Choristodera is an extinct order of semiaquatic diapsid reptiles that ranged from the Middle Jurassic, or possibly Late Triassic, to at least the early Miocene. It was named by Edward Drinker Cope in 1884. Choristoderes have been found in North America, Asia, and Europe, and possibly also North Africa and East Timor. The most common fossils are typically found from the Late Cretaceous to the lower Eocene. Cladists have placed them between basal diapsids and basal archosauromorphs, but the phylogenetic position of Choristodera is still uncertain. It has also been proposed that they represent basal lepidosauromorphs. Most recently, workers have placed Choristodera within Archosauromorpha.
|Life restoration of a Hyphalosaurus|
Champsosauridae is the best-known family of the Choristodera and typifies the group. Champsosaurus was first described from Late Cretaceous strata of Montana by Cope in 1876. Champsosaurs resembled modern gharials (gavials) or false gharials. The skull of these animals have a long, thin snout filled with small, sharp conical teeth. This is due to champsosaurs and gharials occupying similar Ecological niches: hunting small aquatic prey in rivers and swamps. This is a classic example of convergent evolution. More primitive choristoderes have shorter, broader snouts.
There are major differences seen between choristoderes and gharials and other crocodilians, however, particularly in the skull. The orbits are found well forward on the skull, and the rear of the skull is bulbous, hugely expanded and consists of complex bony arches surrounding empty space. These spaces probably contained massive jaw muscles. Other hypotheses for the spaces, such as an otic sensory organ housing, have been tossed around with little support. The external nares are found on the tip of the rostrum. This indicates that champsosaurs breathed while submerged by extending their rostrum through the water surface while their body rested on the bed of the lake or stream. Crocodylians and phytosaurs have their nares located dorsally on their rostrum or skull respectively. This position allows them to rest submerged just below the surface.
Champsosaur skulls are actually very similar to lizard skulls, though heavily modified. This has led some researchers to consider champsosaurids as lepidosauromorphs. However, champsosaurs lack the complex quadrate of lepidosaurians. With features of both diapsid groups, the phylogenetic position of Choristodera is highly confused.
Other features found in choristoderes include heavily ossified gastralia and modified distal limbs, not just the manus and pes, used as paddles. In addition, champsosaur ribs are short and massive, as in other aquatic reptiles. The thorax is dorso-ventrally flattened, and the tail is laterally compressed to aid in swimming. Skin impressions found with champsosaur fossils show non-overlapping scales of very small size and the skin containing no scutes like those found in crocodilians (see crocodile exoskeleton).
Most choristoderes have rather simple teeth, but 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. There is some tooth differentiation, with the anterior teeth being larger than the posterior ones. Choristoderes have retained palatal teeth, indicating food manipulation in the mouth.
As a whole, choristoderes were exceptionally well adapted to life in the water. In forms like Champsosaurus, only adult females could crawl ashore to lay eggs on land, with males and juveniles being incapable of doing so, while Hyphalosaurus and Monjurosuchus are known to have been ovoviviparous. Champsosaurid remains are known from as far as East Timor.
Strict consensus of 10 most parsimonious trees from the analysis of Matsumoto et al. (2019):
Long considered to be a morphologically conservative group, recent phylogenetic analyses and descriptions of new taxa have revolutionized understanding of this taxon.
The order Choristodera comprises two monophyletic groups and three basal taxa. Primitive choristoderes are characterized by small body size, a large, dorsally directed orbit and closed lower temporal fenestrae.
Cteniogenys is the most basal choristodere. Like Monjurosuchus, it is a small-bodied, lizard-like animal. The webbed feet of Monjurosuchus reflect its aquatic lifestyle. Cteniogenys is known from North America and Europe, while Monjurosuchus is known from China.
Lazarussuchus was considered the most basal choristoderan when first described, and hence a late-surviving basal choristodere. However, Matsumoto et al. (2013) recovered Lazarussuchus in a clade along with hyphalosaurids and monjurosuchids, as did the cladistic analysis of Coeruleodraco by Matsumoto et al. (2019). This taxon is known from Cenozoic deposits in France and the Czech Republic. One species, L. dvoraki, persisted into the early Miocene in the Czech Republic.
The two small-bodied choristoderes, Shokawa and Hyphalosaurus form the clade Hyphalosauridae within Choristodera. Their elongate necks and tails represent a unique shared derived condition. They resemble small plesiosaurs or nothosaurs. Shokawa is known from Japan, and Hyphalosaurus is known from China.
