The braincases of Argentinean theropod dinosaurs: Osteology and phylogenetic implications.

PAULINA CARABAJAL, A.

Bristol, Inglaterra

Otro; Society of Vertebrate Paleontology anual meeting; 2009

Society of Vertebrate Paleontology-University of Bristol

Most cranial descriptions do not include the braincase, which is why comparatively little braincase information is used in taxonomic diagnoses and phylogenetic analyses. Argentina is, within the framework of South America, the territory with the greatest number of known theropods. Nevertheless, the number of known skeletons with preserved braincases is less than ten. Although the statistical significance of such a small sample is poor, the Argentinean taxa represent different stages of theropod evolution throughout the Mesozoic. Theropod braincases share with prosauropods a number of plesiomorphic characters that not found in sauropod braincases, which have a number of unique characters. Twenty-five differences were identified to distinguish theropod and sauropod braincases. Theropod braincases develop mostly axially (in contrast with sauropods, which develop mostly transversely), and the dorsoventral height tends to vary in correlation with the degree of dorsoventral development of the basicranium. The variability observed in theropod braincases is high, although many characters are apparently dependent on body size. In order to determine how braincase characters influence the resolution of relationships within the Theropoda, a phylogenetic analysis was done using 29 taxa. Fifty-seven of the 113 braincase characters are new for this study, and another 37 characters are modified (and the rest taken) from previous analyses. Braincase characters support well the monophyly of the clades Theropoda, Neotheropoda, Abelisauridae and Tyrannosauridae, and the resulting trees include Herrerasaurus within the Theropoda. The inclusion of braincase characters in previous matrices results in trees whose topology is not modified significantly, which implies that braincase characters produce trees that are congruent with trees obtained with the rest of the cranial and postcranial characters. The phylogenetic analysis shows that braincase characters have a high degree of homoplasy. This does not agree with the low level of homoplasy that would be expected if the braincase is the most conservative part of the skull, as has sometimes been stated.