the limited size and elongated shape of that of modern cold-blooded reptiles. However, in 1973, McGowan, while studying the natural endocast of a well-preserved specimen, pointed out that the telencephalon was not very small. The visual lobes were large, as could be expected from the eye size. The olfactory lobes were, though not especially large, well-differentiated; the same was true of the cerebellum. | Ichthyosauria | Wikipedia | 345 | 314101 | https://en.wikipedia.org/wiki/Ichthyosauria | Biology and health sciences | Prehistoric marine reptiles | Animals |
Pathologies
Though fossils revealing ichthyosaur behavior remain rare, one ichthyosaur fossil is known to have sustained bites to the snout region. Discovered in Australia, and analyzed by Benjamin Kear et alii in 2011, measurements of the wounds reveal that the bite marks were inflicted by another ichthyosaur, likely of the same species, a probable case of face biting during a conflict. The wounds show signs of healing in the form of bone growth, meaning that the victim survived the attack. Another, very large ichthyosaur close to nine metres in length was found in Svalbard; it was nearly complete save for its tail. Scrutiny of the find revealed that while hunting ammonites (as evidenced by an ammonite shell in the throat region), the ichthyosaur was ambushed and attacked, likely by a pliosaurid (known from the same habitat), which severed its tail. The ichthyosaur then sank to the depths, drowning and eventually becoming fossilized in the deep water. The find was revealed to the public in the National Geographic special Death of a Sea Monster.
Geological formations
The following is a list of geological formations in which ichthyosaur fossils have been found: | Ichthyosauria | Wikipedia | 248 | 314101 | https://en.wikipedia.org/wiki/Ichthyosauria | Biology and health sciences | Prehistoric marine reptiles | Animals |
A skeleton key (also known as a passkey) is a type of master key in which the serrated edge has been removed in such a way that it can open numerous locks, most commonly the warded lock. The term derives from the fact that the key has been reduced to its essential parts.
Master keys
A skeleton key is a key that has been filed or cut to create one that can be used to unlock a variety of warded locks each with a different configuration of wards. This can usually be done by removing most of the center of the key, allowing it to pass by the wards without interference, operating the lock. To counteract the illicit creation of such keys, locksmiths can put wards not just in the center but on the outside as well, making the creation of a skeleton key more difficult.
Lever lock skeleton keys are used in a lock with usually three or five levers and a set of wards that come into contact with the bit of the key only on the sides—the top is for pushing the levers to their correct heights while the warded section of the key just has to pass uninterrupted to allow the key to rotate fully. A master key system of lever locks has the same lever heights in all locks. Each door will have different wards and can only be opened by the correctly warded key or the master key. A skeleton key has the warded section of the key removed so that it opens all the doors of a system.
Some applications, such as a building with multiple entrance doors, have numerous locks that are keyed alike; one key will open every door. A keyed-alike system is different from a master key system as none of the locks have a key that can open only that lock.
Skeleton keys have often been associated with attempts to defeat locks for illicit purposes, to release handcuffs for example, and standard keys have been filed down for that purpose. Legitimate skeleton or master keys are used in many modern contexts where lock operation is required and the original key has been lost or is not available. In hotels without electronic locks, skeleton keys are used by housekeeping services to enter the rooms. | Skeleton key | Wikipedia | 434 | 4053383 | https://en.wikipedia.org/wiki/Skeleton%20key | Technology | Mechanisms | null |
In universal algebra and in model theory, a structure consists of a set along with a collection of finitary operations and relations that are defined on it.
Universal algebra studies structures that generalize the algebraic structures such as groups, rings, fields and vector spaces. The term universal algebra is used for structures of first-order theories with no relation symbols. Model theory has a different scope that encompasses more arbitrary first-order theories, including foundational structures such as models of set theory.
From the model-theoretic point of view, structures are the objects used to define the semantics of first-order logic, cf. also Tarski's theory of truth or Tarskian semantics.
For a given theory in model theory, a structure is called a model if it satisfies the defining axioms of that theory, although it is sometimes disambiguated as a semantic model when one discusses the notion in the more general setting of mathematical models. Logicians sometimes refer to structures as "interpretations", whereas the term "interpretation" generally has a different (although related) meaning in model theory; see interpretation (model theory).
In database theory, structures with no functions are studied as models for relational databases, in the form of relational models.
History
In the context of mathematical logic, the term "model" was first applied in 1940 by the philosopher Willard Van Orman Quine, in a reference to mathematician Richard Dedekind (1831 – 1916), a pioneer in the development of set theory. Since the 19th century, one main method for proving the consistency of a set of axioms has been to provide a model for it.
Definition
Formally, a structure can be defined as a triple consisting of a domain a signature and an interpretation function that indicates how the signature is to be interpreted on the domain. To indicate that a structure has a particular signature one can refer to it as a -structure.
Domain
The domain of a structure is an arbitrary set; it is also called the of the structure, its (especially in universal algebra), its (especially in model theory, cf. universe), or its . In classical first-order logic, the definition of a structure prohibits the empty domain.
Sometimes the notation or is used for the domain of but often no notational distinction is made between a structure and its domain (that is, the same symbol refers both to the structure and its domain.)
Signature
The signature of a structure consists of: | Structure (mathematical logic) | Wikipedia | 494 | 4055928 | https://en.wikipedia.org/wiki/Structure%20%28mathematical%20logic%29 | Mathematics | Model theory | null |
a set of function symbols and relation symbols, along with
a function that ascribes to each symbol a natural number
The natural number of a symbol is called the arity of because it is the arity of the interpretation of
Since the signatures that arise in algebra often contain only function symbols, a signature with no relation symbols is called an algebraic signature. A structure with such a signature is also called an algebra; this should not be confused with the notion of an algebra over a field.
Interpretation function
The interpretation function of assigns functions and relations to the symbols of the signature. To each function symbol of arity is assigned an -ary function on the domain. Each relation symbol of arity is assigned an -ary relation on the domain. A nullary (-ary) function symbol is called a constant symbol, because its interpretation can be identified with a constant element of the domain.
When a structure (and hence an interpretation function) is given by context, no notational distinction is made between a symbol and its interpretation For example, if is a binary function symbol of one simply writes rather than
Examples
The standard signature for fields consists of two binary function symbols and where additional symbols can be derived, such as a unary function symbol (uniquely determined by ) and the two constant symbols and (uniquely determined by and respectively).
Thus a structure (algebra) for this signature consists of a set of elements together with two binary functions, that can be enhanced with a unary function, and two distinguished elements; but there is no requirement that it satisfy any of the field axioms. The rational numbers the real numbers and the complex numbers like any other field, can be regarded as -structures in an obvious way:
In all three cases we have the standard signature given by
with and
The interpretation function is:
is addition of rational numbers,
is multiplication of rational numbers,
is the function that takes each rational number to and
is the number and
is the number
and and are similarly defined.
But the ring of integers, which is not a field, is also a -structure in the same way. In fact, there is no requirement that of the field axioms hold in a -structure.
A signature for ordered fields needs an additional binary relation such as or and therefore structures for such a signature are not algebras, even though they are of course algebraic structures in the usual, loose sense of the word. | Structure (mathematical logic) | Wikipedia | 482 | 4055928 | https://en.wikipedia.org/wiki/Structure%20%28mathematical%20logic%29 | Mathematics | Model theory | null |
The ordinary signature for set theory includes a single binary relation A structure for this signature consists of a set of elements and an interpretation of the relation as a binary relation on these elements.
Induced substructures and closed subsets
is called an (induced) substructure of if
and have the same signature
the domain of is contained in the domain of and
the interpretations of all function and relation symbols agree on
The usual notation for this relation is
A subset of the domain of a structure is called closed if it is closed under the functions of that is, if the following condition is satisfied: for every natural number every -ary function symbol (in the signature of ) and all elements the result of applying to the -tuple is again an element of
For every subset there is a smallest closed subset of that contains It is called the closed subset generated by or the hull of and denoted by or . The operator is a finitary closure operator on the set of subsets of .
If and is a closed subset, then is an induced substructure of where assigns to every symbol of σ the restriction to of its interpretation in Conversely, the domain of an induced substructure is a closed subset.
The closed subsets (or induced substructures) of a structure form a lattice. The meet of two subsets is their intersection. The join of two subsets is the closed subset generated by their union. Universal algebra studies the lattice of substructures of a structure in detail.
Examples
Let be again the standard signature for fields. When regarded as -structures in the natural way, the rational numbers form a substructure of the real numbers, and the real numbers form a substructure of the complex numbers. The rational numbers are the smallest substructure of the real (or complex) numbers that also satisfies the field axioms.
The set of integers gives an even smaller substructure of the real numbers which is not a field. Indeed, the integers are the substructure of the real numbers generated by the empty set, using this signature. The notion in abstract algebra that corresponds to a substructure of a field, in this signature, is that of a subring, rather than that of a subfield. | Structure (mathematical logic) | Wikipedia | 465 | 4055928 | https://en.wikipedia.org/wiki/Structure%20%28mathematical%20logic%29 | Mathematics | Model theory | null |
The most obvious way to define a graph is a structure with a signature consisting of a single binary relation symbol The vertices of the graph form the domain of the structure, and for two vertices and means that and are connected by an edge. In this encoding, the notion of induced substructure is more restrictive than the notion of subgraph. For example, let be a graph consisting of two vertices connected by an edge, and let be the graph consisting of the same vertices but no edges. is a subgraph of but not an induced substructure. The notion in graph theory that corresponds to induced substructures is that of induced subgraphs.
Homomorphisms and embeddings
Homomorphisms
Given two structures and of the same signature σ, a (σ-)homomorphism from to is a map that preserves the functions and relations. More precisely:
For every n-ary function symbol f of σ and any elements , the following equation holds:
.
For every n-ary relation symbol R of σ and any elements , the following implication holds:
where , is the interpretation of the relation symbol of the object theory in the structure , respectively.
A homomorphism h from to is typically denoted as , although technically the function h is between the domains , of the two structures , .
For every signature σ there is a concrete category σ-Hom which has σ-structures as objects and σ-homomorphisms as morphisms.
A homomorphism is sometimes called strong if:
For every n-ary relation symbol R of the object theory and any elements such that , there are such that and
The strong homomorphisms give rise to a subcategory of the category σ-Hom that was defined above.
Embeddings
A (σ-)homomorphism is called a (σ-)embedding if it is one-to-one and
for every n-ary relation symbol R of σ and any elements , the following equivalence holds:
(where as before , refers to the interpretation of the relation symbol R of the object theory σ in the structure , respectively).
Thus an embedding is the same thing as a strong homomorphism which is one-to-one.
The category σ-Emb of σ-structures and σ-embeddings is a concrete subcategory of σ-Hom. | Structure (mathematical logic) | Wikipedia | 481 | 4055928 | https://en.wikipedia.org/wiki/Structure%20%28mathematical%20logic%29 | Mathematics | Model theory | null |
Induced substructures correspond to subobjects in σ-Emb. If σ has only function symbols, σ-Emb is the subcategory of monomorphisms of σ-Hom. In this case induced substructures also correspond to subobjects in σ-Hom.
Example
As seen above, in the standard encoding of graphs as structures the induced substructures are precisely the induced subgraphs. However, a homomorphism between graphs is the same thing as a homomorphism between the two structures coding the graph. In the example of the previous section, even though the subgraph H of G is not induced, the identity map id: H → G is a homomorphism. This map is in fact a monomorphism in the category σ-Hom, and therefore H is a subobject of G which is not an induced substructure.
Homomorphism problem
The following problem is known as the homomorphism problem:
Given two finite structures and of a finite relational signature, find a homomorphism or show that no such homomorphism exists.
Every constraint satisfaction problem (CSP) has a translation into the homomorphism problem. Therefore, the complexity of CSP can be studied using the methods of finite model theory.
