New Mexico Geological Society Annual Spring Meeting — Abstracts

A juvenile specimen (NMMNH P-33893) of a pachycephalosaurid from the Fruitland Formation of Hunter Wash, Bisti/De-na-zin Wilderness Area (Hunter Wash local fauna), northwestern New Mexico (NMMNH locality L-4716) consists of a complete frontoparietal dome. The specimen is the first genuine report of a pachycephalosaur from the Fruitland Formation and represents only the fifth pachycephalosaur to be collected from the state.

The specimen is considered a juvenile because it resembles juvenile specimens of Stegoceras in that the frontoparietal is small (length =75.5 mm), the frontoparietal suture and the suture dividing the frontals are distinguishable across the ventral surface of the skull roof (but not on the dorsal surface), the supratemporal fenestra are relatively large, the dome is restricted to the anterior 2/3 of the frontoparietals, and the sutural contacts for the marginal skull roofing bones are shallow resulting in a prominent parietosquamosal shelf.

The specimen is tentatively referred to a new genus and species of Pachycephalosauridae reported from the De-na-zin Member, Kirtland Formation (Williamson and Carr, in press). It lacks synapomorphies of Stegoceras (e.g., high frontonasal boss bordered laterally by grooves) and can be referred to Pachycephalosauridae by presence of a rostrocaudally-inclined roof of the temporal fossa. NMMNH P-33893 shares with the holotype of the new pachycephalosaurid genus and species a narrow exposure of the parietals between the squamosals and a lack of nodes on the parietal. However the specimen lacks squamosals that carry diagnostic autapomorphies of the new taxon and therefore, a referral to the Asian genera Prenocephale and Tylocephale cannot be precluded.

This new pachycephalosaur specimen helps to reconstruct a hypothetical sequence of ontogenetic development of the dome in Pachycephalosauridae in which the skull roof is initially flat and the dome arises and develops with maturity. Juvenile pachycephalosaurids have a restricted dome and retain open temporal fenestrae and a prominent parietosquamosal shelf. The ontogenetic variability of the dome in pachycephalosaurids has important implications for pachycephalosaur taxonomy and phylogeny.