New Mexico Geological Society Annual Spring Meeting — Abstracts

In 1978, the Museum of Northern Arizona (MNA) excavated a remarkably complete skull and jaws of the ceratopsian dinosaur Pentaceratops sternbergii from the Fruitland Formation in northwestern New Mexico. This specimen, MNA PI. 1747, was on display in the NMMNH's Cretaceous Seacoast hall for a decade, exhibited much as it was found-crushed and slightly disarticulated. Before the specimen was returned to MNA, we made a mold and cast the disarticulated elements and assembled them in a new, threedimensional resin cast mount now on display in the NMMNH's remodeled Cretaceous hall. This mount preserves the specimen as it was found but, unlike the previous exhibit, is now free-standing so that the skull can be examined from all angles. By doing this we avoided the pitfalls associated with reconstruction or "correcting" deformation. The restoration allows us to re-evaluate much of the cranial anatomy of Pentaceratops. The skull was almost entirely complete, lacking only the nasal horn. One significant result of this project reveal that the marginal epoccipitals cannot be placed on the frill with confidence, even though this is one of the best-preserved frills of Pentaceratops known. The XI epoccipitals, however, between the parietals at the posterior edge of the frill, are well preserved and confirm previous diagnoses of the genus. Another significant finding is that frill length is approximately 200% of "face" length. That is, a preorbitallength of 740 mm on the NMMNH cast compares with a "frill length" (measured from the anterior margin of the orbit to the posterior edge of the parietal) of 1480 mm, for a total skull length approaching 2.2 m. This suggests that the specimen of Pentaceratops from the San Juan basin on display at the Sam Noble Museum of Natural History in Norman, (OMNH 10165), while complete, is reconstructed correctly with a total skull length of approximately 3 m. Other useful comparative measurements made on the cast of the MNA specimen include occipital condyle diameter (78 mm), squamosal length (1140 mm) and dimensions of the sheared supratemporal fenestra on both the right (660 mm long, 600 mm wide) and better-preserved left (370 mm long, 26 mm wide) side. This is a relatively rare fossils of Pentaceratops that includes lower jaws, and the left lower jaw is approximately 790 mm long as preserved.