Another application is in database theory, where a relational model of a database is essentially the same thing as a relational structure. It turns out that a conjunctive query on a database can be described by another structure in the same signature as the database model. A homomorphism from the relational model to the structure representing the query is the same thing as a solution to the query. This shows that the conjunctive query problem is also equivalent to the homomorphism problem.
Structures and first-order logic
Structures are sometimes referred to as "first-order structures". This is misleading, as nothing in their definition ties them to any specific logic, and in fact they are suitable as semantic objects both for very restricted fragments of first-order logic such as that used in universal algebra, and for second-order logic. In connection with first-order logic and model theory, structures are often called models, even when the question "models of what?" has no obvious answer.
Satisfaction relation
Each first-order structure has a satisfaction relation defined for all formulas in the language consisting of the language of together with a constant symbol for each element of which is interpreted as that element.
This relation is defined inductively using Tarski's T-schema. | Structure (mathematical logic) | Wikipedia | 509 | 4055928 | https://en.wikipedia.org/wiki/Structure%20%28mathematical%20logic%29 | Mathematics | Model theory | null |
A structure is said to be a model of a theory if the language of is the same as the language of and every sentence in is satisfied by Thus, for example, a "ring" is a structure for the language of rings that satisfies each of the ring axioms, and a model of ZFC set theory is a structure in the language of set theory that satisfies each of the ZFC axioms.
Definable relations
An -ary relation on the universe (i.e. domain) of the structure is said to be definable (or explicitly definable cf. Beth definability, or -definable, or definable with parameters from cf. below) if there is a formula such that
In other words, is definable if and only if there is a formula such that
is correct.
An important special case is the definability of specific elements. An element of is definable in if and only if there is a formula such that
Definability with parameters
A relation is said to be definable with parameters (or -definable) if there is a formula with parameters from such that is definable using Every element of a structure is definable using the element itself as a parameter.
Some authors use definable to mean definable without parameters, while other authors mean definable with parameters. Broadly speaking, the convention that definable means definable without parameters is more common amongst set theorists, while the opposite convention is more common amongst model theorists.
Implicit definability
Recall from above that an -ary relation on the universe of is explicitly definable if there is a formula such that
Here the formula used to define a relation must be over the signature of and so may not mention itself, since is not in the signature of If there is a formula in the extended language containing the language of and a new symbol and the relation is the only relation on such that then is said to be implicitly definable over
By Beth's theorem, every implicitly definable relation is explicitly definable.
Many-sorted structures | Structure (mathematical logic) | Wikipedia | 434 | 4055928 | https://en.wikipedia.org/wiki/Structure%20%28mathematical%20logic%29 | Mathematics | Model theory | null |
Structures as defined above are sometimes called s to distinguish them from the more general s. A many-sorted structure can have an arbitrary number of domains. The sorts are part of the signature, and they play the role of names for the different domains. Many-sorted signatures also prescribe which sorts the functions and relations of a many-sorted structure are defined on. Therefore, the arities of function symbols or relation symbols must be more complicated objects such as tuples of sorts rather than natural numbers.
Vector spaces, for example, can be regarded as two-sorted structures in the following way. The two-sorted signature of vector spaces consists of two sorts V (for vectors) and S (for scalars) and the following function symbols:
If V is a vector space over a field F, the corresponding two-sorted structure consists of the vector domain , the scalar domain , and the obvious functions, such as the vector zero , the scalar zero , or scalar multiplication .
Many-sorted structures are often used as a convenient tool even when they could be avoided with a little effort. But they are rarely defined in a rigorous way, because it is straightforward and tedious (hence unrewarding) to carry out the generalization explicitly.
In most mathematical endeavours, not much attention is paid to the sorts. A many-sorted logic however naturally leads to a type theory. As Bart Jacobs puts it: "A logic is always a logic over a type theory." This emphasis in turn leads to categorical logic because a logic over a type theory categorically corresponds to one ("total") category, capturing the logic, being fibred over another ("base") category, capturing the type theory.
Other generalizations
Partial algebras | Structure (mathematical logic) | Wikipedia | 354 | 4055928 | https://en.wikipedia.org/wiki/Structure%20%28mathematical%20logic%29 | Mathematics | Model theory | null |
Both universal algebra and model theory study classes of (structures or) algebras that are defined by a signature and a set of axioms. In the case of model theory these axioms have the form of first-order sentences. The formalism of universal algebra is much more restrictive; essentially it only allows first-order sentences that have the form of universally quantified equations between terms, e.g. x y (x + y = y + x). One consequence is that the choice of a signature is more significant in universal algebra than it is in model theory. For example, the class of groups, in the signature consisting of the binary function symbol × and the constant symbol 1, is an elementary class, but it is not a variety. Universal algebra solves this problem by adding a unary function symbol −1.
In the case of fields this strategy works only for addition. For multiplication it fails because 0 does not have a multiplicative inverse. An ad hoc attempt to deal with this would be to define 0−1 = 0. (This attempt fails, essentially because with this definition 0 × 0−1 = 1 is not true.) Therefore, one is naturally led to allow partial functions, i.e., functions that are defined only on a subset of their domain. However, there are several obvious ways to generalize notions such as substructure, homomorphism and identity.
Structures for typed languages
In type theory, there are many sorts of variables, each of which has a type. Types are inductively defined; given two types δ and σ there is also a type σ → δ that represents functions from objects of type σ to objects of type δ. A structure for a typed language (in the ordinary first-order semantics) must include a separate set of objects of each type, and for a function type the structure must have complete information about the function represented by each object of that type.
Higher-order languages
There is more than one possible semantics for higher-order logic, as discussed in the article on second-order logic. When using full higher-order semantics, a structure need only have a universe for objects of type 0, and the T-schema is extended so that a quantifier over a higher-order type is satisfied by the model if and only if it is disquotationally true. When using first-order semantics, an additional sort is added for each higher-order type, as in the case of a many sorted first order language. | Structure (mathematical logic) | Wikipedia | 509 | 4055928 | https://en.wikipedia.org/wiki/Structure%20%28mathematical%20logic%29 | Mathematics | Model theory | null |
Structures that are proper classes
In the study of set theory and category theory, it is sometimes useful to consider structures in which the domain of discourse is a proper class instead of a set. These structures are sometimes called class models to distinguish them from the "set models" discussed above. When the domain is a proper class, each function and relation symbol may also be represented by a proper class.
In Bertrand Russell's Principia Mathematica, structures were also allowed to have a proper class as their domain. | Structure (mathematical logic) | Wikipedia | 104 | 4055928 | https://en.wikipedia.org/wiki/Structure%20%28mathematical%20logic%29 | Mathematics | Model theory | null |
The banded bullfrog (Kaloula pulchra) is a species of frog in the narrow-mouthed frog family Microhylidae. Native to Southeast Asia, it is also known as the Asian painted frog, digging frog, Malaysian bullfrog, common Asian frog, and painted balloon frog. In the pet trade, it is sometimes called the chubby frog. Adults measure and have a dark brown back with stripes that vary from copper-brown to salmon pink.
The banded bullfrog lives at low altitudes and is found in both urban and rural settings, as well as in forest habitats. They bury themselves underground during dry periods and emerge after heavy rainfall to emit calls and breed. They feed primarily on ants and termites; predators of adults and tadpoles include snakes, dragonfly larvae, and snails. When threatened, they inflate their lungs and secrete a noxious white substance. The species is prevalent in the pet trade and is a potential invasive species being introduced in Taiwan, the Philippines, Guam, Singapore, Borneo, and Sulawesi.
Taxonomy and etymology
The banded bullfrog was first described in 1831 by the British zoologist John Edward Gray, as Kaloula pulchra (pulchra meaning "beautiful" in Latin). Cantor (1847) described the species under the name Hylaedactylus bivittatus, which was synonymized with K. pulchra by Günther (1858). The subspecies K. p. hainana was described by Gressitt (1938) as having a shorter snout and hind legs compared to the nominate subspecies, K. p. pulchra. A former subspecies in Sri Lanka, originally named K. p. taprobanica by Parker (1934), has since been reclassified as a separate species, Uperodon taprobanicus. Bourret (1942) described a subspecies K. p. macrocephala that is now considered by several authors to be a distinct species, K. macrocephala.
According to Darrel Frost's Amphibian Species of the World, common names for Kaloula pulchra include the Malaysian narrowmouth toad, Asian painted frog, digging frog, painted bullfrog, Malaysian bullfrog, painted burrowing frog, common Asian bullfrog, painted balloon frog, and painted microhylid frog. It is also known as the chubby frog in the pet trade. | Banded bullfrog | Wikipedia | 493 | 4056309 | https://en.wikipedia.org/wiki/Banded%20bullfrog | Biology and health sciences | Frogs and toads | Animals |
Description
The banded bullfrog is medium-sized with a stocky, triangular body and a short snout. Males grow to a snout–vent length (SVL) of and females are slightly larger, reaching an SVL of . Other than the slight difference in length, there is very limited sexual dimorphism. They have a body weight of . The back is dark brown with stripes that vary from copper-brown to salmon pink, and the abdomen is cream-colored.
Tadpoles are about long after hatching and reach an SVL of about at the end of metamorphosis. They have an oval body that is brown or black with a pale belly, a round snout, and a moderately long, tapered tail with yellow speckles and tall fins. The eyes are relatively small and the side of the head, with black or dark gray irises and a golden ring around the pupil. They do not possess any tail filament. During metamorphosis, their eyes increase in size and bulge and they develop slender limbs and digits with rounded tips. The tadpoles metamorphose beginning at two weeks.
Distribution and habitat
The species is native to Southeast Asia. It is common over a range from northeastern India, and Nepal, to southern India and Sri Lanka to southern China (especially Hainan) and Myanmar, and south to the islands of maritime Southeast Asia. Its wide distribution, compared to the related species Kaloula assamensis, has been attributed to its burrowing ability.
The banded bullfrog has been found at elevations between sea level and above sea level. It can occur in both urban and rural settings, and in forest habitats.
As an invasive species
The banded bullfrog is a potential invasive species. It has been introduced through both the pet trade and maritime transport, and has become established in Taiwan, the Philippines, Guam, Singapore, Borneo, and Sulawesi. Some specimens have been observed in Australia and New Zealand. Its introduction into the Philippines was likely accidental, via contamination of plant nursery materials or stowaways on ships and boats.
Several species, likely introduced through the pet trade, were observed in Florida in 2006 and 2008; however, as of 2011, the population is under control and there is no evidence of reproduction. The frog was observed at an airport in Perth, Australia, and at a cargo port in New Zealand, but no established invasive population has been found in either country as of 2019.
Behaviour and ecology | Banded bullfrog | Wikipedia | 500 | 4056309 | https://en.wikipedia.org/wiki/Banded%20bullfrog | Biology and health sciences | Frogs and toads | Animals |
Breeding is stimulated by heavy monsoon rains, after which the frogs relocate from underground to rain pools or ponds. They are more commonly found on wetter nights, and while they are not reproductively active during dry periods, their gonads remain ripe so that they can mate soon after rainfall. In India, the male frogs call after the monsoon season begins in April or May. The pulses of the calls recorded in India were 28–56 per second with a frequency range of 50–1760 Hz. In Thailand the dominant frequency was 250 Hz (duration 560–600 ms long) and 18–21 pulses per call.
Their form is suited for walking and burrowing rather than jumping. They are able to survive dry conditions by burying themselves in the ground and waiting for rain; the burrowing also helps them avoid predators. When burrowing they dig their way down hindlimb first and use their forelimbs to push themselves several inches under the soil, where they can remain for the duration of the dry season. Banded bullfrogs hide under leaf litter during the daylight hours and eat in the evening. They have been found in trees and have been observed hunting termites in them.
