Choristodera

Skeleton of Champsosaurus

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.[2][3] A year later, Simoedosaurus was described by Paul Gervais from Upper Paleocene deposits at Cernay, near Rheims, France. These remained the only described choristoderes for over a century, until new taxa were described in the late 20th century.[4] Beginning in the late 1970s, additional taxa were described by Soviet-Mongolian teams from Lower Cretaceous sediments of Mongolia. In studies from 1989 to 1991, Susan E. Evans described new material of Cteniogenys from the Middle Jurassic of Britain. The genus had been 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 bodied choristodere, different from the crocodile-like forms previously known.[5]

Skeletons of Hyphalosaurus

Neochoristoderes such as Champsosaurus are the best-known group of the Choristodera. They resembled modern gharials (gavials) or false gharials. The skull of these animals have a long, thin snout filled with small, sharp conical teeth. Other choristoderes belong to the paraphyletic assemblage of "non-neochoristoderes", which are mostly small lizard like forms, though Shokawa, Khurendukhosaurus and Hyphalosaurus possess long plesiosaur like necks.[4]

Choristoderes are exclusively found in freshwater deposits, often associated with turtles, fish, frogs, salamanders and crocodyliformes. They appear to have been almost exclusively found in warm temperate climates, with the range of neochoristoderes extending to the high Canadian Arctic during Coniacian-Santonian stages of the Late Cretaceous (~89-83 Million years ago), a time of extreme warmth. Due to the morphological simililarities between choristoderes and crocodyliformes, it has often been assumed that they existed in competition. However "non-neochoristoderes" are morphologically dissimilar to aquatic crocodyliformes and were more likely in competition with other taxa. For the more crocodile-like neochoristoderes, there appears to have been niche differentiation, with gharial-like neochoristoderans occurring in association with blunt snouted crocodyliformes, but not in association with long snouted forms.[4]

Choristoderes are universally agreed to be members of Neodiapsida, but their exact placement in the clade is uncertain, due to their mix of primitive and derived features, and a long ghost lineage (absence of a fossil record) after their split from other reptiles. In a 2016 analysis of neodiapsid relationships they were recovered as members of Sauria, in a polytomy with Lepidosauromorpha and Archosauromorpha with being the earliest diverging members of either group also being plausible.[21] Historically, the 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.[6] However, during the 2010's, the "non-neochoristoderes" from the Early Cretaceous of Asia (with the exception of Heishanosaurus) alongside Lazarussuchus from the Cenozoic of Europe were recovered (with weak support) as belonging to a monophyletic clade, which were informally named the "Allochoristoderes" by Dong and colleagues in 2020, characterised by the shared trait of completely closed lower temporal fenestrae, with Cteniogenys from the Middle-Late Jurassic of Europe and North America being consistently recovered as the basalmost choristodere.[22] The long necked "non-neochoristoderes" Shokawa and Hyphalosaurus have often been recovered as a clade, dubbed the Hyphalosauridae by Gao and Fox in 2005.[23] 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.[24]

Phylogeny from the analysis of Dong and colleagues (2020):[22]

Skeleton of Monjurosuchus splendens on display at the Beijing Museum of Natural History

Choristoderes must have diverged from all other known reptile groups prior to the end of the Permian period, over 250 million years ago, based on their primitive phylogenetic position. In 2015, Rainer R. Schoch reported a new small (~ 20 cm long) diapsid from the Middle Triassic (Ladinian) Lower Keuper of Southern Germany, known from both cranial and postcranial material, which he claimed represented the oldest known choristodere.[25] 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 oldest unequivocal choristoderan is the small lizard-like Cteniogenys, the oldest known remains of which are known from the late Middle Jurassic (Bathonian ~168-166 million years ago) of Britain, with remains also known from the Upper Jurassic of Portugal and the United States, with broadly similar remains also known from the late Middle Jurassic (Callovian) of Kyrgyzstan[4] and the Bathonian of western Siberia,[26] European Russia,[27] as well as possibly the Bathonian of North Africa.[1]

Choristoderes underwent a major evolutionary radiation in the Lower Cretaceous of Asia, 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 neosuchians. The only remains outside of Asia known from this time period is a partial femur from the Yellow Cat Member of the Cedar Mountain Formation in North America, they appear to be absent from the well sampled European localities of the Berriasian aged Purbeck Group, England and the Barremian aged La Huérguina Formation, Spain. In the Late Cretaceous, the neochoristodere Champsosaurus is abundant across North America.[4] Vertebrae from the Cenomanian of Germany[28] and the Campanian aged Grünbach Formation of Austria indicate the presence of choristoderes in Europe during this time period, however, there are no records from Asia. Fragmentary remains found in the Campanian aged Oldman and Dinosaur Park formations in Alberta, Canada, also possibly suggest the presence of small bodied "non-choristoderes" in North America the Late Cretaceous. Champsosaurus survived the K-Pg extinction, alongside the addition of fellow neochoristodere Simoedosaurus were found in Europe, Asia and North America during the Paleocene, however they became extinct during the early Eocene.[4] Small bodied "non-choristoderes", which are absent from the fossil record after the Early Cretaceous (with the exception of the possible North American remains), unexpectedly reappear in the form of the lizard-like Lazarussuchus from the late Paleocene of France.[12] The European endemic Lazarussuchus is the last known choristodere, survivng the extinction of neochoristoderes during the Eocene, with the last known species being L. dvoraki from the Early Miocene of the Czech Republic.[4][29] Remains of Lazarussuchus have also been reported the late Miocene (~11.6 million years ago) of southern Germany.[30]

• "Choristodera page". Palæos. Archived from the original on 16 April 2007.
• "Choristodera-champsosaurs". Mikko's Phylogeny Archive.
• "Archosauria: Systematics". U.C. Berkeley.
• "Two-Headed Reptile Fossil Found". BBC News.
• "Ghost lineages". Archived from the original on 18 April 2008. — brief article on the fossil record of choristoderes