Abstract
We provide morphological, quantitative, and qualitative studies of cranial endocasts of mesotheriid notoungulates solving previous open debate on notoungulate endocasts. For that purpose, we use the most accurate digital reconstructions methods. We confirm that mesotheriids have endocasts similar in shape and gyrification to those of other rodent-like notoungulates (i.e., Hegetotheriidae and Interatheriidae) and living cavy rodents (e.g., Dolicavia minuscula, Hydrochoerus, and Cavia). We identify these similarities as evolutionary response to potentially similar ecological constraints. Based on the encephalization quotient (EQ) of several notoungulate families (i.e., Mesotheriidae, Interatheriidae, Notohippidae, Toxodontiidae, and Hegetotheriidae), there seems to be no increase in terms of EQ or neocortical complexity through time in that group. In addition, comparison with several Holarctic 'euungulates' leads us to propose differential predation pressure as a potential driver for EQ. Among notoungulates, braincase comparison between well-known Oligocene-Pleistocene mesotheriids and other families identifies lifestyle as an additional possible driver for EQ, with lower values for semifossorial taxa, in a similar way to rodents. Finally, the observed stability of mesotheriid EQ (from the Oligocene to the Pliocene) would match a conservative lifestyle further reflected by their highly invariant appendicular skeleton.