A New Species of Sphaerotholus (Dinosauria: Pachycephalosauria) from the Upper Cretaceous Hell Creek Formation of Montana

WOODRUFF, D. Cary; Royal Ontario Museum, University of Toronto, Toronto, ON, Canada. Great Plains Dinosaur Museum, Malta, MT. EVANS, David C; Royal Ontario Museum, University of Toronto, Toronto, ON, Canada.

Abstract

The Hell Creek Formation of western North America is home to some of our most iconic of non-avian dinosaurs, such as Tyrannosaurus and Triceratops. The Hell Creek Formation represents the last geologic interval in North America before the Chicxulub bolide impact; thus, it signifies a vital interval regarding pre-impact dinosaur paleobiology. The debate over increasing or decreasing dinosaurian pre-K/Pg diversity has ebbed and flowed, adding additional importance to the Hell Creek Formation ecosystem. While formerly reconstructed as extremely speciose, recent studies on the ceratopsians, tyrannosaurids, and pachycephalosaurids has caused the formation to potentially 'lose' four genera and seventeen species. While the larger bodied taxa appear to be less speciose than previously perceived, several new smaller bodied taxa have been described which still contribute to an increase in Hell Creek Formation dinosaur diversity.

The pachycephalosaurids of the Hell Creek Formation are popularly perceived to be Pachycephalosaurus and its ontogimorphs. Yet this an inaccurate reflection as the smaller statured Sphaerotholus buchholtzae co-occurred alongside Pachycephalosaurus. Here we report a new, and second species of Sphaerotholus. This new species of Sphaerotholus is distinguished by six characters, the most notable being three posterior and lateral node rows, and the lack of parietosquamosal nodes – compared to S. buchholtzae which possesses singular rows and parietosquamosal nodes. Recovered from the middle of the Hell Creek Formation, this new taxon appears to have co-occurred with both S. buchholtzae and Pachycephalosaurus. This new taxon signifies that not only was the genus Sphaerotholus more diverse during the Hell Creek Formation, but so too were Late Cretaceous North American pachycephalosaurids. Furthermore, while the larger bodied taxa of the Hell Creek Formation may not have been as speciose as previously thought, newer, smaller bodied taxa are highlighting that the formation may have still retained higher dinosaurian gamma diversity.