New Mexico Geological Society Annual Spring Meeting — Abstracts

The ceratopsid dinosaur Pentaceratops sternbergi is an index fossil of the Late Campanian Kirtlandian land-vertebrate “age” found almost exclusively in the Upper Cretaceous Kirtland Formation (Hunter Wash Member) of the San Juan Basin, New Mexico. A single record of a subadult Pentaceratops (San Diego Museum of Natural History 43470) is known from the Williams Fork Formation in northwestern Colorado but consists only of an incomplete and disarticulated skull. Here we preliminarily describe the most complete subadult Pentaceratops discovered to date (New Mexico Museum of Natural History P-68578). The specimen was found in the Hunter Wash Member of the Kirtland Formation in the Bisti/De-Na-Zin Wilderness Study Area during a 2011 paleontological survey funded by the Bureau of Land Management. It is partially articulated and consists of cranial elements and a nearly complete postcranial skeleton. We confidently assign NMMNH P-68578 to Pentaceratops sternbergi based on its diagnostic parietal and squamosals. Moreover, there is no evidence of any other ceratopsid in the Hunter Wash Member of the Kirtland Formation. The right and left squamosals are typical of Pentaceratops with subtriangular-shaped episquamosals fused along the outer margins. The median ramus of the parietal is slender with a U-shaped posterior margin. Length measurements of the humerus (460 mm), ulna (405 mm) and femur (670 mm) suggest that this animal was just over half the size of a mature adult Pentaceratops. This is the most complete subadult specimen of Pentaceratops, thus it provides important insight into the ontogeny of the genus.