New Mexico Geological Society Annual Spring Meeting — Abstracts

Pentaceratops sternbergi is a large chasmosaurine ceratopsid dinosaur found only in the Upper Certaceous (Campanion) Fruitland and lower Kirtland formations of New Mexico, well represented by at least 12 skulls and partial to nearly complete skeletons. It is characterized by a large, keyhole-shaped media embayment surrounded by four large epoccipitals on the dorsal margin of the parietosquamosal frill. In 1998 the New Mexico Museum of Natural History (NMMNH) collected a partial skull and associated partial skeleton of P. sternbergi (NMMNH-27468) from the Hunter Wash Member, Kirtland Formation in the Bisti/De-na-zi Wilderness area. Recovered skull elements include a largely complete parietal, one of the most complete known for this taxon, incomplete left and right squamosals, and a right jugal. We refer this specimen to P. sternbergi because it preserves the diagnostic medial embayment and associated epoccipitals on the parietal. However, the specimen also shows features that are unusual for P. sternbergi. The parietal of P-27468 is small compared to all other specimens. Also, the parietal differs from the other specimens in the unusual laterally-constricted shape and small size of the dorsal medial embayment, the aberrant positioning of the adjacent epoccipitals away from their usual location. We propose two hypotheses (not mutually exclusive) to explain the unique conformation of this specimen; first, that the parietal of P-27468 is pathologic, and second, that it represents an early ontogenetic stage not previously documented.