Dental shape variation of Neogene Pachyrukhinae (Mammalia, Notoungulata, Hegetotheriidae): systematics and evolutionary implications for the late Miocene Paedotherium species

Paedotherium was the last representative of the pachyrukhines (Notoungulata, Hegetotheriidae), a small rabbit- or caviomorph rodent-like clade, with hypsodont and simplified dentition. In contrast to the Pliocene Paedotherium species, the Miocene species are less known and mainly represented by teeth. Different systematic studies of Paedotherium based on the same set of subtle trait variations of the cheek teeth have arrived at different proposals. With the aim of testing the validity and analysing the morphological variation of Paedotherium species, and assessing the definition and diagnostic value of teeth traits, we performed a dual approach: a traditional qualitative study and exploratory geometric morphometric analyses of the upper and lower cheek teeth (both, partial and complete series). In addition, the configurations of cheek tooth shape were used as landmark characters, in combination with traditional characters, to perform a phylogenetic analysis. We propose the validity of the late Miocene P. borrelloi Zetti, and the assignment of north-western Argentinian representatives of Paedotherium to P. aff. minor. Paedotherium borrelloi is morphologically more similar and phylogenetically more closely related to the Pliocene P. bonaerense than to other Paedotherium species. Our results suggest that during the late Miocene the coexistence of two Paedotherium lineages was already established, whose main morphological differences persisted or even intensified in Pliocene representatives, supporting a more complex evolutionary scenario than previously believed.