Choristodera (from the Greek χωριστός chōristos + δέρη dérē, 'separated neck'[2]) is an extinct order of semiaquatic diapsid reptiles that ranged from the Middle Jurassic, or possibly Triassic, to the late Miocene (168 to 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.[3][4] 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.[5] 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.[6]
Choristoderes vary substantially in size, the smallest genera like Cteniogenys and Lazarussuchus had a length of only around 30 cm (12 in), and the largest known choristoderan, Kosmodraco dakotensis is estimated to have had a total length of around 5 m (16 ft).[7][1] Neochoristoderes such as Champsosaurus are the best-known group of the Choristodera. They resembled modern crocodilians, especially gharials. The skull of these animals have a long, thin snout filled with small, sharp conical teeth. Other choristoderes are referred to collectively as "non-neochoristoderes", which are mostly small lizard-like forms, though Shokawa, Khurendukhosaurus and Hyphalosaurus possess long plesiosaur like necks. The grouping of "non-neochoristoderes" is paraphyletic (not containing all descendants of a common ancestor), as the lizard-like bodyform represents the ancestral morphology of the group.[5]