Diet, predators, and parasites
In the wild, the banded bullfrog primarily eats ants and termites. It also feeds on other small invertebrates including flies, crickets, moths, grasshoppers, and earthworms. Its relatively small head and mouth mostly limit its diet to small and slow-moving prey. The feeding cycle from opening of the mouth to closing is about 150 milliseconds and is relatively symmetrical, meaning that the bullfrog spends an equal amount of time extending its tongue and bringing the prey into the mouth. Banded bullfrogs kept as pets can be fed insects such as crickets, mealworms, insect larvae, and beetles. | Banded bullfrog | Wikipedia | 373 | 4056309 | https://en.wikipedia.org/wiki/Banded%20bullfrog | Biology and health sciences | Frogs and toads | Animals |
Snakes such as the kukri snake are predators of adult banded bullfrogs. For eggs and tadpoles, predators include dragonfly larvae and snails such as the golden apple snail. Banded bullfrogs display deimatic behaviour when threatened, greatly inflating their bodies in an attempt to distract or startle predators. By inflating its body and bending its head down, the bullfrog can appear larger than its actual size. It also secretes a noxious white substance through its skin that is distasteful, though non-toxic, to predators. The secretion contains a trypsin inhibitor and can induce hemolysis (rupturing of red blood cells).
Parasites include parasitic worms that have been found in the frog's intestinal mesentery and leeches that attach to the frog's back.
Pet trade
Commonly sold in pet stores, banded bullfrogs thrive in terrariums with substrate choices consisting of peat–soil mixes or moss mixtures. In contrast to the ant and termite diets of wild bullfrogs, captive bullfrogs typically feed on slightly larger insects such as crickets or mealworms.
A survey of internet pet trade listings between 2015 and 2018 in Europe and the United States found that there were three to four times as many offers as requests for the banded bullfrog, with no evidence of captive breeding. In the Philippines, traders collect the frogs locally. Low interest in the Philippine pet trade has been attributed to the bullfrog's muted colours and burrowing behavior. Máximo and colleagues hypothesize that the species has been illegally sold in South America for decades, based on identifications in Argentina during the 1980s and in Brazil in 2020.
Conservation status
The International Union for Conservation of Nature listed the species as least concern due to its extensive distribution, tolerance of a wide range of environments, and predicted large population. In many regions, the banded bullfrog is captured for consumption, but this does not appear to have a substantial impact on its population. | Banded bullfrog | Wikipedia | 419 | 4056309 | https://en.wikipedia.org/wiki/Banded%20bullfrog | Biology and health sciences | Frogs and toads | Animals |
Human sexual reproduction, to produce offspring, begins with fertilization. Successful reproduction typically involves sexual intercourse between a healthy, sexually mature and fertile male and female. During sexual intercourse, sperm cells are ejaculated into the vagina through the penis, resulting in fertilization of an ovum to form a zygote.
While normal cells contain 46 chromosomes (23 pairs), gamete cells contain only half that number, and it is when these two cells merge into one combined zygote cell that genetic recombination occurs. The zygote then undergoes a defined development process that is known as human embryogenesis, and this starts the typical 38-week gestation period for the embryo (and eventually foetus) that is followed by childbirth.
Assisted reproductive technology also exists, like IVF, some of which involve alternative methods of fertilization, which do not involve sexual intercourse; the fertilization of the ovum may be achieved by artificial insemination methods.
Biological and legal requirements
In order for human reproduction to be achieved, an individual must have undergone puberty first, requiring ovulation in females and the spermarche in males to have occurred prior to engaging in sexual intercourse or achieving pregnancy through non-penetrative means. Before puberty, humans are infertile, as their genitals lack reproductive function (only being able to discharge urine).
Legal factors also play a vital role in the achievement of human reproduction: a minor under the age of consent cannot give legal consent to sexual intercourse or artificial alternatives to reproduction, the former case of which is liable to have the older party charged with statutory rape, depending on jurisdictions. Even for minors above the age of consent, comprehensive sex education advises both consenting parties to use contraception to avoid both sexually transmitted infections and early, unplanned/unwanted pregnancies. Pregnancy in girls under the age of 15 is especially discouraged due to their reproductive systems having yet to reach full maturity.
Anatomy
Male reproductive system | Human reproduction | Wikipedia | 409 | 4058774 | https://en.wikipedia.org/wiki/Human%20reproduction | Biology and health sciences | Human reproduction | Biology |
The male reproductive system contains two main divisions: the testicles where sperm are produced, and the penis where semen is ejaculated through the urethra. In humans, both of these organs are outside the abdominal cavity. Having the testicles outside the abdomen facilitates temperature regulation of the sperm, which require specific temperatures to survive about 2-3 °C less than the normal body temperature i.e. 37 °C. In particular, the extraperitoneal location of the testicles may result in a 2-fold reduction in the heat-induced contribution to the spontaneous mutation rate in male germinal tissues compared to tissues at 37 °C. If the testicles remain too close to the body, it is likely that the increase in temperature will harm the spermatozoa formation, making conception more difficult. This is why the testes are carried in an external scrotum rather than within the abdomen; they normally remain slightly cooler than body temperature, facilitating sperm production.
Male germ cells produced in the testes are able to perform special DNA repair processes during meiosis that act to repair DNA damages and to maintain the integrity of the genomes that are to be passed on to progeny. Two of these DNA repair processes are homologous recombinational repair and non-homologous end joining.
Female reproductive system
The female reproductive system likewise contains two main divisions: the external genitalia (the vulva) and the internal genitalia.
The ovum meets with the sperm cell: a sperm may penetrate and merge with the egg, fertilizing it with the help of certain hydrolytic enzymes present in the acrosome. The fertilization usually occurs in the fallopian tubes, but can happen in the uterus itself. The zygote then becomes implanted in the lining of the uterus, where it begins the processes of embryogenesis and morphogenesis. When the fetus is developed enough to survive outside of the uterus, the cervix dilates and contractions of the uterus propel it through the birth canal, which is the vagina, and thereby gives external life to the newborn infant. This process is called childbirth. | Human reproduction | Wikipedia | 450 | 4058774 | https://en.wikipedia.org/wiki/Human%20reproduction | Biology and health sciences | Human reproduction | Biology |
The ova, which are the female sex cells, are much larger than the spermatozoon and are normally formed within the ovaries of the female fetus before birth. They are mostly fixed in location within the ovary until their transit to the uterus, and contain nutrients for the later zygote and embryo. Over a regular interval known as the menstrual cycle, in response to hormonal signals, a process of oogenesis matures one ovum which is released and sent down the fallopian tube. If not fertilized, this egg is flushed out of the system through menstruation.
Oocytes (female germ cells) located in the primordial follicle of the ovary are in a non-growing prophase arrested state, but are able to undergo highly efficient homologous recombinational repair of DNA damages including double-strand breaks. This capability allows the maintenance of genome integrity and protection of the health of offspring.
Process of fertilization
Human reproduction normally begins with copulation, though it may be achieved through artificial insemination, and is followed by nine months of pregnancy before childbirth. Pregnancy can be avoided with the use of contraceptives such as condoms and intrauterine devices.
Copulation
Human reproduction naturally takes place as internal fertilization by sexual intercourse. During this process, the man inserts his erect penis into the woman's vagina and then either partner initiates rhythmic pelvic thrusts until the man achieves orgasm, which leads to ejaculation of semen containing sperm into the vaginal canal. The sperm and the ovum are known as the gametes (each containing half the genetic information of the parent, created through meiosis). The sperm (being one of approximately 250 million sperm in a typical ejaculation) travels through the vagina and cervix into the uterus or fallopian tubes. Only 1 in 14 million of the ejaculated sperm will reach the fallopian tube. The egg simultaneously moves through the fallopian tube away from the ovary. One of the sperm encounters, penetrates and fertilizes the ovum, creating a zygote. Upon fertilization and implantation, gestation of the fetus then occurs within the uterus. | Human reproduction | Wikipedia | 482 | 4058774 | https://en.wikipedia.org/wiki/Human%20reproduction | Biology and health sciences | Human reproduction | Biology |
Pregnancy rates for sexual intercourse are highest during the menstrual cycle time from some 5 days before until 1 to 2 days after ovulation. For optimal pregnancy chance, there are recommendations of sexual intercourse every 1 or 2 days, or every 2 or 3 days. Studies have shown no significant difference between different sex positions and pregnancy rate, as long as it results in ejaculation into the vagina.
Alternative methods
As an alternative to natural sexual intercourse, there exists artificial insemination, where sperm is introduced into the female reproductive system without the insertion of the penis. There are also many methods of assisted reproductive technology, such as in vitro fertilization, where one or more egg cells are retrieved from a woman's ovaries and co-incubated with sperm outside the body. The resulting embryo can then be reinserted into the womb of the woman.
Pregnancy
Pregnancy is the period of time during which the fetus develops, dividing via mitosis inside the uterus. During this time, the fetus receives all of its nutrition and oxygenated blood from the mother, filtered through the placenta, which is attached to the fetus' abdomen via an umbilical cord. This drain of nutrients can be quite taxing on the mother, who is required to ingest slightly higher levels of calories. In addition, certain vitamins and other nutrients are required in greater quantities than normal, often creating abnormal eating habits. Gestation period is about 266 days in humans. While in the uterus, the baby first endures a very brief zygote stage, then the embryonic stage, which is marked by the development of major organs and lasts for approximately eight weeks, then the fetal stage, which revolves around the development of bone cells while the fetus continues to grow in size. It is estimated that about 3-5% of couples are infertile and the fecunditity of couples is around 30% for each menstrual cycle.
Labor and birth | Human reproduction | Wikipedia | 405 | 4058774 | https://en.wikipedia.org/wiki/Human%20reproduction | Biology and health sciences | Human reproduction | Biology |
Labor is separated into 4 stages. The first stage involves latent phase and active phase separated by the dilation of the cervix for 6 to 10 cm. The second stage is the pushing stage. The third stage involves the delivery of the placenta. And the last stage is the contraction of the uterus. Once the fetus is sufficiently developed, chemical signals begin the process of birth, which begins with the fetus being pushed out of the birthing canal. The newborn, which is called an infant in humans, should typically begin respiration on its own shortly after birth. Not long after, the placenta eventually falls off on its own. The person assisting the birth may also sever the umbilical cord.
Discovery of mechanism
While most ancient human societies believed that sexual intercourse was necessary for reproduction, the reasons some sex did not result in children, and the mechanism by which mating produced children were not understood. The theory of preformationism was popular in Ancient Greece and Christendom for centuries. Because they are too small to see with the naked eye, it was only after his invention of the microscope that Antonie van Leeuwenhoek discovered spermatozoa in 1677. Mitosis and meiosis were not discovered until the late 1800s. | Human reproduction | Wikipedia | 258 | 4058774 | https://en.wikipedia.org/wiki/Human%20reproduction | Biology and health sciences | Human reproduction | Biology |
A search engine is a software system that provides hyperlinks to web pages and other relevant information on the Web in response to a user's query. The user inputs a query within a web browser or a mobile app, and the search results are often a list of hyperlinks, accompanied by textual summaries and images. Users also have the option of limiting the search to a specific type of results, such as images, videos, or news.
For a search provider, its engine is part of a distributed computing system that can encompass many data centers throughout the world. The speed and accuracy of an engine's response to a query is based on a complex system of indexing that is continuously updated by automated web crawlers. This can include data mining the files and databases stored on web servers, but some content is not accessible to crawlers.
There have been many search engines since the dawn of the Web in the 1990s, but Google Search became the dominant one in the 2000s and has remained so. It currently has a 90% global market share. Other search engines with a smaller market share include Bing at 4%, Yandex at 2%, and Yahoo at 1%. Other search engines not listed have less than a 3% market share. The business of websites improving their visibility in search results, known as marketing and optimization, has thus largely focused on Google.
History
Pre-1990s
In 1945, Vannevar Bush described an information retrieval system that would allow a user to access a great expanse of information, all at a single desk. He called it a memex. He described the system in an article titled "As We May Think" that was published in The Atlantic Monthly. The memex was intended to give a user the capability to overcome the ever-increasing difficulty of locating information in ever-growing centralized indices of scientific work. Vannevar Bush envisioned libraries of research with connected annotations, which are similar to modern hyperlinks.