The named clade Neochoristodera includes Champsosaurus, Tchoiria, Simoedosaurus, and Ikechosaurus. This group of large-bodied reptiles is characterized by elongate snouts and relatively small orbits. Although it reflects our early understanding of the Choristodera as a whole, it in fact represents a highly derived condition.
The neochoristoderan taxa Champsosauridae and Simoedosauridae are found from the Late Cretaceous to the middle Eocene, indicating that the group survived the Cretaceous–Paleogene extinction event. Champsosaurus teeth, vertebrae, and other bones are common fossils in Cretaceous deposits such as the Dinosaur Park Formation of Alberta and the Lance Formation of Wyoming. The presence of champsosaurs as far north as the North Slope of Alaska implies warmer temperatures during the Cretaceous. The Paleocene species, Champsosaurus gigas, is the largest known champsosaur. The most complete skeleton was found at the Wannagan Creek site in North Dakota, but is found throughout North American Paleocene strata in the Dakotas, Wyoming, and Montana. C. gigas is unusual among Paleocene reptiles in that it is far larger than known Mesozoic ancestors, at about 3 meters in length (10 ft) versus 1.5 meters (5 ft) for the largest Cretaceous champsosaurs.
Simoedosaurus is known from Europe, western Asia and North America, Tchoria is known from Mongolia and Ikechosaurus is known from China. Champsosaurus is known from North America, where its distinctive, hourglass-shaped vertebral centra are important biostratigraphic indicators.
Pachystropheus rhaeticus may or may not be a choristodere. If a member of Choristodera, it extends the fossil record of the choristoderes from the Middle Jurassic back 45 mya and implies a significant ghost lineage. However, Pachystropheus lacks any cranial material, and all of the postcranial adaptations that link it to choristoderes may merely reflect convergent adaptations to an aquatic lifestyle. All unambiguous choristoderes were freshwater animals, while Pachystropheus was recovered from European marine transgressive sequences. Pachystropheus may be a choristodere, or it may be a type of thalattosaur, a large bodied, long necked group of marine reptiles. More complete fossil material is needed in order to resolve the placement of Pachystropheus.
The ultimate extinction of choristoderes may have been due to a number of possible causes, such as falling temperatures and habitat loss. Competition with crocodilians has occasionally been cited as a possible source of decline, but the co-existence of large choristoderes with aquatic crocodilians, as well as a fairly broad range of morphology in both clades, render this unresolved, if not unlikely.
Actiosaurus (meaning "coast lizard") is an extinct genus of reptile first described by Henri Sauvage in 1883 from Antully bonebed, Autun (Triassic of France). The type species is A. gaudryi (commonly misspelled A. gaudrii after Boulenger). Little is known of it, and it is considered a nomen dubium. Actiosaurus was originally described as a dinosaur in 1883 and was reinterpreted as an ichthyosaur in 1908. Actiosaurus may instead represent the remains of a choristodere. Fischer et al. (2014) considered A. gaudryi to be a species inquirenda, and noted the similarity of its bones to the limb bones of choristoderes.Champsosaurus
Champsosaurus is an extinct genus of diapsid reptiles belonging to the order Choristodera, that existed in the Late Cretaceous and Paleogene periods. It consists of seven species: C. albertensis, C. ambulator, C. gigas, C. laramiensis, C. lindoei, C. natator, and C. tenuis. The name Champsosaurus is thought to come from χαμψαι [champsai] (said in an Ancient Greek source to be an Egyptian word for "crocodiles"), and σαύρος [sauros] (Greek for "lizard"). The largest species, C. gigas, grew to 3-3.5 m (10–12 ft) in length.Coeruleodraco
Coeruleodraco is an extinct genus of choristoderan that lived in China during the Late Jurassic (Oxfordian).Coeruleodraco is significant as the most complete Jurassic choristodere taxon ever found worldwide, considering that Cteniogenys is based on fragmentary remains. Although similar to Philydrosaurus in its proportions and postcranial characters, it is distinct in retaining several apparently plesiomorphic characters, including a short snout, paired external nares and an open lower temporal fenestra.Cteniogenys
Cteniogenys is a genus of choristodere, a morphologically diverse group of aquatic reptiles.Czatkowiella
Czatkowiella is an extinct genus of long-necked archosauromorph known from Early Triassic (Olenekian age) rocks of Czatkowice 1, Poland. It was first named by Magdalena Borsuk−Białynicka and Susan E. Evans in 2009 and the type species is Czatkowiella harae.Hyphalosaurus
Hyphalosaurus (meaning "submerged lizard") is a genus of freshwater aquatic reptiles which represent a major part of the Jehol Biota. They lived during the early Cretaceous period (Aptian age), about 122 million years ago. The genus contains two species, H. lingyuanensis and H. baitaigouensis, both from the Yixian Formation of Liaoning Province, China. They are among the best-known animals from the Jehol Biota, with thousands of fossil specimens representing all growth stages in scientific and private collections.Ikechosaurus
Ikechosaurus is an extinct genus of choristoderan which existed in China and Mongolia during the Cretaceous period. It contains the species Ikechosaurus sunailinae and Ikechosaurus gaoi.
