The braincase of the North American therizinosaurian Nothronychus mckinleyi (Dinosauria, Theropoda)

Abstract

ABSTRACTThe description and comparison of the posterior braincase of Nothronychus mckinleyi (MSM P-2117) from the Upper Cretaceous (Turonian) Moreno Hills Formation of east-central New Mexico is updated. As a result of the enlarged basicranial pneumatic system, this region of the skeleton has undergone extensive rearrangement of the cranial nerves and blood vessels from the plesiomorphic theropod condition. Nothronychus and Falcarius possessed a number of avian-associated characters or avian-trending characters in the braincase and endocranium characteristic of highly derived coelurosaurs. These traits, in some cases hypothesized, include the posterior endocranium nearly filled with nervous tissue, a laterally directed enlarged optic tectum, an enlarged flocculus, an intracranial trigeminal ganglion, and the abducens canal entering the endocranium separate from the infundibulum with no associated cavernous sinus. The angle of the occipital plate with the basal plate of the basicranium appears to be perpendicular and, therefore, intermediate between more basal theropods and extant birds. However, the posterior endocranial cavity does not appear flexed, which would result in a posteroventral rotation of the endocranial cavity in Nothronychus, a character also seen in the Cretaceous hesperornithiform bird Enaliornis and in contrast to extant birds. Therefore, the evolution of the coelurosaurian brain and braincase was trending in an avian direction before the rest of the skeleton. The results presented here support the model that therizinosaurs possessed weak bite forces as compared to obligatory carnivorous theropods.