Exploring the palaeoneurology of the extinct spiny rat Eumysops chapalmalensis (Hystricognathi: Echimyidae): a comparative phylogenetic analysis of brain size and shape

This study examines the brain morphology of Pliocene specimens of the South American Echimyidae rodent Eumysops chapalmalensis through virtual endocasts and comparisons with extant members of the family. Shape variation is analysed through geometric morphometrics, and the relative sizes of the brain and its components are estimated using phylogenetic regression models. The influence of phylogeny and ecological variables on the brain variation is analysed through phylogenetic comparative analyses. Results indicate that Eumysops chapalmalensis exhibits low encephalization and a high spatial packing of its brain, suggesting the independence of these two measures of relative brain size. Brain shape of Eumysops chapalmalensis departs from the extant species in morphospaces evidencing a distinctive morphology characterized by a strong flexion, posterolateral expansion of the neocortex, and large petrosal lobules. The low phylogenetic signal observed in the brain shape of echimyids, coupled with the significant influence of ecological factors, suggests that adaptive pressures probably played a more influential role in shaping the distinctive brain morphology of Eumysops chapalmalensis. This unique morphology may be associated with the occupation of a niche not explored by extant echimyids—that of a species with high body mass, terrestrial to fossorial habits, and agile movements in open environments.