Abelisaurid theropod braincase neumaticity: phylogenetic implications based on argentinean specimens

PAULINA CARABAJAL, ARIANA

Plaza Huincul, Neuquén

Jornada; Reunión Anual de Comunicaciones de la Asociación Paleontológica Argentina; 2008

Museo Carmen Funes

In phylogenetic analyses of Theropoda, the braincase of abelisaurids was considered apneumatic, which is regarded as a plesiomorphic state. In Abelisauridae, there is neither a tympanic caudal recess nor a basipterigoid recess, and the basipterigoid processes as well as the paraoccipital processes are massive structures. Nevertheless, recent studies show the presence of three clasic pneumatic cavities in the abelisaurid basicranium: the lateral tympanic recess, the basisphenoid recess and the subselar recess. The basisphenoid recess is deeply excavated on the ventral surface of the basisphenoid in Carnotaurus Bonaparte and Abelisaurus Bonaparte & Novas. In Ilokelesia Coria & Salgado and Aucasaurus Coria, Chiappe & Dingus, this recess internally has two parasagital cavities that invade the basisphenoid at the level of the neck of the occipital condyle. In Carnotaurus and Abelisaurus, there is a lateral tympanic recess on the lateral surface of the basisphenoid. In the latter, the recess is a wide depression, divided into two chambers (the internal carotid artery foramen opens in the anterodorsalmost chamber). Finally, in each of Abelisaurus and Carnotaurus, there is a deeply excavated subselar recess anteroventral to the cultriform process that is also partially visible in the anterior view of the braincase. Therefore, the presence of three well developed pneumatic recesses in each of the basicrania of Abelisaurus and Carnotaurus (and other non-South American forms), and further evidence of these pneumatic cavities in fragmentary material of both Ilokelesia and Aucasaurus, rule out the apneumatic feature for the abelisaur braincase. Consequently, the presence of a pneumatic braincase turns out to be a synapomorphic feature for all Neotheropoda and probably all Theropoda.