Link analysis eventually became a crucial component of search engines through algorithms such as Hyper Search and PageRank.
1990s: Birth of search engines
The first internet search engines predate the debut of the Web in December 1990: WHOIS user search dates back to 1982, and the Knowbot Information Service multi-network user search was first implemented in 1989. The first well documented search engine that searched content files, namely FTP files, was Archie, which debuted on 10 September 1990. | Search engine | Wikipedia | 492 | 4059023 | https://en.wikipedia.org/wiki/Search%20engine | Technology | Internet | null |
Prior to September 1993, the World Wide Web was entirely indexed by hand. There was a list of webservers edited by Tim Berners-Lee and hosted on the CERN webserver. One snapshot of the list in 1992 remains, but as more and more web servers went online the central list could no longer keep up. On the NCSA site, new servers were announced under the title "What's New!".
The first tool used for searching content (as opposed to users) on the Internet was Archie. The name stands for "archive" without the "v". It was created by Alan Emtage, computer science student at McGill University in Montreal, Quebec, Canada. The program downloaded the directory listings of all the files located on public anonymous FTP (File Transfer Protocol) sites, creating a searchable database of file names; however, Archie Search Engine did not index the contents of these sites since the amount of data was so limited it could be readily searched manually.
The rise of Gopher (created in 1991 by Mark McCahill at the University of Minnesota) led to two new search programs, Veronica and Jughead. Like Archie, they searched the file names and titles stored in Gopher index systems. Veronica (Very Easy Rodent-Oriented Net-wide Index to Computerized Archives) provided a keyword search of most Gopher menu titles in the entire Gopher listings. Jughead (Jonzy's Universal Gopher Hierarchy Excavation And Display) was a tool for obtaining menu information from specific Gopher servers. While the name of the search engine "Archie Search Engine" was not a reference to the Archie comic book series, "Veronica" and "Jughead" are characters in the series, thus referencing their predecessor.
In the summer of 1993, no search engine existed for the web, though numerous specialized catalogs were maintained by hand. Oscar Nierstrasz at the University of Geneva wrote a series of Perl scripts that periodically mirrored these pages and rewrote them into a standard format. This formed the basis for W3Catalog, the web's first primitive search engine, released on September 2, 1993. | Search engine | Wikipedia | 439 | 4059023 | https://en.wikipedia.org/wiki/Search%20engine | Technology | Internet | null |
In June 1993, Matthew Gray, then at MIT, produced what was probably the first web robot, the Perl-based World Wide Web Wanderer, and used it to generate an index called "Wandex". The purpose of the Wanderer was to measure the size of the World Wide Web, which it did until late 1995. The web's second search engine Aliweb appeared in November 1993. Aliweb did not use a web robot, but instead depended on being notified by website administrators of the existence at each site of an index file in a particular format.
JumpStation (created in December 1993 by Jonathon Fletcher) used a web robot to find web pages and to build its index, and used a web form as the interface to its query program. It was thus the first WWW resource-discovery tool to combine the three essential features of a web search engine (crawling, indexing, and searching) as described below. Because of the limited resources available on the platform it ran on, its indexing and hence searching were limited to the titles and headings found in the web pages the crawler encountered.
One of the first "all text" crawler-based search engines was WebCrawler, which came out in 1994. Unlike its predecessors, it allowed users to search for any word in any web page, which has become the standard for all major search engines since. It was also the search engine that was widely known by the public. Also, in 1994, Lycos (which started at Carnegie Mellon University) was launched and became a major commercial endeavor.
The first popular search engine on the Web was Yahoo! Search. The first product from Yahoo!, founded by Jerry Yang and David Filo in January 1994, was a Web directory called Yahoo! Directory. In 1995, a search function was added, allowing users to search Yahoo! Directory. It became one of the most popular ways for people to find web pages of interest, but its search function operated on its web directory, rather than its full-text copies of web pages.
Soon after, a number of search engines appeared and vied for popularity. These included Magellan, Excite, Infoseek, Inktomi, Northern Light, and AltaVista. Information seekers could also browse the directory instead of doing a keyword-based search. | Search engine | Wikipedia | 475 | 4059023 | https://en.wikipedia.org/wiki/Search%20engine | Technology | Internet | null |
In 1996, Robin Li developed the RankDex site-scoring algorithm for search engines results page ranking and received a US patent for the technology. It was the first search engine that used hyperlinks to measure the quality of websites it was indexing, predating the very similar algorithm patent filed by Google two years later in 1998. Larry Page referenced Li's work in some of his U.S. patents for PageRank. Li later used his Rankdex technology for the Baidu search engine, which was founded by him in China and launched in 2000.
In 1996, Netscape was looking to give a single search engine an exclusive deal as the featured search engine on Netscape's web browser. There was so much interest that instead, Netscape struck deals with five of the major search engines: for $5 million a year, each search engine would be in rotation on the Netscape search engine page. The five engines were Yahoo!, Magellan, Lycos, Infoseek, and Excite.
Google adopted the idea of selling search terms in 1998 from a small search engine company named goto.com. This move had a significant effect on the search engine business, which went from struggling to one of the most profitable businesses in the Internet.
Search engines were also known as some of the brightest stars in the Internet investing frenzy that occurred in the late 1990s. Several companies entered the market spectacularly, receiving record gains during their initial public offerings. Some have taken down their public search engine and are marketing enterprise-only editions, such as Northern Light. Many search engine companies were caught up in the dot-com bubble, a speculation-driven market boom that peaked in March 2000. | Search engine | Wikipedia | 343 | 4059023 | https://en.wikipedia.org/wiki/Search%20engine | Technology | Internet | null |
2000s–present: Post dot-com bubble
Around 2000, Google's search engine rose to prominence. The company achieved better results for many searches with an algorithm called PageRank, as was explained in the paper Anatomy of a Search Engine written by Sergey Brin and Larry Page, the later founders of Google. This iterative algorithm ranks web pages based on the number and PageRank of other web sites and pages that link there, on the premise that good or desirable pages are linked to more than others. Larry Page's patent for PageRank cites Robin Li's earlier RankDex patent as an influence. Google also maintained a minimalist interface to its search engine. In contrast, many of its competitors embedded a search engine in a web portal. In fact, the Google search engine became so popular that spoof engines emerged such as Mystery Seeker.
By 2000, Yahoo! was providing search services based on Inktomi's search engine. Yahoo! acquired Inktomi in 2002, and Overture (which owned AlltheWeb and AltaVista) in 2003. Yahoo! switched to Google's search engine until 2004, when it launched its own search engine based on the combined technologies of its acquisitions.
Microsoft first launched MSN Search in the fall of 1998 using search results from Inktomi. In early 1999, the site began to display listings from Looksmart, blended with results from Inktomi. For a short time in 1999, MSN Search used results from AltaVista instead. In 2004, Microsoft began a transition to its own search technology, powered by its own web crawler (called msnbot).
Microsoft's rebranded search engine, Bing, was launched on June 1, 2009. On July 29, 2009, Yahoo! and Microsoft finalized a deal in which Yahoo! Search would be powered by Microsoft Bing technology.
active search engine crawlers include those of Google, Sogou, Baidu, Bing, Gigablast, Mojeek, DuckDuckGo and Yandex.
Approach
A search engine maintains the following processes in near real time:
Web crawling
Indexing
Searching | Search engine | Wikipedia | 428 | 4059023 | https://en.wikipedia.org/wiki/Search%20engine | Technology | Internet | null |
Web search engines get their information by web crawling from site to site. The "spider" checks for the standard filename robots.txt, addressed to it. The robots.txt file contains directives for search spiders, telling it which pages to crawl and which pages not to crawl. After checking for robots.txt and either finding it or not, the spider sends certain information back to be indexed depending on many factors, such as the titles, page content, JavaScript, Cascading Style Sheets (CSS), headings, or its metadata in HTML meta tags. After a certain number of pages crawled, amount of data indexed, or time spent on the website, the spider stops crawling and moves on. "[N]o web crawler may actually crawl the entire reachable web. Due to infinite websites, spider traps, spam, and other exigencies of the real web, crawlers instead apply a crawl policy to determine when the crawling of a site should be deemed sufficient. Some websites are crawled exhaustively, while others are crawled only partially".
Indexing means associating words and other definable tokens found on web pages to their domain names and HTML-based fields. The associations are made in a public database, made available for web search queries. A query from a user can be a single word, multiple words or a sentence. The index helps find information relating to the query as quickly as possible. Some of the techniques for indexing, and caching are trade secrets, whereas web crawling is a straightforward process of visiting all sites on a systematic basis.
Between visits by the spider, the cached version of the page (some or all the content needed to render it) stored in the search engine working memory is quickly sent to an inquirer. If a visit is overdue, the search engine can just act as a web proxy instead. In this case, the page may differ from the search terms indexed. The cached page holds the appearance of the version whose words were previously indexed, so a cached version of a page can be useful to the website when the actual page has been lost, but this problem is also considered a mild form of linkrot. | Search engine | Wikipedia | 451 | 4059023 | https://en.wikipedia.org/wiki/Search%20engine | Technology | Internet | null |
Typically when a user enters a query into a search engine it is a few keywords. The index already has the names of the sites containing the keywords, and these are instantly obtained from the index. The real processing load is in generating the web pages that are the search results list: Every page in the entire list must be weighted according to information in the indexes. Then the top search result item requires the lookup, reconstruction, and markup of the snippets showing the context of the keywords matched. These are only part of the processing each search results web page requires, and further pages (next to the top) require more of this post-processing.
Beyond simple keyword lookups, search engines offer their own GUI- or command-driven operators and search parameters to refine the search results. These provide the necessary controls for the user engaged in the feedback loop users create by filtering and weighting while refining the search results, given the initial pages of the first search results.
For example, from 2007 the Google.com search engine has allowed one to filter by date by clicking "Show search tools" in the leftmost column of the initial search results page, and then selecting the desired date range. It is also possible to weight by date because each page has a modification time. Most search engines support the use of the Boolean operators AND, OR and NOT to help end users refine the search query. Boolean operators are for literal searches that allow the user to refine and extend the terms of the search. The engine looks for the words or phrases exactly as entered. Some search engines provide an advanced feature called proximity search, which allows users to define the distance between keywords. There is also concept-based searching where the research involves using statistical analysis on pages containing the words or phrases you search for. | Search engine | Wikipedia | 370 | 4059023 | https://en.wikipedia.org/wiki/Search%20engine | Technology | Internet | null |
The usefulness of a search engine depends on the relevance of the result set it gives back. While there may be millions of web pages that include a particular word or phrase, some pages may be more relevant, popular, or authoritative than others. Most search engines employ methods to rank the results to provide the "best" results first. How a search engine decides which pages are the best matches, and what order the results should be shown in, varies widely from one engine to another. The methods also change over time as Internet usage changes and new techniques evolve. There are two main types of search engine that have evolved: one is a system of predefined and hierarchically ordered keywords that humans have programmed extensively. The other is a system that generates an "inverted index" by analyzing texts it locates. This first form relies much more heavily on the computer itself to do the bulk of the work.
Most Web search engines are commercial ventures supported by advertising revenue and thus some of them allow advertisers to have their listings ranked higher in search results for a fee. Search engines that do not accept money for their search results make money by running search related ads alongside the regular search engine results. The search engines make money every time someone clicks on one of these ads.
Local search
Local search is the process that optimizes the efforts of local businesses. They focus on change to make sure all searches are consistent. It is important because many people determine where they plan to go and what to buy based on their searches.
Market share
Google is by far the world's most used search engine, with a market share of 90%, and the world's other most used search engines were Bing at 4%, Yandex at 2%, Yahoo! at 1%. Other search engines not listed have less than a 3% market share. In 2024, Google's dominance was ruled an illegal monopoly in a case brought by the US Department of Justice.
