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Calsoyasuchus (meaning "Dr. Calsoyas' crocodile") is a genus of CrocodylomorphaMesoeucrocodylia is the clade that includes all the living crocodilians and their closest extinct relatives. that lived in the Early Jurassic. Its Fossil were found in the Sinemurian-Pliensbachian-age Kayenta Formation on Navajo Nation land in Coconino County, Arizona, United States. Formally described as C. valliceps, it is known from a single incomplete skull which is unusually Synapomorphy for such an early crocodile relative. This genus was described in 2002 by Ronald Tykoski and colleagues; the specific name means "valley head" and refers to a deep groove along the midline of the and . It has often been interpreted as the earliest diverging member of Goniopholididae, but other studies have recovered it in various other positions.
The skull was long, low, and curved so that both extremities were higher than the middle. The premaxilla bones that formed the end of the snout were enlarged to form a wide tip; there were at least four teeth in the right premaxilla and five in the left. The left maxilla (main tooth-bearing bone of the upper jaw) is more complete than the right, and had at least 29 teeth. There was a deep groove along the midline of the nasals and the frontals; the frontals were fused into a single bone, as is seen in other adult mesoeucrocodylians. Unlike derived neosuchians, it had external antorbital fenestrae. Tykoski and colleagues subjected the skull to CT scanning, which revealed internal cavities and air passages, and showed that it had a double-walled secondary palate similar to that of Eucrocodylia, and similar pneumatic cavities as well.
In 2011, Calsoyasuchus was recovered as the basalmost goniopholidid and the sister taxon to all other goniopholidids.
Its placement as a goniopholid is not uncontested. A cladistic analysis of Crocodylomorpha published by Wilberg et al. (2019) in their discussion of crocodylomorph paleobiology recovers Calsoyasuchus as a close relative of the basal Mesoeucrocodylia Hsisosuchus.Eric W. Wilberg, Alan H. Turner, Christopher A. Brochu. Evolutionary structure and timing of major habitat shifts in Crocodylomorpha. Scientific Reports, 2019; 9 (1) DOI: 10.1038/s41598-018-36795-1 A 2022 analysis found it to be the sister group of Thalattosuchia.
Phylogeny
Tykoski and colleagues performed a cladistics with their new taxon, and found that it grouped with Goniopholis, Sunosuchus, and, most closely, with Eutretauranosuchus in a weakly supported clade, Goniopholididae. They noted that the skull of Calsoyasuchus is very similar to some goniopholid skulls from the younger, Upper Jurassic Morrison Formation. Calsoyasuchus pushes back the earliest occurrence of goniopholids from the Late Jurassic into the Early Jurassic, and not only helps to bridge a temporal gap between groups of crocodyliforms, but also a morphological gap. It also implies that some groups of crocodyliforms have long undiscovered histories.
Paleoecology
During the Sinemurian and Pliensbachian ages of the Early Jurassic, the Kayenta Formation had a diverse fauna, with the remains of , , , at least five other taxa of crocodylomorpha, , theropod, sauropodomorph, and , and early relatives of mammals (tritylodontidae and ).
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