Choristodera

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. 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):