New Mexico Geological Society Annual Spring Meeting — Abstracts

The Colorado City Formation of West Texas preserves the oldest extensive Late Triassic vertebrate fauna in the southwestern U.S.A., a fauna that is the basis of the Otischalkian land-vertebrate faunachron. The richest and most diverse vertebrate fauna found in the Colorado City Formation is the assemblage from Trilophosaurus quarry 1. We report here significant additions to this fauna as a result of continued screenwashing and picking of the matrix from the spoil pile at the quarry. New records from the quarry we report here include possible semionotid fish, diverse archosauriform teeth, a possible cynodont, a probable sauropodmorph, and a sphenodontian. The cynodont is fragmentary but potentially represents the oldest known from the Chinle Group. The sauropodmorph is a probable prosauripod, based on two teeth that appear relatively advanced (“Azendohsaurus- grade”) are the oldest such fossils known. The sphenodonid is represented by two jaw fragments and is the oldest sphenodontian from North America. Dozens of fragmentary reptiliam vertebrae recovered doubtless represent other new or poorly known taxa. Another significant fossil recovered is an extremely small (near hatchling) dentulous maxillary(?) fragment of T. buttneri that further demonstrates that T. buttneri and T. jacobsi are distinct species. The fauna as a whole is dominated by amniotes (~79% of catalogued specimens), followed by osteichthyans (14.4%) and amphibians (5-6%) with chondrichthyans (1.1%) an extremely minor component. This is the oldest Late Triassic microvertebrate fauna in North America and suggests that dinosaurs and the sphenodonts both have evolutionary origins in the Ladinian.