New Mexico Geological Society Annual Spring Meeting — Abstracts

A partial skull (NMMNH P-27403) from the Upper Cretaceous (upper Campanian) De-na-zin Member, Kirtland Formation, San Juan Basin, represents a pachycephalosaur (Dinosauria: Pachycephalosauria). Previous identifications of Pachycephalosauria from the San Juan Basin have been questionable or erroneous and were based only on isolated teeth. Therefore, this represents the first certain report of Pachycephalosauria in New Mexico.

The skull of P-27403 includes most of the fronto-parietal dome as well as most of the right supraorbital II, the right postorbital and postfrontal, the right squamosal and much of the basicranium. The new specimen differs from all previously described pachycephalosaur taxa in North America. The skull lacks a distinct squamosal shelf as is seen in Stegoceras and Stygimoloch and exhibits derived "pachycephalosamine" features such as closed supratemporal fenestrae, a high dome, and marked anteroventral rotation of the occiput that unite it with Prenocephale and Pachycephalosaurus. The new specimen further resembles Prenocephale prenes from the Nemegt Formation, Mongolia, in its possession of a single row of low conical nodes along the posterior and lateral margins of the squamosal.

The new pachycephalosaur specimen is a new member of the Willow Wash local fauna that also includes the dinosaurs Parasaurolophus tubicen, Naashoibitosaurus ostromi, Pentaceratops sternbergi, and a new genus and species of Ankylosauridae. The new pachycephalosaur specimen increases the distinctiveness of the Willow Wash local fauna compared to near contemporaneous faunas of western North America. This marked faunal difference supports the argument for relatively strong provinciality among terrestrial faunas during the Late Cretaceous of western North America.