What are Choristoderes? (you know…Champsosaurus, Cteniogenys, Doswellia, etc.)

The Choristordera constitute a clade of elongated aquatic to semi-aquatic, lizard-like to croc-like diapsid reptiles. Traditional taxa include: Champsosaurus, Cteniogenys, Lazarrusuchus and Hyphalosaurus. The first two-headed fossil reptile came from this clade.

What Wiki Sez:
Cladists have placed [choristoderes] 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.”

So we have an enigma taxa, an ideal opportunity to use the large study to narrow down choristodere outgroup relations.

Several choristoderes

Figure 1. Several choristoderes (in white), their predecessor and sisters (in yellow).

Choristoderes are Pararchosauriformes
The large study nested choristoderes within the Archosauriformes and within the Pararchosauriform branch between Youngoides (the RC91 specimen) and Proterochampsa.

A section of the large study focusing on choristodere relations.

Figure 2. A section of the large study focusing on choristodere relations.

Doswellia was also a Choristodere
Doswellia (Weems 1980) has been considered an enigma taxon, different enough from all other known taxa to create more questions than answers. Dilkes and Sues (2009) proposed a nesting with Proterochampsa, which is confirmed here.

Parsimonly Rules
Side by side, the resemblance of several choristoderes to Youngina, Doswellia and parasuchians is clear and reasonable. In the present taxon list, there is no more parsimonious nesting to be found. Think of choristoderes as successors to Youngoides (RC91 specimen), a taxon that has never been tested with choristoderes before.

The Dorsal Naris
Most choristoderes have a dorsal naris, similar to Cerritosaurus, parasuchians and Proterochampsa. Champsosaurus has a naris at the tip of it snorkel like snout. This appears to be a reversal because the premaxilla has no ascending process.

Another Appearance of the Antorbital Fenestra
This nesting highlights an important taxonomic fact: the antorbital fenestra appeared in reptiles at least four times. Parasuchians and Cerritosaurus had an antorbital fenestra. Precursors, including choristoderes, did not. This means the antorbital fenestra in parasuchians and their kin developed independently of the antorbital fenestra in Euarchosauriformes, such as Proterosuchus and its successors.

The Longevity and Variety Within the Choristodera
Choristoderes appeared in the Late Triassic, but probably originated in the Late Permian, along with their sister taxa. Some survived into the Early Miocene. Despite the longevity of this clade, relatively few modifications to the basic body plan appeared. Oh, sure, the lateral temporal fenestra disappeared in Doswellia and Lazarussuchus. The rostrum elongated in Champsosaurus. The neck elongated in Hyphalosaurus. The unguals were enlarged in Lazarussuchus, which means it was probably more terrestrial than its aquatic sisters and may have climbed trees. Doswellia was the giant of the clade, reaching 1.6 m in length, or slightly larger than Champsosaurus at 1.5 m. No choristoderes developed an herbivorous diet, a mammal-like dentition, a bipedal stance or wings.

Traditional enigmas, choristoderes were a monophyletic clade that nested between Youngoides and Parasuchia + Proterochampsa, close to the base of the Archosauriformes. Relatively conservative in morphology, choristoderes were a relatively minor presence throughout the Mesozoic and into the Cenozoic.

As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.

Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.

Brown B 1905. The osteology of Champsosaurus Cope. Memoirs of the AMNH 9 (1):1-26. http://digitallibrary.amnh.org/dspace/handle/2246/63
Cope ED 1876. 
On some extinct reptiles and Batrachia from the Judith River and Fox Hills beds of Montana: Proceedings of the Academy of Natural Sciences, Philadelphia. 28, p. 340-359.
Dilkes D and Sues H-D 2009. 
Redescription and phylogenetic relationships of Doswellia kaltenbachi (Diapsida: Archosauriformes) from the Upper Triassic of Virginia. Journal of Vertebrate Paleontology 29(1):58-79
Evans SE and Hecht MK 1993.A history of an extinct reptilian clade, the Choristodera: longevity, Lazarus-Taxa, and the fossil record. Evolutionary Biology 27:323–338.
Foster JR and Trujillo KC 2000.
New occurrences of Cteniogenys (Reptilia, Choristodera) in the Late Jurassic of Wyoming and South Dakota. Brigham Young University Geology Studies 45:11-18.
Gao K-Q, Tang Z-L and Wang X-L 1999
A long-necked reptile from the Upper Jurassic/Lower Cretaceous of Liaoning Province, northeastern China. Vertebrata PalAsiatica 37:1–8.
Gilmore CW 1928. 
Fossil lizards of North America. Memoirs of the National Academy of Sciences 22(3):1-201.
Hecht MK 1992. A new choristodere (Reptilia, Diapsida) from the Oligocene of France: an example of the Lazarus effect. Geobios 25:115–131. doi:10.1016/S0016-6995(09)90041-9.
Matsumoto R and Evans SE 2010. Choristoderes and the freshwater assemblages of Laurasia. Journal of Iberain Geology 36(2):253-274. online pdf
Weems RE 1980. 
An unusual newly discovered archosaur from the Upper Triassic of Virginia, U.S.A. Transactions of the American Philosophical Society, New Series 70(7):1-53



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