New Mexico Geological Society Annual Spring Meeting — Abstracts

We report an isolated complete lateral tooth, nearly complete right metatarsal II and right thoracic rib pertaining to a tyrannosaurid theropod from the Menefee Formation near Cuba in northwestern New Mexico. The Menefee Formation is an early Campanian terrestrial unit that yields diverse vertebrate fossil assemblage, notably of turtles and dinosaurs. The overall morphology of the isolated tyrannosaurid tooth, including small, block-shaped anterior and posterior denticles and the labiolingually twisted anterior carina, closely resembles the teeth of other tyrannosaurids from younger, Upper Cretaceous terrestrail deposits of Alberta and Montana. The presence of the isolated tyrannosaurid tooth in the Menefee Formation marks one of the earliest occurrences of these iconic theropods in the southwestern part of North America and adds to the previous assumption of the origin of Tyrannosauridae in Late Cretaceous Laramidia. Further, the tooth is labioligually compressed, unlike the bullet-shaped teeth of Tyrannosaurus rex, which provides new information about the dental morphology of tyrannosaurids. The right metatarsal II preserves most of the shaft and nearly complete distal condyles. The shaft is not exceptionally well-preserved; however,, it maintains its original morphology, which is identical to that of other tyrannosaurids, including a large, teardrop-shaped articular surface for metatarsal III. Both distal condyles are clearly visible; however, the lateral is only partially preserved, missing the entire lateral surface. The medial condyle is nearly complete. The ligament pit is shallow but extensive. Ventral to the ligament pit is a characteristic medioventral notch that lies nearly on the long axis of the ligament pit. This characteristic medioventral notch is present in the metatarsal II of the albertosaurine tyrannosaurids Albertosaurus sarcophagus and Gorgosaurus libratus and also in Appalachian taxa such as Appalachiosaurus montgomeriensis. The isolated thoracic rib is missing the head and part of the anterior and posterior shaft. However, the preserved shaft has a characteristic tyrannosaurid morphology, which includes a deep intercostal groove and prominent flange-like structure located on the anterolateral side of the shaft. At present, the genus-level taxonomic identity of the tooth, metatarsal II and the thoracic rib cannot be ascertained because other skeletal elements are unknown. However, given their relatively small size, the bones most likely belong to a juvenile animal.