Compared to other neochoristoderes, Ikechosaurus has a rather simple dentition, lacking the speciations seen in latter species. It also has parasphenoid palatal teeth, a feature not seen in any other choristodere.Irenosaurus
Irenosaurus is a genus of choristodere, a type of amphibious reptile. It is known from a single fragmentary postcranial skeleton (PIN 3386/2), discovered in the Aptian-age Lower Cretaceous Hühteeg Formation at Hüren Dukh, central Mongolia. The type species is I. egloni, which was originally described as a new species of the related choristodere Tchoiria in 1983 by Efimov. Efimov transferred the species to the new genus Irenosaurus in 1988. Evans and Hecht (1993) questioned the separation of the taxon from Tchoiria namsarai from the same locality on the grounds that the differences between the two may not have been greater than those of various species of the choristodere Champsosaurus. A later review by Efimov and Storrs (2000) retained the two as separate, noting that some characteristics of Irenosaurus are more like Khurendukhosaurus, also known from the same site, at the same time recognizing the difficulties of distinguishing three genera from the same locality. Irenosaurus was a small choristodere, approximately 1 to 1.5 metres (3.3 to 4.9 ft) long. It is known from what are interpreted as lake deposits.Khurendukhosaurus
Khurendukhosaurus is a genus of choristodere, a type of amphibious reptile. It is known from Lower Cretaceous rocks of Mongolia and Russia. Two species have been named. The type species, K. orlovi, was named in 1984 by Sigogneau–Russell and Efimov for the fragmentary postcranial skeleton PIN 3386/3. This specimen was discovered in the Aptian-age Lower Cretaceous Hühteeg Svita Formation at Hüren Dukh, central Mongolia. The lake deposits at this site also contain fossils of the choristoderes Irenosaurus and Tchoiria. Other postcranial bones of K. orlovi have been found at this site as well.Second species K. bajkalensis was named by Efimov in 1996 for PIN 2234/201, consisting of a scapulocoracoid and a rib. These bones were found in the Lower Cretaceous Murtoi Formation at Lake Gusinoye, Buryatia, Russia. The first Russian choristodere, Efimov and Storrs (2000) found it difficult to distinguish from K. orlovi based on the small amount of material. Skutschas (2008) reported on additional material which supported the placement of the Russian taxon within Khurendukhosaurus, but found the species K. bajkalensis to be dubious within the genus.Khurendukhosaurus was a small choristodere, approximately 1 metre (3.3 ft) long at most. Efimov and Storrs regarded it as a basal member of Choristodera, but Skutschas was unable to confirm this in a phylogenetic analysis. It may have been related to the hyphalosaurids, a group of long–necked choristoderes. The neural spines of the tail are elongate, suggesting that it swam by using a tall tail.Lazarussuchus
Lazarussuchus (meaning "Lazarus's crocodile") is a genus of basal choristodere, a type of amphibious reptile, known from France, Germany, and the Czech Republic. Fossils have been found in Late Paleocene, Late Oligocene, and Early Miocene deposits, meaning that the genus lasted about 40 million years. Two species have been named: the type species L. inexpectatus ("unexpected") (Hecht, 1992) from the late Oligocene of France and Germany and L. dvoraki (for Zdeněk Dvořák) from the early Miocene of the Czech Republic. It was not a large animal; the skull of L. inexpectatus was only about 4.53 centimeters long (1.78 in). A complete specimen of Lazarussuchus with preserved soft tissue was found from the Late Paleocene of France, but has not been assigned to a species.Monjurosuchidae
Monjurosuchidae is a family of prehistoric reptiles belonging to the order Choristodera. Until the description of Philydrosaurus in 2005, the only genus in the family was Monjurosuchus. Both lived in what is now China during the Early Cretaceous.Monjurosuchus
Monjurosuchus is a genus of choristoderan reptile that lived in what is now China and Japan during the Early Cretaceous. It has large eyes, a rounded skull, robust legs with short claws, and a long, thin tail. Fossils have been found that preserve soft tissue, showing that it had soft skin and webbed feet. Monjurosuchus has been placed in the family Monjurosuchidae along with Philydrosaurus, but the relationships of early choristoderes remains unclear.Neochoristodera
