BRAINCASE STRUCTURE IN THE OLDEST KNOWN SKULL OF A THERIAN MAMMAL - IMPLICATIONS FOR MAMMALIAN SYSTEMATICS AND CRANIAL EVOLUTION

Vincelestes neuquenianus, from the Early Cretaceous of Argentina, is the only non-tribosphenic therian mammal in which the side wall of the braincase is known. Because it has both a large alisphenoid and a well-developed anterior lamina (lamina obturans) separated by a distinct interdigitating suture, Vincelestes supports the non-homology of the sheet-like ossifications of the alisphenoid and lamina obturans. Likewise, because it shows the primitive tetrapod relation of the alisphenoid (processus ascendens) to the maxillary branch of the trigeminal nerve (V2), as also seen in Early Jurassic mammals, it indicates that this is the outgroup condition for modern therians, retained in pouch young of didelphid marsupials. Both the embryonic lamina ascendens of the ala temporalis and the greater part of the adult alisphenoid of modern mammals are homologous with the embryonic processus ascendens and adult epipterygoid of other amniotes. Developmental studies suggest that the anterior position of the maxillary nerve with respect to the ala temporalis, seen in monotremes and a majority of marsupials and eutherians, is due to the phylogenetic anteromedial shift of the nerve with respect to the ala. The more anterior exit of the maxillary nerve is possibly related to the forward expansion of the external adductor jaw musculature, which lies posterolateral to the nerve, and to reduction of the internal adductor jaw musculature, which lies anteromedial to the nerve in sauropsids but ventral to the nerve in mammals. The greatly reduced alisphenoid and enlarged anterior lamina of monotremes and multituberculates is a derived trait which may indicate close relationship.