The braincase endocast of Sparassodonta (Mammalia, Metatheria) reveals that some of today’s morphological characters of marsupial brains were already present in stem Marsupialia

Abstract

In this study, we analyze virtual tridimensional models of braincase endocasts of various extinct South American mammalian predators to further understand brain evolution in stem Marsupialia. Our sample includes ten species and four families (Hathliacynidae, Proborhyaenidae, Thylacosmilidae, Borhyaenidae) of Sparassodonta (Mammalia: Metatheria). We complement anatomical information on the encephalic vasculature, such as the postglenoid venous system, the accessory diploic sinus, and the pericarotid venous network in Sparassodonta. The neuroanatomy of most sparassodonts is overall similar to extant marsupials with an elongated and straight endocast, with the exception of the sabretooth Thylacosmilus atrox that exhibits a gyrencephalic, globular, and inclined endocast in relation to the reduction of its braincase, which parallels the rostro-caudal compression of the cranium. This is another peculiarity of Thylacosmilus compared to other members of the group. Based on analyses of encephalization quotient (EQ) and phylogenetic EQ, small hathliacynids have similar encephalization to extant didelphids, whereas large borhyaenoids are outliers in having a small braincase endocast for their body size. Contrary to previous studies and considering the updated body mass estimates used in this study, the EQ of Thylacosmilus is low and within the expected range of values for stem marsupials. The reconstruction of the ancestral state in the common ancestor of Marsupialia shows that the olfactory bulbs represented approximately 8–9% of the total volume of the braincase endocast, similar to extant marsupials. In short, the analysis of the braincase endocasts of Sparassodonta suggests that some morphological characters seen in extant marsupial brains were already present in stem Marsupialia.