Ichthyosaur

Ichthyosaurs (Ancient Greek for "fish lizard" – and ) are large extinct marine reptiles. Ichthyosaurs belong to the order known as Ichthyosauria or Ichthyopterygia ('fish flippers' – a designation introduced by Sir Richard Owen in 1842, although the term is now used more for the parent clade of the Ichthyosauria).

Ichthyosaurs 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. Ichthyosaurs 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. Ichthyosaurs 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.

Science became aware of the existence of ichthyosaurs during the early nineteenth century, when the first complete skeletons were found in England. In 1834, the order Ichthyosauria was named. Later that century, many excellently preserved ichthyosaur fossils were discovered in Germany, including soft-tissue remains. Since the late twentieth century, there has been a revived interest in the group, leading to an increased number of named ichthyosaurs from all continents, with over fifty valid genera being now known.

Ichthyosaur species varied from in length. Ichthyosaurs 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. Ichthyosaurs were air-breathing, warm-blooded, and bore live young. They may have had a layer of blubber for insulation.

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 ''Lithophylacii Brittannici Ichnographia'' 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.

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 what is now called 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 now independent Natural History Museum and has the inventory number 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.

Home felt 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 in-between fishes and crocodiles, not in an evolutionary sense, but as regarded its place in the '' scala naturae'', 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 Olm, and lizards; he now 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 ἰχθύς, ''ichthys'', "fish". This name at the time was an invalid '' nomen nudum'' 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 now lost skeleton. Conybeare considered that ''Ichthyosaurus'' had priority relative to ''Proteosaurus''. Although this is incorrect by present standards, the latter name became a "forgotten" '' nomen oblitum''. 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

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.

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 noted 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 quickly increased to a few hundred each year. Ultimately, over four thousand were uncovered, forming the bulk of ichthyosaur specimens displayed today. The sites were also a '' Konservat-Lagerstätte'', 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

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 now 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 today seen as 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. In 2003, McGowan and Motani published the first modern textbook on the Ichthyosauria and their closest relatives.

Evolutionary history

Origin

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.

Until the 1980s, a close relationship was assumed between the Ichthyosauria and the Sauropterygia, another marine reptile group, within an overarching Euryapsida. Today, however, this is usually rejected, with the Euryapsida being seen as an unnatural polyphyletic assemblage of reptiles that happen to share some adaptations to a swimming lifestyle. An exception is a study in 1997 by John Merck, which showed monophyletic archosauromorph euryapsids.

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:

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''. This diversity suggests an even earlier origin. 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

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 by Elizabeth Nicholls, has been estimated to be as much as in length—if correct, the largest marine reptile known to date.

In the Late Triassic, ichthyosaurs attained the peak of their 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. From this epoch, the original British discoveries were made, so that among ichthyosaurs, their names are the ones most familiar to the general public. Genera include '' Eurhinosaurus'', '' Ichthyosaurus'', '' Leptonectes'', ''Stenopterygius'', and the large predator ''Temnodontosaurus'', along with the basal parvipelvian ''Suevoleviathan'', which was little changed from its Norian ancestors. The general morphological variability had been strongly reduced, however. Giant forms, suction feeders and durophagous species were absent. All of these animals were streamlined, dolphin-like forms, although the more basal animals were perhaps more elongated than the advanced and compact ''Stenopterygius'' and ''Ichthyosaurus''. The latter belonged to a parvipelvian subgroup, the Thunnosauria. These were fully adapted to an efficient thunniform locomotion, propelling themselves with the end of the tail only, equipped with a vertical tail fin. Their front flippers had more than five fingers. Another parvipelvian branch was the Eurhinosauria such as ''Leptonectes'' and ''Eurhinosaurus'', which were specialised forms having very elongated and pointy 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

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. The ichthyosaurs thus would have disappeared much earlier than other large Mesozoic reptile groups that generally survived until the end of the Cretaceous. Two major explanations were given for this. Firstly, it could have been a matter of chance. The second explanation had the extinction as caused by competition. Less hydrodynamically efficient animals, such as the Mosasauridae and long-necked plesiosaurs, flourished. The ichthyosaurian overspecialisation could be a contributing factor to their extinction, possibly being unable to 'keep up' with the fast-swimming and highly evasive new teleost fish, which had become dominant at this time, against which the sit-and-wait ambush strategies of the mosasaurids 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 referred to ''Platypterygius'' in fact represented diverse species. In 2012, at least eight lineages spanned the Jurassic-Cretaceous boundary, among them ''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.

The demise of the ichthyosaurs has recently 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 then present, the 'soft-prey specialists' and the 'generalists', leaving only an apex predator group, which probably was not particularly specialized. The second major ichthyosaur extinction took place during the Cenomanian-Turonian boundary event, an ' anoxic event', eliminating the apex predators, after which just a single lineage survived, ''Platypterygius hercynicus'', which disappeared shortly afterwards, 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. If so, it was not caused by mosasaurid competition; large mosasaurs did not appear until 3 million years after the ichthyosaur extinction, likely to fill the resulting ecological void. 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.

Taxonomy

This taxonomy was presented by Michael Werner Maisch in his 2010 review of ichthyosaur classification.
* Clade Ichthyopterygia (''sensu'' Motani (1999))
** Genus ''Chaohusaurus''
** Family Grippiidae
** Family Parvinatatoridae
** Family Thaisauridae
** Family Utatsusauridae
** ? Family Omphalosauridae
** ? Genus '' Isfjordosaurus''
** Order Ichthyosauria (''sensu'' Motani (1999))
*** Family Quasianosteosauridae
*** Parvorder Hueneosauria
**** Nanorder Mixosauria
***** Family Wimaniidae
***** Family Mixosauridae
**** Nanorder Longipinnati
***** Family Toretocnemidae
***** Family Cymbospondylidae
***** Hyporder Merriamosauria
****** Family Merriamosauridae
****** Family Besanosauridae
****** Family Shastasauridae
****** Family Shonisauridae
****** Family Californosauridae
****** Minorder Parvipelvia
******* Family Hudsonelpidiidae
******* Family Macgowaniidae
******* Suborder Neoichthyosauria
******** Family Temnodontosauridae
******** Family Leptonectidae
******** Family Suevoleviathanidae
******** Infraorder Thunnosauria
********* Family Ichthyosauridae
********* Family Stenopterygiidae
********* Family Ophthalmosauridae

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

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). 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 porpoise-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 fin on their backs and a more or less vertical fin 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.

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.

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.

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. 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

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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.

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

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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.

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.

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 have ten fingers per hand. 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.

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.

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