The Choristoderan of the Cedar Mountain Formation

The Cedar Mountain Formation of eastern Utah contains rock layers dated to the early and middle Cretaceous Period, approximately 144-95 million years ago. Most of the fossils found here belong to medium-sized and large-sized dinosaurs, while fossils belonging to smaller animals are much scarcer. This article concerns one of these smaller animals – an as-yet-unidentified species of choristoderan reptile.

The choristoderans (pronounced as Kore-RISS-toe-DEER-rans), also known colloquially as “champsosaurs”, were a peculiar group of prehistoric semi-aquatic freshwater reptiles. Although they have been regularly studied since their fossils were first discovered during the 1800s, paleontologists still aren’t sure how exactly to classify them. What’s known for certain is that they belonged to a group of vertebrates called the “diapsids”, meaning that they had two holes in their skull behind each eye socket – lizards, snakes, tuataras, crocodilians, pterosaurs, dinosaurs, and birds are all classified as diapsids. However, beyond this broad classification, things get trickier. A multitude of analyses have failed to definitively place Choristodera within the reptile tree. Some paleontologists say that choristoderans belong within the diapsid sub-group Archosauromorpha (the group containing the relatives of crocodiles, pterosaurs, dinosaurs, and birds), while others say that they lay somewhere between archosauromorphs and lepidosauromorphs (the group containing the relatives of lizards, snakes, and tuataras), and yet others say that choristoderans exist outside both of those groups, perhaps somewhere at the foundation where both of these branches split. This uncertainty largely has to do with the fact that choristoderans possess a combination of basal and derived features which are seen in both archosauromorphs and lepidosauromorphs, which makes it very difficult to say where these animals belong (1).

The first choristoderans appeared during the middle Jurassic Period about 168 million years ago, and they lasted throughout the remainder of the Mesozoic Era. Their heyday occurred during the early Cretaceous Period from about 144 to 100 MYA, after which they went into a long steady decline. They were fortunate to survive the K-T Extinction, but they were always second fiddle to their crocodile neighbors. Most of the surviving species went extinct about 50 MYA during the lower Eocene, with the remainder just barely hanging on. The last of the choristoderans completely went extinct around 11.6 million years ago (2).

I’ve published another article on this blog concerning choristoderans, and the genus Champsosaurus in particular, which you can read here.

One specimen of a choristoderan was discovered at Dalton Wells, Utah. The rock layer in which it was found belonged to the upper half of the Yellow Cat Member of the Cedar Mountain Formation and is dated to 135-132 million years ago. The only fossil of this animal is the proximal third of a left femur, which is currently housed in the collections of Brigham Young University (collection ID code: BYU 14202) (3).

The order Choristodera is divided into four families: Champsosauridae, Hyphalosauridae, Monjurosuchidae, and Simoedosauridae. The more primitive the species, the more lizard-like it is in form; the more derived, then the more crocodilian it is in appearance. According to Britt et al (2006), the creature that the femur specimen belonged to was a member of the clade Neochoristodera, which is composed of the more advanced members of the choristoderan line. These include the families Champsosauridae and Simoedosauridae. However, while the fossil fragment has been identified as belonging to Neochoristodera, the extreme scarcity of finds makes a more precise classification difficult. Therefore, we don’t yet know if this creature was a champsosaurid or a simoedosaurid, and we certainly cannot ascribe a new genus or species name to it until more fossils are found which can give us more knowledge about its anatomy. All we can do for the moment is make an educated guess about its size. Based upon the size of this femur fragment and comparing it with the size of other choristoderan femurs, it is hypothesized that this individual reached approximately 5 to 6.5 feet (1.5 to 2 meters) long (4).

Given its size and the habitat in which it lived, the creature likely occupied the same ecological niche as small crocodilians or the modern-day Asian Water Monitor Lizard (Varanus salvator).

Below is a reconstruction of what this enigmatic choristoderan might have looked like, based upon the appearance of related champsosaurids and simoedosaurids. Since no skull was found, the head was based upon the choristoderans Ikechosaurus and Tchoiria. Ikechosaurus lived in Mongolia and China during the early to middle Cretaceous Period about 140-112 million years ago – the same time as the fossil specimen recovered from Utah. Tchoiria was also found in Mongolia in rocks dated to the middle Cretaceous 120-112 million years ago. It’s possible that the choristoderan which once lived in Utah around 135 MYA looked similar to these genera, but until more specimens are found (preferably ones with skull elements) we cannot be sure.

The illustration was made with No.2 pencil on printer paper. © Jason R. Abdale (November 18, 2022).

