New Mexico Geological Society Annual Spring Meeting — Abstracts

We document a new chasmosaurine ceratopsid from the Upper Cretaceous Hall Lake Member of the McRae Formation, New Mexico. The chasmosaurine fossils consists of much of the skull, several vertebrae, ribs, and incomplete forelimbs. The fossils were collected from a red mudstone bed ~24 m above the base of the Hall Lake Member, south of McRae Canyon, Sierra County. Based on the occurrence of a tyrannosaur equivalent in body size to Tyrannosaurus rex and the sauropod Alamosaurus sanjuanensis, the Hall Lake Member is dated as Lancian (late Maastrichtian). The new chasmosaurine taxon is distinguished by a short but robust supraorbital horncore that is anteroposteriorly wide and mediolaterally compressed, premaxilla with a short pronounced ridge on the lateral surface, pterygoid with flat posteromedial ridge, robust jugal with pronounced posterolateral ridge, robust epijugal, and a long fenestrated frill with a strongly convex median parietal bar, and transversely narrow squamosal with a pointed end and elongate episquamosals. Cladistic analysis recoverses the McRae Formation chasmosaurine ceratopsian as a sister to Pentaceratops and places it within the Coahuilaceratops+Utahceratops clade based on the transversely narrow squamosal and the transversely expanded frill posteriorly. The discovery of the new taxon adds to the diversity of chasmosaurine ceratopsians during the final stage of the Late Cretaceous and to the poorly known dinosaur fauna of the McRae Formation.