Russia and East Asia | Search engine | Wikipedia | 401 | 4059023 | https://en.wikipedia.org/wiki/Search%20engine | Technology | Internet | null |
In Russia, Yandex has a market share of 62.6%, compared to Google's 28.3%. Yandex is the second most used search engine on smartphones in Asia and Europe. In China, Baidu is the most popular search engine. South Korea-based search portal Naver is used for 62.8% of online searches in the country. Yahoo! Japan and Yahoo! Taiwan are the most popular choices for Internet searches in Japan and Taiwan, respectively. China is one of few countries where Google is not in the top three web search engines for market share. Google was previously more popular in China, but withdrew significantly after a disagreement with the government over censorship and a cyberattack. Bing, however, is in the top three web search engines with a market share of 14.95%. Baidu is top with 49.1% of the market share.
Europe
Most countries' markets in the European Union are dominated by Google, except for the Czech Republic, where Seznam is a strong competitor.
The search engine Qwant is based in Paris, France, where it attracts most of its 50 million monthly registered users from.
Search engine bias
Although search engines are programmed to rank websites based on some combination of their popularity and relevancy, empirical studies indicate various political, economic, and social biases in the information they provide and the underlying assumptions about the technology. These biases can be a direct result of economic and commercial processes (e.g., companies that advertise with a search engine can become also more popular in its organic search results), and political processes (e.g., the removal of search results to comply with local laws). For example, Google will not surface certain neo-Nazi websites in France and Germany, where Holocaust denial is illegal.
Biases can also be a result of social processes, as search engine algorithms are frequently designed to exclude non-normative viewpoints in favor of more "popular" results. Indexing algorithms of major search engines skew towards coverage of U.S.-based sites, rather than websites from non-U.S. countries.
Google Bombing is one example of an attempt to manipulate search results for political, social or commercial reasons.
Several scholars have studied the cultural changes triggered by search engines, and the representation of certain controversial topics in their results, such as terrorism in Ireland, climate change denial, and conspiracy theories.
Customized results and filter bubbles | Search engine | Wikipedia | 498 | 4059023 | https://en.wikipedia.org/wiki/Search%20engine | Technology | Internet | null |
There has been concern raised that search engines such as Google and Bing provide customized results based on the user's activity history, leading to what has been termed echo chambers or filter bubbles by Eli Pariser in 2011. The argument is that search engines and social media platforms use algorithms to selectively guess what information a user would like to see, based on information about the user (such as location, past click behaviour and search history). As a result, websites tend to show only information that agrees with the user's past viewpoint. According to Eli Pariser users get less exposure to conflicting viewpoints and are isolated intellectually in their own informational bubble. Since this problem has been identified, competing search engines have emerged that seek to avoid this problem by not tracking or "bubbling" users, such as DuckDuckGo. However many scholars have questioned Pariser's view, finding that there is little evidence for the filter bubble. On the contrary, a number of studies trying to verify the existence of filter bubbles have found only minor levels of personalisation in search, that most people encounter a range of views when browsing online, and that Google news tends to promote mainstream established news outlets.
Religious search engines
The global growth of the Internet and electronic media in the Arab and Muslim world during the last decade has encouraged Islamic adherents in the Middle East and Asian sub-continent, to attempt their own search engines, their own filtered search portals that would enable users to perform safe searches. More than usual safe search filters, these Islamic web portals categorizing websites into being either "halal" or "haram", based on interpretation of Sharia law. ImHalal came online in September 2011. Halalgoogling came online in July 2013. These use haram filters on the collections from Google and Bing (and others).
While lack of investment and slow pace in technologies in the Muslim world has hindered progress and thwarted success of an Islamic search engine, targeting as the main consumers Islamic adherents, projects like Muxlim (a Muslim lifestyle site) received millions of dollars from investors like Rite Internet Ventures, and it also faltered. Other religion-oriented search engines are Jewogle, the Jewish version of Google, and Christian search engine SeekFind.org. SeekFind filters sites that attack or degrade their faith. | Search engine | Wikipedia | 472 | 4059023 | https://en.wikipedia.org/wiki/Search%20engine | Technology | Internet | null |
Search engine submission
Web search engine submission is a process in which a webmaster submits a website directly to a search engine. While search engine submission is sometimes presented as a way to promote a website, it generally is not necessary because the major search engines use web crawlers that will eventually find most web sites on the Internet without assistance. They can either submit one web page at a time, or they can submit the entire site using a sitemap, but it is normally only necessary to submit the home page of a web site as search engines are able to crawl a well designed website. There are two remaining reasons to submit a web site or web page to a search engine: to add an entirely new web site without waiting for a search engine to discover it, and to have a web site's record updated after a substantial redesign.
Some search engine submission software not only submits websites to multiple search engines, but also adds links to websites from their own pages. This could appear helpful in increasing a website's ranking, because external links are one of the most important factors determining a website's ranking. However, John Mueller of Google has stated that this "can lead to a tremendous number of unnatural links for your site" with a negative impact on site ranking.
Comparison to social bookmarking
Technology
Archie
The first web search engine was Archie, created in 1990 by Alan Emtage, a student at McGill University in Montreal. The author originally wanted to call the program "archives", but had to shorten it to comply with the Unix world standard of assigning programs and files short, cryptic names such as grep, cat, troff, sed, awk, perl, and so on.
The primary method of storing and retrieving files was via the File Transfer Protocol (FTP). This was (and still is) a system that specified a common way for computers to exchange files over the Internet. It works like this: Some administrator decides that he wants to make files available from his computer. He sets up a program on his computer, called an FTP server. When someone on the Internet wants to retrieve a file from this computer, he or she connects to it via another program called an FTP client. Any FTP client program can connect with any FTP server program as long as the client and server programs both fully follow the specifications set forth in the FTP protocol. | Search engine | Wikipedia | 486 | 4059023 | https://en.wikipedia.org/wiki/Search%20engine | Technology | Internet | null |
Initially, anyone who wanted to share a file had to set up an FTP server in order to make the file available to others. Later, "anonymous" FTP sites became repositories for files, allowing all users to post and retrieve them.
Even with archive sites, many important files were still scattered on small FTP servers. These files could be located only by the Internet equivalent of word of mouth: Somebody would post an e-mail to a message list or a discussion forum announcing the availability of a file.
Archie changed all that. It combined a script-based data gatherer, which fetched site listings of anonymous FTP files, with a regular expression matcher for retrieving file names matching a user query. (4) In other words, Archie's gatherer scoured FTP sites across the Internet and indexed all of the files it found. Its regular expression matcher provided users with access to its database.
Veronica
In 1993, the University of Nevada System Computing Services group developed Veronica. It was created as a type of searching device similar to Archie but for Gopher files. Another Gopher search service, called Jughead, appeared a little later, probably for the sole purpose of rounding out the comic-strip triumvirate. Jughead is an acronym for Jonzy's Universal Gopher Hierarchy Excavation and Display, although, like Veronica, it is probably safe to assume that the creator backed into the acronym. Jughead's functionality was pretty much identical to Veronica's, although it appears to be a little rougher around the edges.
The Lone Wanderer
The World Wide Web Wanderer, developed by Matthew Gray in 1993 was the first robot on the Web and was designed to track the Web's growth. Initially, the Wanderer counted only Web servers, but shortly after its introduction, it started to capture URLs as it went along. The database of captured URLs became the Wandex, the first web database.
Matthew Gray's Wanderer created quite a controversy at the time, partially because early versions of the software ran rampant through the Net and caused a noticeable netwide performance degradation. This degradation occurred because the Wanderer would access the same page hundreds of times a day. The Wanderer soon amended its ways, but the controversy over whether robots were good or bad for the Internet remained. | Search engine | Wikipedia | 474 | 4059023 | https://en.wikipedia.org/wiki/Search%20engine | Technology | Internet | null |
In response to the Wanderer, Martijn Koster created Archie-Like Indexing of the Web, or ALIWEB, in October 1993. As the name implies, ALIWEB was the HTTP equivalent of Archie, and because of this, it is still unique in many ways.
ALIWEB does not have a web-searching robot. Instead, webmasters of participating sites post their own index information for each page they want listed. The advantage to this method is that users get to describe their own site, and a robot does not run about eating up Net bandwidth. The disadvantages of ALIWEB are more of a problem today. The primary disadvantage is that a special indexing file must be submitted. Most users do not understand how to create such a file, and therefore they do not submit their pages. This leads to a relatively small database, which meant that users are less likely to search ALIWEB than one of the large bot-based sites. This Catch-22 has been somewhat offset by incorporating other databases into the ALIWEB search, but it still does not have the mass appeal of search engines such as Yahoo! or Lycos.
Excite
Excite, initially called Architext, was started by six Stanford undergraduates in February 1993. Their idea was to use statistical analysis of word relationships in order to provide more efficient searches through the large amount of information on the Internet.
Their project was fully funded by mid-1993. Once funding was secured. they released a version of their search software for webmasters to use on their own web sites. At the time, the software was called Architext, but it now goes by the name of Excite for Web Servers.
Excite was the first serious commercial search engine which launched in 1995. It was developed in Stanford and was purchased for $6.5 billion by @Home. In 2001 Excite and @Home went bankrupt and InfoSpace bought Excite for $10 million.
Some of the first analysis of web searching was conducted on search logs from Excite
Yahoo!
In April 1994, two Stanford University Ph.D. candidates, David Filo and Jerry Yang, created some pages that became rather popular. They called the collection of pages Yahoo! Their official explanation for the name choice was that they considered themselves to be a pair of yahoos. | Search engine | Wikipedia | 471 | 4059023 | https://en.wikipedia.org/wiki/Search%20engine | Technology | Internet | null |
As the number of links grew and their pages began to receive thousands of hits a day, the team created ways to better organize the data. In order to aid in data retrieval, Yahoo! (www.yahoo.com) became a searchable directory. The search feature was a simple database search engine. Because Yahoo! entries were entered and categorized manually, Yahoo! was not really classified as a search engine. Instead, it was generally considered to be a searchable directory. Yahoo! has since automated some aspects of the gathering and classification process, blurring the distinction between engine and directory.
The Wanderer captured only URLs, which made it difficult to find things that were not explicitly described by their URL. Because URLs are rather cryptic to begin with, this did not help the average user. Searching Yahoo! or the Galaxy was much more effective because they contained additional descriptive information about the indexed sites.
Lycos
At Carnegie Mellon University during July 1994, Michael Mauldin, on leave from CMU, developed the Lycos search engine.
Types of web search engines
Search engines on the web are sites enriched with facility to search the content stored on other sites. There is difference in the way various search engines work, but they all perform three basic tasks.
Finding and selecting full or partial content based on the keywords provided.
Maintaining index of the content and referencing to the location they find
Allowing users to look for words or combinations of words found in that index.
The process begins when a user enters a query statement into the system through the interface provided.
There are basically three types of search engines: Those that are powered by robots (called crawlers; ants or spiders) and those that are powered by human submissions; and those that are a hybrid of the two.
Crawler-based search engines are those that use automated software agents (called crawlers) that visit a Web site, read the information on the actual site, read the site's meta tags and also follow the links that the site connects to performing indexing on all linked Web sites as well. The crawler returns all that information back to a central depository, where the data is indexed. The crawler will periodically return to the sites to check for any information that has changed. The frequency with which this happens is determined by the administrators of the search engine.
Human-powered search engines rely on humans to submit information that is subsequently indexed and catalogued. Only information that is submitted is put into the index. | Search engine | Wikipedia | 501 | 4059023 | https://en.wikipedia.org/wiki/Search%20engine | Technology | Internet | null |
In both cases, when you query a search engine to locate information, you're actually searching through the index that the search engine has created —you are not actually searching the Web. These indices are giant databases of information that is collected and stored and subsequently searched. This explains why sometimes a search on a commercial search engine, such as Yahoo! or Google, will return results that are, in fact, dead links. Since the search results are based on the index, if the index has not been updated since a Web page became invalid the search engine treats the page as still an active link even though it no longer is. It will remain that way until the index is updated.
