Comparative Endocranial Anatomy, Encephalization, and Phylogeny of Notoungulata (Placentalia, Mammalia)

Cranial endocasts are one of the most direct tools available to obtain information about the endocranial cavity of fossil mammals, but few anatomical comparisons have analyzed endocranial data within a phylogenetic framework. Our study combines analyses of new digital endocasts from high-resolution X-ray micro-computed tomography (µCT) for two notoungulates, Cochilius sp. and Notostylops murinus, with data from published anatomical descriptions and endocasts (prior µCT digital endocasts and natural and extracted plaster endocasts), resulting in 22 endocranial characters (11 new) scored on 20 notoungulates and five outgroups. These data permitted a comparative study of cranial endocasts of notoungulates, an enigmatic group of extinct South American mammals, shedding light on several important issues concerning their phylogenetic relationships. The new endocranial data were integrated into a craniodental and postcranial matrix, resulting in 259 characters across 65 taxa, the most comprehensive character and taxon sampling to date and the only one including a substantial suite of endocranial characters. We further performed ancestral state reconstructions for endocranial characters using parsimony optimization on the resulting phylogenetic trees. Body mass estimates and endocranial volumes were used to calculate Encephalization Quotients (EQs) and Phylogenetic Encephalization Quotients (PEQs), which then were optimized onto the phylogenies. Endocast data from ancestral character state reconstructions bear on relationships among meridiungulates, revealing that many endocast characters may be synapomorphies for Notoungulata and other clades within the group. Notoungulates, in general, display relatively low EQs when compared with Holarctic herbivorous mammals, potentially due to their long history of evolution in isolation in South America during the Cenozoic.