Braincases of abelisaurid theropods from the Upper Cretaceous of north Patagonia

  The braincases of the abelisaurid theropod dinosaurs Abelisaurus comahuensis and Aucasaurus garridoi are described and compared. These two taxa share the presence of a floccular recess that is ‘8’‐shaped, the absence of a medullar eminence in the floor of the endocranial cavity and the possession of a well‐developed, narrow and tall dorsal longitudinal sinus. The basisphenoidal recess is continuous dorsocaudally with two pneumatic cavities, which are separated medially by a thin septum. A connection between the metotic canal and the columellar recess that probably represents the exit of cranial nerve IX is observed in Abelisaurus . Aucasaurus has a skull roof with a wide sagittal crest and a low parietal eminence that differentiates it from other derived abelisaurids such as Abelisaurus , Carnotaurus and Majungasaurus . The abelisaurid braincases described herein show strong consistency in their general morphology and structure. However, there is some variation, and these detailed descriptions are important for understanding the variation and distribution of braincase characters within Abelisauridae.