So why will the same search on different search engines produce different results? Part of the answer to that question is because not all indices are going to be exactly the same. It depends on what the spiders find or what the humans submitted. But more important, not every search engine uses the same algorithm to search through the indices. The algorithm is what the search engines use to determine the relevance of the information in the index to what the user is searching for.
One of the elements that a search engine algorithm scans for is the frequency and location of keywords on a Web page. Those with higher frequency are typically considered more relevant. But search engine technology is becoming sophisticated in its attempt to discourage what is known as keyword stuffing, or spamdexing.
Another common element that algorithms analyze is the way that pages link to other pages in the Web. By analyzing how pages link to each other, an engine can both determine what a page is about (if the keywords of the linked pages are similar to the keywords on the original page) and whether that page is considered "important" and deserving of a boost in ranking. Just as the technology is becoming increasingly sophisticated to ignore keyword stuffing, it is also becoming more savvy to Web masters who build artificial links into their sites in order to build an artificial ranking. | Search engine | Wikipedia | 403 | 4059023 | https://en.wikipedia.org/wiki/Search%20engine | Technology | Internet | null |
Modern web search engines are highly intricate software systems that employ technology that has evolved over the years. There are a number of sub-categories of search engine software that are separately applicable to specific 'browsing' needs. These include web search engines (e.g. Google), database or structured data search engines (e.g. Dieselpoint), and mixed search engines or enterprise search. The more prevalent search engines, such as Google and Yahoo!, utilize hundreds of thousands computers to process trillions of web pages in order to return fairly well-aimed results. Due to this high volume of queries and text processing, the software is required to run in a highly dispersed environment with a high degree of superfluity.
Another category of search engines is scientific search engines. These are search engines which search scientific literature. The best known example is Google Scholar. Researchers are working on improving search engine technology by making them understand the content element of the articles, such as extracting theoretical constructs or key research findings. | Search engine | Wikipedia | 204 | 4059023 | https://en.wikipedia.org/wiki/Search%20engine | Technology | Internet | null |
Jaekelopterus is a genus of predatory eurypterid, a group of extinct aquatic arthropods. Fossils of Jaekelopterus have been discovered in deposits of Early Devonian age, from the Pragian and Emsian stages. There are two known species: the type species J. rhenaniae from brackish to fresh water strata in the Rhineland, and J. howelli from estuarine strata in Wyoming. The generic name combines the name of German paleontologist Otto Jaekel, who described the type species, and the Greek word πτερόν (pteron) meaning "wing".
Based on the isolated fossil remains of a large chelicera (claw) from the Klerf Formation of Germany, J. rhenaniae has been estimated to have reached a size of around 2.3–2.6 metres (7.5–8.5 ft), making it the largest arthropod ever discovered, surpassing other large arthropods such as fellow eurypterids Acutiramus and Pterygotus; the millipede Arthropleura. J. howelli was much smaller, reaching 80 centimetres (2.6 ft) in length.
In overall appearance, Jaekelopterus is similar to other pterygotid eurypterids, possessing a large, expanded telson (the hindmost segment of the body) and enlarged pincers and forelimbs. Both species of Jaekelopterus were first described as species of the closely related Pterygotus but were raised as a separate genus based on an observed difference in the genital appendage. Though this feature has since proved to be a misidentification, other features distinguishing the genus from its relatives have been identified, including a telson with a triangular shape and a different inclination of the denticles of the claws.
The chelicerae and compound eyes of Jaekelopterus indicate it was active and powerful with high visual acuity, most likely an apex predator in the ecosystems of Early Devonian Euramerica. Although eurypterids such as Jaekelopterus are often called "sea scorpions", the strata in which Jaekelopterus fossils have been found suggest that it lived in fresh water environments.
Description | Jaekelopterus | Wikipedia | 482 | 14341914 | https://en.wikipedia.org/wiki/Jaekelopterus | Biology and health sciences | Fossil arthropods | Animals |
Jaekelopterus is the largest known eurypterid and the largest known arthropod to have ever existed. This was determined based on a chelicera (claw) from the Emsian Klerf Formation of Willwerath, Germany, that measures long, but is missing a quarter of its length, suggesting that the full chelicera would have been long. If the ratio of body length to chelicera length matches those of other giant pterygotids, such as Acutiramus and Pterygotus, where the ratio between claw size and body length is relatively consistent, the organism that possessed the chelicera would have measured between in length. With the chelicerae extended, another metre would be added to this length. This estimate exceeds the maximum body size of all other known giant arthropods by almost half a metre even if the extended chelicerae are not included.
Jaekelopterus is similar to other pterygotid eurypterids in its overall morphology, distinguished by its triangular telson (the hindmost segment of its body) and inclined principal denticles on its cheliceral rami (the moving part of the claws). The pterygotids, a group of highly derived ("advanced") eurypterids, differ from other groups in several features, especially in the chelicerae and the telson. The chelicerae of the Pterygotidae are enlarged and robust, clearly adapted for active prey capture, with chelae (pincers) more similar to the claws of some modern crustaceans, with well-developed teeth on the claws, relative to the chelicerae of other eurypterid groups. Another feature distinguishing the group from other eurypterid groups is their flattened and expanded telsons, likely used as rudders when swimming. | Jaekelopterus | Wikipedia | 387 | 14341914 | https://en.wikipedia.org/wiki/Jaekelopterus | Biology and health sciences | Fossil arthropods | Animals |
J. howelli, known from over 30 specimens, has an almost identical pattern of denticulation on the chelicerae as J. rhenaniae and also preserves a flattened posterior margin of the telson, which results in a triangular shape, as in J. rhenaniae. Its serrated telson margin and the massive elongation of the second intermediate denticle clearly distinguishes it from J. rhenaniae. Furthermore, the type A genital appendage is not bifurcated at its end. J. howelli is much smaller than J. rhenaniae, reaching 80 centimetres (2.6 ft) in length.
History of research
Jaekelopterus was originally described as a species of Pterygotus, P. rhenaniae, in 1914 by German palaeontologist Otto Jaekel based on an isolated fossil pretelson (the segment directly preceding the telson) he received that had been discovered at Alken in Lower Devonian deposits of the Rhineland in Germany. Jaekel considered the pretelson to be characteristic of Pterygotus, other discovered elements differing little from previously known species of that genus, such as P. buffaloensis, and he estimated the length of the animal in life to be about 1 metre (1.5 metres if the chelicerae are included, 3.3 and 4.9 ft).
Based on more comprehensive material, including genital appendages, chelicerae and fragments of the metastoma (a large plate that is part of the abdomen) and telson discovered by German palaeontologist Walter R. Gross near Overath, Germany, Norwegian palaeontologist Leif Størmer provided a more comprehensive and detailed description of the species in 1936. Størmer interpreted the genital appendages as being segmented, distinct from other species of Pterygotus. | Jaekelopterus | Wikipedia | 388 | 14341914 | https://en.wikipedia.org/wiki/Jaekelopterus | Biology and health sciences | Fossil arthropods | Animals |
British palaeontologist Charles D. Waterston erected the genus Jaekelopterus in 1964 to accommodate Pterygotus rhenaniae, which he considered sufficiently distinct from other species of Pterygotus to warrant its own genus, primarily due to the abdominal appendages of Jaekelopterus being segmented as opposed to those of Pterygotus. Waterston diagnosed Jaekelopterus as a pterygotid with segmented genital appendages, a trapezoid prosoma, narrow and long chelicerae with terminal teeth almost at right angles to the rami and the primary teeth slightly angled anteriorly and with a telson with an expanded terminal spine and dorsal keel. The generic name honours Otto Jaekel; the Greek word πτερόν (pteron), meaning "wing", is a common epithet in eurypterid names.
In 1974, Størmer erected a new family to house the genus, Jaekelopteridae, due to the supposed considerable differences between the genital appendage of Jaekelopterus and other pterygotids. This diverging feature has since been proven to simply represent a misinterpretation by Størmer in 1936, the genital appendage of Jaekelopterus in fact being unsegmented like that of Pterygotus. As such, the family Jaekelopteridae has subsequently been rejected and treated as synonymous with the family Pterygotidae.
Another species of Pterygotus, P. howelli, was named by American palaeontologist Erik Kjellesvig-Waering and Størmer in 1952 based on a fossil telson and tergite (the dorsal part of a body segment) from Lower Devonian deposits of the Beartooth Butte Formation in Wyoming. The species name howelli honours Dr. Benjamin Howell of Princeton University, who loaned the fossil specimens examined in the description to Kjellesvig-Waering and Størmer. This species was assigned to Jaekelopterus as Jaekelopterus howelli by Norwegian palaeontologist O. Erik Tetlie in 2007.
Classification | Jaekelopterus | Wikipedia | 456 | 14341914 | https://en.wikipedia.org/wiki/Jaekelopterus | Biology and health sciences | Fossil arthropods | Animals |
Jaekelopterus is classified within the family Pterygotidae in the superfamily Pterygotioidea. Jaekelopterus is similar to Pterygotus, virtually only distinct in features of its genital appendage and potentially its telson. The close similarities between the two genera have prompted some researchers to question if the pterygotids are oversplit on the generic level. Based on some similarities in the genital appendage, American palaeontologists James C. Lamsdell and David A. Legg suggested in 2010 that Jaekelopterus, Pterygotus and even Acutiramus could be synonyms of each other. Though differences have been noted in chelicerae, these structures were questioned as the basis of generic distinctions in eurypterids by Charles D. Waterston in 1964 since their morphology is dependent on lifestyle and varies throughout ontogeny (the development of the organism following its birth). Whilst telson morphology can be used to distinguish genera in eurypterids, Lamsdell and Legg noted that the triangular telson of Jaekelopterus might still fall within the morphological range of the paddle-shaped telsons present in Pterygotus and Acutiramus. Genital appendages can vary even within genera; for instance, the genital appendage of Acutiramus changes from species to species, being spoon-shaped in earlier species and then becoming bilobed and eventually beginning to look similar to the appendage of Jaekelopterus. Lamsdell and Legg concluded that an inclusive phylogenetic analysis with multiple species of Acutiramus, Pterygotus and Jaekelopterus is required to resolve whether the genera are synonyms of each other. | Jaekelopterus | Wikipedia | 366 | 14341914 | https://en.wikipedia.org/wiki/Jaekelopterus | Biology and health sciences | Fossil arthropods | Animals |
The cladogram below is based on the nine best-known pterygotid species and two outgroup taxa (Slimonia acuminata and Hughmilleria socialis). Jaekelopterus had previously been classified as a basal sister taxon to the rest of the Pterygotidae since its description as a separate genus by Waterston in 1964 due to its supposedly segmented genital appendages (fused and undivided in other pterygotids), but restudy of the specimens in question revealed that the genital appendage of Jaekelopterus also was undivided. The material examined and phylogenetic analysis conducted by British palaeontologist Simon J. Braddy, German palaeontologist Markus Poschmann and O. Erik Tetlie in 2007 revealed that Jaekelopterus was not a basal pterygotid, but one of the most derived taxa in the group. The cladogram also contains the maximum sizes reached by the species in question, which was suggested to possibly have been an evolutionary trait of the group per Cope's rule ("phyletic gigantism") by Braddy, Poschmann and Tetlie.
Palaeobiology
Gigantism
The pterygotid eurypterids include many of the largest known eurypterids, such as Pterygotus and Acutiramus. Several factors have been suggested that might have contributed to the unprecedented large size of Jaekelopterus, its relatives and other large Paleozoic invertebrates, such as predation, courtship behaviour, competition and environmental resources.
