Neochoristodera

Long-jawed choristoderes form a monophyletic group. They are generally divided into two main families: Champsosauridae, which includes Champsosaurus, and Simoedosauridae which includes Simoedosaurus, Liaoxisaurus, Ikechosaurus and Tchoiria. Various taxa of uncertain affinities within this group are known, including a specimen from the Cedar Mountain Formation and from the Kuwajima Formation.[1]

Skeleton of Tchoiria

Neochoristoderes first appear in the Early Cretaceous of Asia, where they co-exist with other choristodere groups like monjurosuchids and hyphalosaurids. Here, a regional absence of aquatic crocodilians, possibly due to colder temperatures, seems to have opened the ecological niche for these choristoderes to occupy a similar ecological niche.[2][3]

Other than a possible specimen from the Cedar Mountain Formation, a large gap occurs between these Early Cretaceous faunas and the Late Cretaceous ones. There are no fossils of neochoristoderes in the Asian Late Cretaceous, but the subsequent distribution of Simoedosaurus in the Paleocene and Eocene implies that there were Asian taxa around this time, seeing as Simoedosauridae is predominantly Asian and Simoedosaurus only propagated widely after the KT event.[4]

Fossils of Simoedosaurus

Choristoderes reappear in the fossil record in the Campanian of North America and Europe (and possibly Timor[5]) under the genus Champsosaurus. This genus survives the KT event unscathed and diversifies in the aftermath, being soon after joined by Simoedosaurus. Both taxa remain at high latitudes in North America and Europe until the Eocene, when they mysteriously disappear.[6]

Competition from crocodilians has been at times implicated in the extinction of neochoristoderes. There appears to be a niche partitioning between neochoristoderes and long-snouted crocodilians such as gharials, which are absent from freshwater sites in Laurasia until well after neochoristoderes disappear, but competition between both groups, if it even existed, is currently unaccounted for. Ultimately, both Champsosaurus and Simoedosaurus co-existed with a variety of broad-snouted species like alligatorids and Borealosuchus.[7][8]

Lateral and dorsal views of a Champsosaurus skeletal mount

Neochoristoderes are thought to be fully aquatic. They possess laterally flattened, muscular tails and paddle-like limbs, their torsos are dorsoventrally flattened with short but massive ribs and their gastralia are heavily ossified, facilitating sinking. While they have their nostrils at the end of the snout as in crocodiles, they are oriented towards the tip instead of dorsally; their eye sockets are similarly forward-facing, implying that these animals did not surface often and probably simply rose the snout in a snorkel-like fashion. As in most choristoderes the skin lacks scutes, instead having small, non-overlapping scales.[9] In Champsosaurus adult females are thought to have particularly ossified limbs and pelvises, implying that they could crawl unto land, while adult males and juveniles could not.[10] As most choristoderes are thought to have been ovoviviparous or viviparous, it is likely that at least some neochoristoderes were too.[11]

Champsosaurus skull

Most neochoristoderes have exceptionally large temporal fenestrae, suggesting powerful bite forces; Champsosaurus is estimated to have a bite force around 1194 to 1910 N, as opposed to the modern gharial's 310-497 N.[12] In spite of this, most are thought to have had a diet similar to that of modern gharials due to the long and slender jaws, though Simoedosaurus has a more robust and shorter snout and could have fed on larger prey.[13]

Compared with other choristoderes, which have rather simple teeth, 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. As with most choristoderes, neochoristoderes have palatal teeth, indicating food manipulation in the mouth.[14]

Other than the possible Timor fossils, nearly all neochoristodere remains occur at high latitudes, suggesting a preference for colder climates.[15] Champsosaurus fossils are known in the Canadian Arctic and Greenland.[16]

Endocast studies on Champsosaurus reveal that the ears were internal and located near the base of the skull.[17] This same study also finds nasal conchae in neochoristoderes aside from Ikechosaurus, suggesting that these animals were capable of regulating their body temperature to some extent.[18]