New trilophosaurid species demonstrates a decline in allokotosaur diversity across the Adamanian-Revueltian boundary in the Late Triassic of western North America

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 estimated holochronozone) and Sonsela Member (Adamanian and Revueltian estimated holochronozone). The Adamanian-Revueltian faunal assemblage transition is hypothesized to represent extinction and faunal turnover in large-bodied archosauromorphs, but poor sampling of known localities has obscured any similar patterns in tetrapods <1 m long. Trilophosaurids are the most common smaller-bodied herbivorous taxa found in Adamanian microvertebrate sites, with four species previously reported (Trilophosaurus buettneri, T. jacobsi, T. dornorum, and Spinosuchus caseanus). The presence of three new allokotosaur taxa in Adamanian microvertebrate sites, including a new species of trilophosaurid described here (Trilophosaurus phasmalophos sp. nov.) demonstrates the high diversity of this group in the Adamanian. Trilophosaurus phasmalophos occurs in microvertebrate-bearing horizons in the Jim Camp Wash beds of the Sonsela Member, making it the first unambiguous trilophosaurid reported from the Revueltian estimated holochronozone. 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 Adamanian-Revueltian boundary marks a reduction in allokotosaur diversity from five Adamanian taxa to one Revueltian taxon. This pattern of faunal turnover and extinction is similar to those of other large-bodied archosauromorph clades across the Adamanian-Revueltian boundary in the American Southwest. Systematic sampling of microvertebrate bonebeds at stratigraphic intervals across this boundary demonstrates that this turnover in allokotosaurs may reveal similar patterns in other groups that have been missed because of collecting biases towards larger-bodied vertebrate fossils.