Factors such as respiration, the energy costs of moulting, locomotion and the actual properties of the exoskeleton restrict the size of arthropods. Other than the robust and heavily sclerotised claws, most of the preserved large body segments of the pterygotids are thin and unmineralised. Even tergites and sternites (the plates that form the surfaces of the abdominal segments) are generally preserved as paper-thin compressions, suggesting that pterygotids were very lightweight in construction. Similar lightweight adaptations can be observed in other Paleozoic giant arthropods, such as the giant millipede-like Arthropleura, and it has been suggested to be vital for the evolution of giant arthropod sizes. A lightweight build decreases the influence of factors that restrict body size. | Jaekelopterus | Wikipedia | 511 | 14341914 | https://en.wikipedia.org/wiki/Jaekelopterus | Biology and health sciences | Fossil arthropods | Animals |
Despite being the largest arthropods, the lightweight build of Jaekelopterus and other giant pterygotid eurypterids meant they likely were not the heaviest. Other giant eurypterids, particularly the deep-bodied walking forms in the Hibbertopteridae, such as the almost 2-metre-long Hibbertopterus, may have rivalled the pterygotids and other giant arthropods in weight, if not surpassed them.
American palaeontologist Alexander Kaiser and South African palaeontologist Jaco Klok suggested in 2008 that the massive size estimates for Jaekelopterus are exaggerated, noting that the size estimates assume that the relative proportions between the chelicerae and body length would stay the same as the animal matured. The denticles (the serrations of the claws) were observed as showing positive allometry (being proportionally larger in larger specimens), which Kaiser and Klok suggest could have occurred in the chelicerae as a whole. Furthermore, the largest coxae (limb segments) found of the same species, measuring wide, suggest a total maximum body length of only . Positive allometry has not been demonstrated in eurypterid chelicerae as a whole in any other eurypterid genus, including in the closest relatives of Jaekelopterus. There are also some undescribed specimens of J. rhenaniae similar in proportions to the large chelicera, including another claw found in the same strata as the original find. In the opinion of Braddy, Poschmann and Tetlie, who replied to Kaiser and Klok the same year, the size estimates around remain the most accurate estimates on the maximum size of the species yet.
Ontogeny | Jaekelopterus | Wikipedia | 368 | 14341914 | https://en.wikipedia.org/wiki/Jaekelopterus | Biology and health sciences | Fossil arthropods | Animals |
Like all other arthropods, eurypterids matured through a sequence of stages called "instars" consisting of periods of ecdysis (moulting) followed by rapid growth. Unlike many arthropods, such as insects and crustaceans, chelicerates (the group to which eurypterids like Jaekelopterus belongs, alongside other organisms such as horseshoe crabs, sea spiders and arachnids) are generally direct developers, meaning that there are no extreme morphological changes after they have hatched. Extant xiphosurans hatch without the full complement of adult opisthosomal appendages (appendages attached to the opisthosoma, the posterior segments of the body), but extant spiders are fully direct developers. Studies of fossil specimens of Strobilopterus and Jaekelopterus suggest that the ontogeny of eurypterids broadly parallelled that of modern horseshoe crabs, but that eurypterids (like arachnids) were true direct developers, hatching with the same number of appendages and segments as adults. | Jaekelopterus | Wikipedia | 234 | 14341914 | https://en.wikipedia.org/wiki/Jaekelopterus | Biology and health sciences | Fossil arthropods | Animals |
Though several fossilised instars of Jaekelopterus howelli are known, the fragmentary and incomplete status of the specimens makes it difficult to study its ontogeny in detail. Despite this, there are some noticeable changes occurring in the chelicerae, telson and metastoma. Four of the J. howelli specimens studied by Lamsdell and Selden (2013) preserve the chelicerae in enough detail to allow for study of the denticles. Two of these chelicerae were assumed to come from juveniles and two were assumed to be from adults. The morphology of the chelicerae is similar across all ages, with the same arrangement and number of denticles, but there were also some noticeable differences. Particularly, the principal denticles grew in size relative to the intermediate denticles, being 1.5 times the size of the intermediate denticles in juveniles, but up to 3.5 times the size of the intermediate denticles in adults. Furthermore, the terminal denticle was far larger and more robust in adult specimens than in juveniles. Perhaps most extreme of all, the second intermediate denticle is not different in size from the other intermediate denticles in juveniles, but it is massively elongated in adults, where it is more than twice the length of any principal denticle. Though such growth in the denticles of pterygotids has been described in other genera, the massive elongation of the second intermediate denticle through ontogeny is unique to Jaekelopterus, particularly to J. howelli.
The metastoma of Jaekelopterus also altered its dimensions as the animal matured. In J. rhenaniae, the relative width of the metastoma decreased through ontogeny. The metastoma in J. howelli is also broader in juveniles than in adults, although the length–width ratios measured in juveniles and adults were not as disparate as assumed, being 1.43 in juveniles and 1.46 in adults. Such a change in metastomal dimensions has been noted in other eurypterid genera as well, such as Stoermeropterus, Moselopterus and Strobilopterus.
Visual system | Jaekelopterus | Wikipedia | 451 | 14341914 | https://en.wikipedia.org/wiki/Jaekelopterus | Biology and health sciences | Fossil arthropods | Animals |
The cheliceral morphology and visual acuity of the pterygotid eurypterids separates them into distinct ecological groups. The primary method for determining visual acuity in arthropods is by determining the number of lenses in their compound eyes and the interommatidial angle (IOA), which is the angle between the optical axes of adjacent lenses. The IOA is especially important as it can be used to distinguish different ecological roles in arthropods, being low in modern active arthropod predators.
Both Jaekelopterus rhenaniae and Pterygotus anglicus had high visual acuity, as suggested by the low IOA and many lenses in their compound eyes. Further studies on the compound eyes of fossilised specimens of J. rhenaniae, including a large specimen with the right eye preserved from the uppermost Siegenian and a small and likely juvenile specimen, confirmed the high visual acuity of the genus. The overall average IOA of Jaekelopterus (0.87°) is comparable to that of modern predatory arthropods. The visual acuity of Jaekelopterus increased with age, the smaller specimens having relatively worse eyesight. This is consistent with other pterygotids, such as Acutiramus, and has been interpreted as indicating that adult Jaekelopterus lived in darker environments, such as in deeper water. Trace fossil evidence of eurypterids also supports such a conclusion, indicating that eurypterids migrated to nearshore environments to mate and spawn.
Jaekelopterus had a frontally overlapping visual field, i.e. stereoscopic vision, typical of predatory animals. Structurally, eurypterid eyes were almost identical to the eyes of horseshoe crabs. The square-like pattern of the receptor cells in the compound eyes of Jaekelopterus is also similar, but not identical, to the pattern in horseshoe crabs, suggesting a specialised visual system. The photoreceptors are unusually large in Jaekelopterus. At around 70 μm, they are far larger than those of humans (1-2 μm) and most arthropods (also 1-2 μm) but they match those of modern horseshoe crabs in size. | Jaekelopterus | Wikipedia | 472 | 14341914 | https://en.wikipedia.org/wiki/Jaekelopterus | Biology and health sciences | Fossil arthropods | Animals |
The unique eyes of modern horseshoe crabs are highly distinct from eyes of other modern arthropods and allow increased edge-perception and enhance contrasts, important for animals in low and scattered light conditions. As the eyes of Jaekelopterus were very similar, it too likely had the same adaptations. With its highly specialised eyes, Jaekelopterus was very well adapted to its predatory lifestyle.
Palaeoecology
The morphology and body construction of Jaekelopterus and other eurypterids in the Pterygotidae suggests they were adapted to a completely aquatic lifestyle. Braddy, Poschmann and Tetlie considered in a 2007 study that it was highly unlikely that an arthropod with the size and build of Jaekelopterus would be able to walk on land. Eurypterids such as Jaekelopterus are often popularly referred to as "sea scorpions", but the deposits from which Jaekelopterus fossils have been discovered suggest that it lived in non-marine aquatic environments. The Beartooth Butte Formation in Wyoming, where J. howelli fossils have been discovered, has been interpreted as a quiet, shallow estuarine environment. This species has been found together with two other eurypterid species: Dorfopterus angusticollis and Strobilopterus princetonii. The fossil sites yielding J. rhenaniae in the Rhineland have also been interpreted as having been part of a shallow aquatic environment with brackish to fresh water.
The chelicerae of Jaekelopterus are enlarged, robust and have a curved free ramus and denticles of different lengths and sizes, all adaptations that correspond to strong puncturing and grasping abilities in extant scorpions and crustaceans. Some puncture wounds on fossils of the poraspid agnathan fish Lechriaspis patula from the Devonian of Utah were likely caused by Jaekelopterus howelli. The latest research indicates that Jaekelopterus was an active and visual predator. Fully grown Jaekelopterus would have been apex predators in their environments and likely preyed upon smaller arthropods (including resorting to cannibalism) and early vertebrates. | Jaekelopterus | Wikipedia | 460 | 14341914 | https://en.wikipedia.org/wiki/Jaekelopterus | Biology and health sciences | Fossil arthropods | Animals |
A powerful and active predator, Jaekelopterus was likely highly agile and possessed high maneuverability. The hydromechanics of the swimming paddles and telsons of Jaekelopterus and other pterygotids suggest that all members of the group were capable of hovering, forward locomotion and quick turns. Though they were not necessarily rapidly swimming animals, they were likely able to give chase to prey in habitats such as lagoons and estuaries. | Jaekelopterus | Wikipedia | 95 | 14341914 | https://en.wikipedia.org/wiki/Jaekelopterus | Biology and health sciences | Fossil arthropods | Animals |
In biology, a hybrid is the offspring resulting from combining the qualities of two organisms of different varieties, subspecies, species or genera through sexual reproduction. Generally, it means that each cell has genetic material from two different organisms, whereas an individual where some cells are derived from a different organism is called a chimera. Hybrids are not always intermediates between their parents such as in blending inheritance (a now discredited theory in modern genetics by particulate inheritance), but can show hybrid vigor, sometimes growing larger or taller than either parent. The concept of a hybrid is interpreted differently in animal and plant breeding, where there is interest in the individual parentage. In genetics, attention is focused on the numbers of chromosomes. In taxonomy, a key question is how closely related the parent species are.
Species are reproductively isolated by strong barriers to hybridization, which include genetic and morphological differences, differing times of fertility, mating behaviors and cues, and physiological rejection of sperm cells or the developing embryo. Some act before fertilization and others after it. Similar barriers exist in plants, with differences in flowering times, pollen vectors, inhibition of pollen tube growth, somatoplastic sterility, cytoplasmic-genic male sterility and the structure of the chromosomes. A few animal species and many plant species, however, are the result of hybrid speciation, including important crop plants such as wheat, where the number of chromosomes has been doubled.
A form of often intentional human-mediated hybridization is the crossing of wild and domesticated species. This is common in both traditional horticulture and modern agriculture; many commercially useful fruits, flowers, garden herbs, and trees have been produced by hybridization. One such flower, Oenothera lamarckiana, was central to early genetics research into mutationism and polyploidy. It is also more occasionally done in the livestock and pet trades; some well-known wild × domestic hybrids are beefalo and wolfdogs. Human selective breeding of domesticated animals and plants has also resulted in the development of distinct breeds (usually called cultivars in reference to plants); crossbreeds between them (without any wild stock) are sometimes also imprecisely referred to as "hybrids".
Hybrid humans existed in prehistory. For example, Neanderthals and anatomically modern humans are thought to have interbred as recently as 40,000 years ago. | Hybrid (biology) | Wikipedia | 493 | 41244 | https://en.wikipedia.org/wiki/Hybrid%20%28biology%29 | Biology and health sciences | Genetics and taxonomy | null |
Mythological hybrids appear in human culture in forms as diverse as the Minotaur, blends of animals, humans and mythical beasts such as centaurs and sphinxes, and the Nephilim of the Biblical apocrypha described as the wicked sons of fallen angels and attractive women.
