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 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). 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.

According to Matsumoto and colleagues (2019), choristoderes are united by the presence of nine synapomorphies (shared traits characteristic of the group), including a median contact of the elongated prefrontal bones of the skull separating the nasal bones from the frontal bones, the dorsal flange of the maxilla is inflected medially (toward the midline of the body), the parietal foramen are absent, the squamosal bones are expanded behind (posterior to) the occipital condyle, the teeth are conical and sub-thecodont (located in shallow sockets), the dentaries are slender with elongated grooves running along the labial (outward facing) surface of the bone, additional sacral vertebrae are present, expanded "spine tables" are present on the vertebrae, and the surfaces of both ends of vertebral centra are flat (amphiplatyan). 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. Choristoderes possessed gastralia (rib-like bones situated in the abdomen) like tuatara and crocodilians.

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.

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 the Coniacian-Santonian stages of the Late Cretaceous (~89-83 Million years ago), a time of extreme warmth. Due to the morphological similarities between choristoderes and crocodyliformes, it has often been assumed that they existed in competition. However "non-neochoristoderes" were smaller than adult 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 narrow snouted forms.