Neochoristodera is a lineage of specialised crocodile-like fully aquatic choristodere reptiles. Noted for their long jaws and large size, these animals were predominant across the northern hemisphere (and possibly Australasia), occurring in freshwater and coastal environments across the Cretaceous and early Cenozoic.Pachystropheus
Pachystropheus (After Greek Pachys, "Thick" and Strophaios, Vertebrae) is a genus of prehistoric reptile, possibly a choristodere (champsosaur), from the Rhaetian (Late Triassic) of southwestern England. It was named by Erika von Huene in 1935; Huene described Pachystropheus as a champsosaur, but this was overlooked for decades until its redescription by Storrs and Gower in 1993. This reevaluation would extend the fossil record of champsosaurs back 45 million years. However, other authors consider attribution of Pachystropheus to Choristodera problematic, stating that it depends on vertebral and girdle characters that are also found in the skeletons of aquatic reptiles other than choristoderes; most of the diagnostic features of choristoderes are skull features, but the presence of these cannot be confirmed in Pachystropheus, as there is no confirmed skull material for this taxon. Silvio Renesto (2005) found similarities in the postcranial skeleton of Pachystropheus and the thalattosaur genus Endennasaurus; according to Renesto, these similarities may indicate that Pachystropheus and Endennasaurus are close relatives, but they might as well simply be a case of a convergent evolution triggered by the aquatic lifestyle of both taxa.Philydrosaurus
Philydrosaurus is an extinct genus of choristoderan which existed in China during the Early Cretaceous. The type species P. proseilus was named in 2005. Philydrosaurus is closely related to the choristodere Monjurosuchus, and both have been placed in the same family Monjurosuchidae. Philydrosaurus was found from the Jiufotang Formation and is slightly younger than Monjurosuchus, which was found from the Yixian Formation.Sauria
The clade Sauria was traditionally a suborder for lizards which, before 1800, were crocodilians. It has been redefined as the group containing the most recent common ancestor of archosaurs and lepidosaurs and all its descendants; as such it was commonly thought that Sauria is a crowned-base grouping of diapsids. However, recent genomic studies and comprehensive studies in the fossil record suggest that turtles are closely related to archosaurs, not to parareptiles as previously thought. Sauria can be seen as a crowned-group of all modern reptiles (including birds) within the larger total group Sauropsida, which also contains various stem-reptile groups.Shokawa
Shokawa ikoi was a 1.8 m long choristoderan diapsid reptile closely resembling and closely related to the smaller choristoderan, Hyphalosaurus. It lived during the Lower Cretaceous in what is now Japan.
As with Hyphalosaurus, S. ikoi had an extremely long, thin neck with a small head. The mouth was filled with long teeth used to snare small fish.
The generic name refers to the village of Shokawa, located in Gifu Prefecture, Japan, while the specific name honors the collector of the first specimen, one Mr. Ikoi Shibata.Simoedosaurus
Simoedosaurus is an extinct reptile known from the Paleocene of North America, Europe and western Asia, and a member of the Choristodera, a group of aquatic reptiles that lived in the Northern Hemisphere from the Jurassic to the early Cenozoic.Tchoiria
Tchoiria is a genus of simoedosaurid choristodere, a type of crocodile-like aquatic reptile. Fossils of this genus have been found in Early Cretaceous-age rocks in Mongolia. Four species have been named, but two have been given their own genera. The type species is T. namsari, based on PIN 3386/1, a partial skull and skeleton discovered in the Aptian-age Lower Cretaceous Hühteeg Formation at Hüren Dukh, central Mongolia. Two other species were named from this locality, with both later being reassigned: T. egloni, now Irenosaurus egloni; and T. magnus, now Ikechosaurus magnus. A fourth species, T. klauseni, was named from a partial skull and skeleton found in rocks of roughly the same age farther west in Mongolia.