Significance
In evolution
Hybridization between species plays an important role in evolution, though there is much debate about its significance. Roughly 25% of plants and 10% of animals are known to form hybrids with at least one other species. One example of an adaptive benefit to hybridization is that hybrid individuals can form a "bridge" transmitting potentially helpful genes from one species to another when the hybrid backcrosses with one of its parent species, a process called introgression. Hybrids can also cause speciation, either because the hybrids are genetically incompatible with their parents and not each other, or because the hybrids occupy a different niche than either parent.
Hybridization is a particularly common mechanism for speciation in plants, and is now known to be fundamental to the evolutionary history of plants. Plants frequently form polyploids, individuals with more than two copies of each chromosome. Whole genome doubling has occurred repeatedly in plant evolution. When two plant species hybridize, the hybrid may double its chromosome count by incorporating the entire nuclear genome of both parents, resulting in offspring that are reproductively incompatible with either parent because of different chromosome counts.
In conservation
Human impact on the environment has resulted in an increase in the interbreeding between regional species, and the proliferation of introduced species worldwide has also resulted in an increase in hybridization. This has been referred to as genetic pollution out of concern that it may threaten many species with extinction. Similarly, genetic erosion from monoculture in crop plants may be damaging the gene pools of many species for future breeding.
The conservation impacts of hybridization between species are highly debated. While hybridization could potentially threaten rare species or lineages by "swamping" the genetically "pure" individuals with hybrids, hybridization could also save a rare lineage from extinction by introducing genetic diversity. It has been proposed that hybridization could be a useful tool to conserve biodiversity by allowing organisms to adapt, and that efforts to preserve the separateness of a "pure" lineage could harm conservation by lowering the organisms' genetic diversity and adaptive potential, particularly in species with low populations. While endangered species are often protected by law, hybrids are often excluded from protection, resulting in challenges to conservation.
Etymology | Hybrid (biology) | Wikipedia | 492 | 41244 | https://en.wikipedia.org/wiki/Hybrid%20%28biology%29 | Biology and health sciences | Genetics and taxonomy | null |
The term hybrid is derived from Latin , used for crosses such as of a tame sow and a wild boar. The term came into popular use in English in the 19th century, though examples of its use have been found from the early 17th century. Conspicuous hybrids are popularly named with portmanteau words, starting in the 1920s with the breeding of tiger–lion hybrids (liger and tigon).
As seen by different disciplines
Animal and plant breeding
From the point of view of animal and plant breeders, there are several kinds of hybrid formed from crosses within a species, such as between different breeds. Single cross hybrids result from the cross between two true-breeding organisms which produces an F1 hybrid (first filial generation). The cross between two different homozygous lines produces an F1 hybrid that is heterozygous; having two alleles, one contributed by each parent and typically one is dominant and the other recessive. Typically, the F1 generation is also phenotypically homogeneous, producing offspring that are all similar to each other.
Double cross hybrids result from the cross between two different F1 hybrids (i.e., there are four unrelated grandparents).
Three-way cross hybrids result from the cross between an F1 hybrid and an inbred line. Triple cross hybrids result from the crossing of two different three-way cross hybrids. Top cross (or "topcross") hybrids result from the crossing of a top quality or pure-bred male and a lower quality female, intended to improve the quality of the offspring, on average.
Population hybrids result from the crossing of plants or animals in one population with those of another population. These include interspecific hybrids or crosses between different breeds. In biology, the result of crossing of two populations is called a synthetic population.
In horticulture, the term stable hybrid is used to describe an annual plant that, if grown and bred in a small monoculture free of external pollen (e.g., an air-filtered greenhouse) produces offspring that are "true to type" with respect to phenotype; i.e., a true-breeding organism.
Biogeography | Hybrid (biology) | Wikipedia | 436 | 41244 | https://en.wikipedia.org/wiki/Hybrid%20%28biology%29 | Biology and health sciences | Genetics and taxonomy | null |
Hybridization can occur in the hybrid zones where the geographical ranges of species, subspecies, or distinct genetic lineages overlap. For example, the butterfly Limenitis arthemis has two major subspecies in North America, L. a. arthemis (the white admiral) and L. a. astyanax (the red-spotted purple). The white admiral has a bright, white band on its wings, while the red-spotted purple has cooler blue-green shades. Hybridization occurs between a narrow area across New England, southern Ontario, and the Great Lakes, the "suture region". It is at these regions that the subspecies were formed. Other hybrid zones have formed between described species of plants and animals.
Genetics
From the point of view of genetics, several different kinds of hybrid can be distinguished.
A genetic hybrid carries two different alleles of the same gene, where for instance one allele may code for a lighter coat colour than the other. A structural hybrid results from the fusion of gametes that have differing structure in at least one chromosome, as a result of structural abnormalities. A numerical hybrid results from the fusion of gametes having different haploid numbers of chromosomes. A permanent hybrid results when only the heterozygous genotype occurs, as in Oenothera lamarckiana, because all homozygous combinations are lethal. In the early history of genetics, Hugo de Vries supposed these were caused by mutation.
Genetic complementation
Genetic complementation is a hybridization test widely used in genetics to determine whether two separately isolated mutants that have the same (or similar) phenotype are defective in the same gene or in different genes (see complementation). If a hybrid organism containing the genomes of two different mutant parental organisms displays a wild type phenotype, it is ordinarily considered that the two parental mutant organisms are defective in different genes. If the hybrid organism displays a distinctly mutant phenotype, the two mutant parental organisms are considered to be defective in the same gene. However, in some cases the hybrid organism may display a phenotype that is only weakly (or partially) wild-type, and this may reflect intragenic (interallelic) complementation.
Taxonomy | Hybrid (biology) | Wikipedia | 453 | 41244 | https://en.wikipedia.org/wiki/Hybrid%20%28biology%29 | Biology and health sciences | Genetics and taxonomy | null |
From the point of view of taxonomy, hybrids differ according to their parentage.
Hybrids between different subspecies (such as between the dog and Eurasian wolf) are called intra-specific hybrids. Interspecific hybrids are the offspring from interspecies mating; these sometimes result in hybrid speciation. Intergeneric hybrids result from matings between different genera, such as between sheep and goats. Interfamilial hybrids, such as between chickens and guineafowl or pheasants, are reliably described but extremely rare. Interordinal hybrids (between different orders) are few, but have been engineered between the sea urchin Strongylocentrotus purpuratus (female) and the sand dollar Dendraster excentricus (male).
Biology
Expression of parental traits
When two distinct types of organisms breed with each other, the resulting hybrids typically have intermediate traits (e.g., one plant parent has red flowers, the other has white, and the hybrid, pink flowers). Commonly, hybrids also combine traits seen only separately in one parent or the other (e.g., a bird hybrid might combine the yellow head of one parent with the orange belly of the other).
Mechanisms of reproductive isolation
Interspecific hybrids are bred by mating individuals from two species, normally from within the same genus. The offspring display traits and characteristics of both parents, but are often sterile, preventing gene flow between the species. Sterility is often attributed to the different number of chromosomes between the two species. For example, donkeys have 62 chromosomes, horses have 64 chromosomes, and mules or hinnies have 63 chromosomes. Mules, hinnies, and other normally sterile interspecific hybrids cannot produce viable gametes, because differences in chromosome structure prevent appropriate pairing and segregation during meiosis, meiosis is disrupted, and viable sperm and eggs are not formed. However, fertility in female mules has been reported with a donkey as the father.
A variety of mechanisms limit the success of hybridization, including the large genetic difference between most species. Barriers include morphological differences, differing times of fertility, mating behaviors and cues, and physiological rejection of sperm cells or the developing embryo. Some act before fertilization; others after it.
In plants, some barriers to hybridization include blooming period differences, different pollinator vectors, inhibition of pollen tube growth, somatoplastic sterility, cytoplasmic-genic male sterility and structural differences of the chromosomes. | Hybrid (biology) | Wikipedia | 511 | 41244 | https://en.wikipedia.org/wiki/Hybrid%20%28biology%29 | Biology and health sciences | Genetics and taxonomy | null |
Speciation
A few animal species are the result of hybridization. The Lonicera fly is a natural hybrid. The American red wolf appears to be a hybrid of the gray wolf and the coyote, although its taxonomic status has been a subject of controversy. The European edible frog is a semi-permanent hybrid between pool frogs and marsh frogs; its population requires the continued presence of at least one of the parent species. Cave paintings indicate that the European bison is a natural hybrid of the aurochs and the steppe bison.
Plant hybridization is more commonplace compared to animal hybridization. Many crop species are hybrids, including notably the polyploid wheats: some have four sets of chromosomes (tetraploid) or six (hexaploid), while other wheat species have (like most eukaryotic organisms) two sets (diploid), so hybridization events likely involved the doubling of chromosome sets, causing immediate genetic isolation.
Hybridization may be important in speciation in some plant groups. However, homoploid hybrid speciation (not increasing the number of sets of chromosomes) may be rare: by 1997, only eight natural examples had been fully described. Experimental studies suggest that hybridization offers a rapid route to speciation, a prediction confirmed by the fact that early generation hybrids and ancient hybrid species have matching genomes, meaning that once hybridization has occurred, the new hybrid genome can remain stable.
Many hybrid zones are known where the ranges of two species meet, and hybrids are continually produced in great numbers. These hybrid zones are useful as biological model systems for studying the mechanisms of speciation. Recently DNA analysis of a bear shot by a hunter in the Northwest Territories confirmed the existence of naturally occurring and fertile grizzly–polar bear hybrids.
Hybrid vigour | Hybrid (biology) | Wikipedia | 358 | 41244 | https://en.wikipedia.org/wiki/Hybrid%20%28biology%29 | Biology and health sciences | Genetics and taxonomy | null |
Hybridization between reproductively isolated species often results in hybrid offspring with lower fitness than either parental. However, hybrids are not, as might be expected, always intermediate between their parents (as if there were blending inheritance), but are sometimes stronger or perform better than either parental lineage or variety, a phenomenon called heterosis, hybrid vigour, or heterozygote advantage. This is most common with plant hybrids. A transgressive phenotype is a phenotype that displays more extreme characteristics than either of the parent lines. Plant breeders use several techniques to produce hybrids, including line breeding and the formation of complex hybrids. An economically important example is hybrid maize (corn), which provides a considerable seed yield advantage over open pollinated varieties. Hybrid seed dominates the commercial maize seed market in the United States, Canada and many other major maize-producing countries.
In a hybrid, any trait that falls outside the range of parental variation (and is thus not simply intermediate between its parents) is considered heterotic. Positive heterosis produces more robust hybrids, they might be stronger or bigger; while the term negative heterosis refers to weaker or smaller hybrids. Heterosis is common in both animal and plant hybrids. For example, hybrids between a lion and a tigress ("ligers") are much larger than either of the two progenitors, while "tigons" (lioness × tiger) are smaller. Similarly, the hybrids between the common pheasant (Phasianus colchicus) and domestic fowl (Gallus gallus) are larger than either of their parents, as are those produced between the common pheasant and hen golden pheasant (Chrysolophus pictus). Spurs are absent in hybrids of the former type, although present in both parents.
Human influence
Anthropogenic hybridization
Hybridization is greatly influenced by human impact on the environment, through effects such as habitat fragmentation and species introductions. Such impacts make it difficult to conserve the genetics of populations undergoing introgressive hybridization. Humans have introduced species worldwide to environments for a long time, both intentionally for purposes such as biological control, and unintentionally, as with accidental escapes of individuals. Introductions can drastically affect populations, including through hybridization.
Management | Hybrid (biology) | Wikipedia | 469 | 41244 | https://en.wikipedia.org/wiki/Hybrid%20%28biology%29 | Biology and health sciences | Genetics and taxonomy | null |
Subsets and Splits
No community queries yet
The top public SQL queries from the community will appear here once available.