Жоғалған келескектер тобы: Хористодералар

Extinct order of reptiles

Choristodera (from the Greek χωριστός chōristos + δέρη dérē, 'separated neck') is an extinct order of semiaquatic diapsid reptiles that ranged from the Middle Jurassic, or possibly Triassic, to the Miocene (168 to 20 or possibly 11.6 million years ago). Choristoderes are morphologically diverse, with the best known members being the crocodile like neochoristoderes such as Champsosaurus. Other choristoderans had lizard like or long necked morphologies. Choristoderes appear to have been confined to the Northern Hemisphere, having been found in North America, Asia, and Europe, and possibly also North Africa. Choristoderes are generally thought to be derived neodiapsids that are close relatives or members of Sauria.

Choristodera was erected in 1876, originally as a suborder of Rhynchocephalia by Edward Drinker Cope to contain Champsosaurus, which was described from Late Cretaceous strata of Montana by Cope in the same paper. A year later, in 1877, Simoedosaurus was described by Paul Gervais from Upper Paleocene deposits at Cernay, near Rheims, France. These remained the only recognised choristoderes for over a century, until new taxa were described in the late 20th century. Beginning in the late 1970s, additional taxa were described by Soviet Mongolian teams from Lower Cretaceous sediments in Mongolia. In studies from 1989 to 1991, Susan E. Evans described new material of Cteniogenys from the Middle Jurassic of Britain. The genus had first been described by Charles W. Gilmore in 1928 from the Late Jurassic of the western United States, and had previously been enigmatic. The studies revealed it to be a small, lizard like choristodere, different from the crocodile like forms previously known.

Choristoderes vary substantially in size, the smallest genera like Cteniogenys and Lazarussuchus had a length of only around , and the largest known choristoderan, Kosmodraco dakotensis is estimated to have had a total length of around All known choristoderans possess or are inferred to possess a novel skull bone not found in other reptiles, referred to as the "neomorphic bone" or neomorph, which is a component of the dermatocranium. Ancestrally, the skull of choristoderes possess elongated upper and lower temporal fenestrae (openings of the skull behind the eye socket), these are greatly expanded in neochoristoderes, most extremely in Champsosaurus, giving the skull a cordiform (heart shaped) appearance when viewed from above. In many "non neochoristoderes" the lower temporal fenestrae are secondarily closed.

The internal skull anatomy of choristoderes is only known for Champsosaurus. The braincase of Champsosaurus is poorly ossified at the front of the skull (anterior), but is well ossified in the rear (posterior) similar to other diapsids. The cranial endocast (space occupied by the brain in the cranial vault) is proportionally narrow in both lateral and dorsoventral axes, with an enlarged pineal body and olfactory bulbs. The optic lobes and flocculi are small in size, indicating only average vision ability at best. The olfactory chambers of the nasal passages and olfactory stalks of the braincase are reasonably large, indicating that Champsosaurus probably had good olfactory capabilities (sense of smell). The nasal passages lack bony turbinates. The semicircular canals of the inner ear are most similar to those of other aquatic reptiles. The expansion of the sacculus indicates that Champsosaurus likely had an increased sensitivity to low frequency sounds and vibrations.

Most choristoderes have rather simple undifferentiated (homodont) teeth, with striated enamel covering the tooth crown but not the base. Neochoristoderes have teeth completely enveloped in striated enamel with an enamel infolding at the base, labiolingually compressed and hooked, the exception being Ikechosaurus which has still rather simple teeth aside from the start of an enamel infolding. Teeth implantation is subthecodont, with teeth being replaced by erosion of a pit in the lingual (side of the tooth facing the tongue) surface of the tooth base. There is some tooth differentiation among neochoristoderes, with the anterior teeth being sharper and more slender than posterior teeth. Choristoderes retain palatal teeth (teeth present on the bones of the roof of the mouth). Unlike most diapsid groups, where palatal teeth are reduced or lost completely, the palatal teeth in choristoderes are extensively developed indicating food manipulation in the mouth, probably in combination with the tongue. In most choristoderes, longitudinal rows of palatal teeth are present on the pterygoid, palatine and vomer, as well as a row on the pterygoid flange. In some neochoristoderes the palatal tooth rows are modified into tooth batteries on raised platforms. The morphology of the palatal teeth is identical to that of the marginal teeth of non neochoristoderes, and the replacement of palatal teeth is nearly identical to the replacement of marginal teeth.