Source Citations

  1. Evans, Susan E. (1988). “The early history and relationships of the Diapsida”. In Benton, M. J. (ed.). The Phylogeny and Classification of the Tetrapods, Volume 1: Amphibians, Reptiles, Birds. Oxford: Clarendon Press, 1988. Pages 223, 238; Evans, Susan E.; Hecht, Max K. (1993). “A History of an Extinct Reptilian Clade, the Choristodera: Longevity, Lazarus-Taxa, and the Fossil Record”. In Hecht M, MacIntyre RJ, Clegg MT (eds.). Evolutionary Biology. Boston: Springer US, 1993. Pages 327-329, 331-333; Ezcurra, Martín D. (2016). “The phylogenetic relationships of basal archosauromorphs, with an emphasis on the systematics of proterosuchian archosauriforms”. PeerJ, volume 4: e1778; Matsumoto, Ryoko; Dong, Liping; Wang, Yuan; Evans, Susan E. (2019). “The first record of a nearly complete choristodere (Reptilia: Diapsida) from the Upper Jurassic of Hebei province, People’s Republic of China”. Journal of Systematic Palaeontology, volume 17, issue 12. Pages 1031-1048.
  2. Matsumoto, Ryoko; Evans, Susan E. (2010). “Choristoderes and the freshwater assemblages of Laurasia”. Journal of Iberian Geology, volume 36, issue 2. Page 253-274; Ezcurra, Martín D. (2016). “The phylogenetic relationships of basal archosauromorphs, with an emphasis on the systematics of proterosuchian archosauriforms”. PeerJ, volume 4: e1778.
  3. Britt, Brooks B.; Scheetz, Rodney D.; Brinkman, Donald B.; Eberth David A. (2006). “A Barremian Neochoristodere from the Cedar Mountain Formation, Utah, U.S.A.”. Journal of Vertebrate Paleontology, volume 26, issue 4. Pages 1005-1008.
  4. Matsumoto, Ryoko; Evans, Susan E. (2010). “Choristoderes and the freshwater assemblages of Laurasia”. Journal of Iberian Geology, volume 36, issue 2. Pages 253-274; Matsumoto, Ryoko; Dong, Liping; Wang, Yuan; Evans, Susan E. (2019). “The first record of a nearly complete choristodere (Reptilia: Diapsida) from the Upper Jurassic of Hebei province, People’s Republic of China”. Journal of Systematic Palaeontology, volume 17, issue 12. Pages 1031-1048; Britt, Brooks B.; Scheetz, Rodney D.; Brinkman, Donald B.; Eberth David A. (2006). “A Barremian Neochoristodere from the Cedar Mountain Formation, Utah, U.S.A.”. Journal of Vertebrate Paleontology, volume 26, issue 4. Pages 1005-1008.

Bibliography

Britt, Brooks B.; Scheetz, Rodney D.; Brinkman, Donald B.; Eberth David A. (2006). “A Barremian Neochoristodere from the Cedar Mountain Formation, Utah, U.S.A.”. Journal of Vertebrate Paleontology, volume 26, issue 4. Pages 1005-1008.

Ezcurra, Martín D. (2016). “The phylogenetic relationships of basal archosauromorphs, with an emphasis on the systematics of proterosuchian archosauriforms”. PeerJ, volume 4: e1778.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4860341/.

Evans, Susan E. (1988). “The early history and relationships of the Diapsida”. In Benton, M. J. (ed.). The Phylogeny and Classification of the Tetrapods, Volume 1: Amphibians, Reptiles, Birds. Oxford: Clarendon Press, 1988. Pages 221-260.

Evans, Susan E.; Hecht, Max K. (1993). “A History of an Extinct Reptilian Clade, the Choristodera: Longevity, Lazarus-Taxa, and the Fossil Record”. In Hecht M, MacIntyre RJ, Clegg MT (eds.). Evolutionary Biology. Boston: Springer US, 1993. Pages 323-338.

Matsumoto, Ryoko; Dong, Liping; Wang, Yuan; Evans, Susan E. (2019). “The first record of a nearly complete choristodere (Reptilia: Diapsida) from the Upper Jurassic of Hebei province, People’s Republic of China”. Journal of Systematic Palaeontology, volume 17, issue 12. Pages 1031-1048.
https://discovery.ucl.ac.uk/id/eprint/10067787/1/PDF%20preprint%20of%20Matsumoto%20et%20al.%20Coeruleodraco.pdf.

Matsumoto, Ryoko; Evans, Susan E. (2010). “Choristoderes and the freshwater assemblages of Laurasia”. Journal of Iberian Geology, volume 36, issue 2. Page 253-274.



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