''Pachystropheus'' (After Greek ''Pachys'', "Thick" and ''Strophaios'', Vertebrae) is a

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In the hierarchy of biological classification, genus com ...

of prehistoric

reptile Reptiles, as most commonly defined are the animals in the class Reptilia ( ), a paraphyletic grouping comprising all sauropsids except birds. Living reptiles comprise turtles, crocodilians, squamates (lizards and snakes) and rhynchocephalians ( ...

, possibly a

choristodere 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 late Miocene (168 to 1 ...

(champsosaur), from the

Rhaetian The Rhaetian is the latest age of the Triassic Period (in geochronology) or the uppermost stage of the Triassic System (in chronostratigraphy). It was preceded by the Norian and succeeded by the Hettangian (the lowermost stage or earliest age ...

(

Late Triassic The Late Triassic is the third and final epoch (geology), epoch of the Triassic geologic time scale, Period in the geologic time scale, spanning the time between annum, Ma and Ma (million years ago). It is preceded by the Middle Triassic Epoch ...

) of southwestern

England England is a country that is part of the United Kingdom. It shares land borders with Wales to its west and Scotland to its north. The Irish Sea lies northwest and the Celtic Sea to the southwest. It is separated from continental Europe b ...

. 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 ''Endennasaurus'' is an extinct genus of thalattosaurian from the Upper Triassic of Italy. It was found in and named after the Endenna cave, composed of Zorzino Limestone in Lombardia.convergent evolution Convergent evolution is the independent evolution of similar features in species of different periods or epochs in time. Convergent evolution creates analogous structures that have similar form or function but were not present in the last com ...

triggered by the aquatic lifestyle of both taxa.

References

Choristodera Triassic diapsids Late Triassic reptiles of Europe Prehistoric marine reptiles Prehistoric reptile genera {{triassic-reptile-stub