Trilophosaurus phasmalophus
Project
ID: 00000C926
Public
Archosauromorph reptiles are the most diverse clade of tetrapods in the Upper Triassic Chinle Formation of western North America, exhibiting the breadth of morphological and ecological evolution achieved by this group in its radiation after the end-Permian extinction. Allokotosauria, including Trilophosauridae and Azendohsauridae, is an extinct archosauromorph group that reached a near Pangean distribution in the Middle Triassic to Late Triassic and evolved a broad range of cranial and dental morphologies. Within the Chinle Formation of western North America, allokotosaurs span the Norian-aged Blue Mesa Member (Adamanian biozone) and Sonsela Member (Adamanian and Revueltian biozones). These biozones are demarcated by the Adamanian-Revueltian faunal assemblage transition, a hypothesized extinction and faunal turnover in large-bodied archosauromorphs, but poor sampling has obscured any similar patterns in tetrapods <1 m long from known localities. Trilophosaurus is the most common smaller-bodied herbivorous taxon found in Adamanian microvertebrate sites, with four species reported (Trilophosaurus buettneri, T. jacobsi, T. dornorum, and Spinosuchus caseanus). The presence of three new allokotosaur taxa in Adamanian microvertebrate sites, including a member of the Azendohsauridae, a new species of trilophosaurid described herein, and another unnamed trilophosaurid, with unique dental morphologies, demonstrate the high diversity of this group in the Adamanian. We describe a new species of trilophosaurid (Trilophosaurus phasmalophos sp. nov.) found in microvertebrate-bearing horizons in the Jim Camp Wash beds of the Sonsela Member that is the first unambiguous trilophosaurid reported from the Revueltian biozone. Unlike in other trilophosaurids, the teeth are bicuspid, but the cusp and cingulum structure and complexity are similar to those of Trilophosaurus dornorum and T. jacobsi. The occurrence of this new taxon demonstrates the dynamics of allokotosaur evolution at the Adamanian-Revueltian boundary with a reduction in diversity from five Adamanian taxa to one Revueltian taxon. The pattern of faunal turnover and extinction seen in allokotosaurs is similar to patterns in other large-bodied archosauromorph clades including aetosaurs and phytosaurs across the Adamanian-Revueltian boundary in the American southwest. Systematic sampling of microvertebrate bonebeds at stratigraphic intervals across this boundary demonstrates this turnover in allokotosaurs and may reveal similar dynamics in other groups that have been missed because of collecting biases towards larger-bodied vertebrate fossils.