Neochoristodera

Neochoristodera is a lineage of specialised crocodile-like fully aquatic choristodere reptiles. Noted for their long jaws and large size, these animals were predominant across the northern hemisphere (and possibly Australasia), occurring in freshwater and coastal environments across the Cretaceous and early Cenozoic.

Neochoristodera Temporal range: Early Cretaceous - Eocene, 130–40 Ma PreꞒ Ꞓ O S D C P T J K Pg N
Skeletal mount of Champsosaurus
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Reptilia
Order:	†Choristodera
Suborder:	†Neochoristodera Cope, 1884
Families
†Champsosauridae Simoedosauridae †Ikechosaurus †Liaoxisaurus †Simoedosaurus †Tchoiria

Systematics

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]

Evolution

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]

Biology

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]

References

1. The first record of a long-snouted choristodere (Reptilia, Diapsida) from the Early Cretaceous of Ishikawa Prefecture, Japan. Historical Biology 27(5):1-12. December 2014. doi:10.1080/08912963.2014.898296
2. The first record of a long-snouted choristodere (Reptilia, Diapsida) from the Early Cretaceous of Ishikawa Prefecture, Japan Article in Historical Biology 27(5):1-12 · December 2014 doi:10.1080/08912963.2014.898296
3. R. Matsumoto and S. E. Evans. 2010. Choristoderes and the freshwater assemblages of Laurasia. Journal of Iberian Geology 36(2):253-274
4. R. Matsumoto and S. E. Evans. 2010. Choristoderes and the freshwater assemblages of Laurasia. Journal of Iberian Geology 36(2):253-274
5. . H. F. Umbgrove, Structural History Of The East Indies
6. R. Matsumoto and S. E. Evans. 2010. Choristoderes and the freshwater assemblages of Laurasia. Journal of Iberian Geology 36(2):253-274
7. R. Matsumoto and S. E. Evans. 2010. Choristoderes and the freshwater assemblages of Laurasia. Journal of Iberian Geology 36(2):253-274
8. The first record of a long-snouted choristodere (Reptilia, Diapsida) from the Early Cretaceous of Ishikawa Prefecture, Japan Article in Historical Biology 27(5):1-12 · December 2014 doi:10.1080/08912963.2014.898296
9. R. Matsumoto and S. E. Evans. 2010. Choristoderes and the freshwater assemblages of Laurasia. Journal of Iberian Geology 36(2):253-274
10. Yoshihiro Katsura, Fusion of sacrals and anatomy in Champsosaurus (Diapsida, Choristodera), doi:10.1080/08912960701374659
11. R. Matsumoto and S. E. Evans. 2010. Choristoderes and the freshwater assemblages of Laurasia. Journal of Iberian Geology 36(2):253-274
12. omateus.googlepages.com/Jacobsetal2009CretaceousSkeletoncoas.pdf
13. R. Matsumoto and S. E. Evans. 2010. Choristoderes and the freshwater assemblages of Laurasia. Journal of Iberian Geology 36(2):253-274
14. Morphology and function of the palatal dentition in Choristodera Article in Journal of Anatomy 228(3):n/a-n/a · November 2015 doi:10.1111/joa.12414
15. The first record of a long-snouted choristodere (Reptilia, Diapsida) from the Early Cretaceous of Ishikawa Prefecture, Japan Article in Historical Biology 27(5):1-12 · December 2014 doi:10.1080/08912963.2014.898296
16. R. Matsumoto and S. E. Evans. 2010. Choristoderes and the freshwater assemblages of Laurasia. Journal of Iberian Geology 36(2):253-274
17. https://curve.carleton.ca/system/files/etd/6208eb7f-08f0-42a9-9a09-6d96779836fb/etd_pdf/bf6cdd9410c74730ae7c6029e7acbc5d/dudgeon-theinternalcranialanatomyofchampsosauruslindoei.pdf
18. https://curve.carleton.ca/system/files/etd/6208eb7f-08f0-42a9-9a09-6d96779836fb/etd_pdf/bf6cdd9410c74730ae7c6029e7acbc5d/dudgeon-theinternalcranialanatomyofchampsosauruslindoei.pdf