An exceptionally preserved specimen of Monjurosuchus preserves pleated skin, which indicates that in life it was probably thin and soft. The preserved scales are small and overlapping, and are smaller on the ventral underside of the body than the dorsal surface. A double row of larger ovoid scales runs along the dorsum (upper midline) of the body. The fossil also preserves webbed feet. Hyphalosaurus was covered in scales of varying shape, depending on their position on the body, with at least one and possibly multiple rows of large ovoid scales running down sides of the trunk and tail. The feet display evidence of webbing, and the tail probably had additional tissue at the top and bottom, allowing it to be used as a fin to propel Hyphalosaurus through the water. Skin impressions of Champsosaurus have also been reported, they consist of small (0.6 0.1 mm) pustulate and rhomboid scales, with the largest scales being located on the lateral sides of the body, decreasing in size dorsally, no osteoderms were present. The Menat specimen of Lazarussuchus preserves some remnants of soft tissue, but no scales, which shows that the hindfoot (pes) was not webbed, and a dark stained region with a crenellated edge is present above the caudal vertebrae of the tail, suggestive of a crest similar to those found in some living reptiles, like the tuatara, lizards and crocodiles. Cteniogenys and Lazarussuchus have been suggested to have fed on invertebrates. However, this proposal has been criticised by other authors, who suggest it is more likely that they represent late stage embryos. A specimen of Hyphalosaurus has been found with small rib bones in its abdominal cavity, suggesting that it took vertebrate prey at least on occasion. but another specimen shows that Hyphalosaurus baitaigouensis also possessed soft shelled eggs, similar to those of lepidosaurs. A possible explanation for this is that Hyphalosaurus was ovoviviparous, with the thin shelled eggs hatching immediately after they were laid, presumably on land, though it has also been suggested that the species employed both viviparous and oviparous reproductive modes. A skeleton of Philydrosaurus has been found with associated post hatchling stage juveniles, suggesting that they engaged in post hatching parental care. Tracks attributed to neochoristoderans dubbed Champsosaurichnus parfeti have also been reported from the Late Cretaceous Laramie Formation of the United States, though only two prints are present and it is not possible to distinguish between a manus (forefoot) or pes (hindfoot).

Historically, the internal phylogenetics of Choristodera were unclear, with the neochoristoderes being recovered as a well supported clade, but the relationships of the "non neochoristoderes" being poorly resolved. The long necked "non neochoristoderes" Shokawa and Hyphalosaurus have often been recovered as a clade, dubbed the Hyphalosauridae by Gao and Fox in 2005. The finding of more complete material of the previously fragmentary Khurendukhosaurus shows that it also has a long neck, and it has also been recovered as part of the clade. Phylogeny from the analysis of Dong and colleagues (2020): After initially being placed in Rhynchocephalia, Cope later suggested a placement in Lacertilla due to the shape of the cervical vertebrae. Louis Dollo in 1891 returned Choristodera to Rhynchocephalia, but in 1893 suggested a close relationship with Pareiasaurus. Alfred Romer in publications in 1956 and 1968 placed Choristodera within the paraphyletic or polyphyletic grouping of "Eosuchia", describing them, as “an offshoot of the basic eosuchian stock”, a classification which was widely accepted. However, the use of computer based cladistics in the 1980s demonstrated the non monophyly of "Eosuchia", making the classification of choristoderes again uncertain. Subsequent studies either suggested placement as archosauromorphs, lepidosauromorphs or members of Diapsida incertae sedis. In a 2016 analysis of neodiapsid relationships by Martín Ezcurra they were recovered as members of the advanced neodiapsid group Sauria, in a polytomy with Lepidosauromorpha and Archosauromorpha, with being the earliest diverging members of either group also being plausible. Pachystropheus from the Late Triassic (Rhaetian) of Britain has been suggested to be a choristodere, but cannot be referred in confidence to the group as it lacks cranial material, on which most diagnostic characters of Choristodera are based. the Bathonian Itat Formation of western Siberia, as well as possibly the Bathonian aged Anoual Formation in Morocco, North Africa. Choristoderes underwent a major evolutionary radiation in Asia during the Early Cretaceous, which represents the high point of choristoderan diversity, including the first records of the gharial like Neochoristodera, which appear to have evolved in the regional absence of aquatic neosuchian crocodyliformes. They appear to be absent from the well sampled European localities of the Berriasian aged Purbeck Group, Great Britain and the Barremian aged La Huérguina Formation, Spain, In the latter half of the Late Cretaceous (Campanian Maastrichtian), the neochoristodere Champsosaurus is found in Utah, Wyoming, Montana, North Dakota, Alberta and Saskatchewan, which were along the western coast of the Western Interior Seaway on the island of Laramidia. and from the Navesink Formation of New Jersey from the latest Cretaceous (Maastrichtian), which formed the separate island of Appalachia. Vertebrae from the Cenomanian of Germany and the Campanian aged Grünbach Formation of Austria indicate the presence of choristoderes in Europe during this time period. The only record of choristoderes from Asia in the Late Cretaceous is a single vertebra from the Turonian of Japan. Fragmentary remains found in the Campanian aged Oldman and Dinosaur Park formations in Alberta, Canada, also possibly suggest the presence of small bodied "non neochoristoderes" in North America during the Late Cretaceous. Champsosaurus survived the K Pg extinction, and together with fellow neochoristodere Simoedosaurus are present in Europe, Asia and North America during the Paleocene, however they became extinct during the early Eocene. Their extinction coincides with major faunal turnover associated with elevated temperatures. The European endemic Lazarussuchus is the last known choristodere, surviving the extinction of neochoristoderes at the beginning of the Eocene, with the youngest known remains being those of L. dvoraki from the Early Miocene of the Czech Republic and as well as possible indeterminate remains of Lazarussuchus reported from the late Miocene (~11.6 million years ago) of southern Germany.