Tracing the Paleobiology of Paedotherium and Tremacyllus (Pachyrukhinae, Notoungulata), the Latest Sciuromorph South American Native Ungulates – Part I: Snout and Masticatory Apparatus.

Inquiring into the paleoecology of extinct forms is always a challenge, particularly when the taxa under study correspond to derived ecomorphs of ancient and completely extinct clades. In this contribution, the configuration of the masticatory apparatus and associated features of the Neogene pachyrukhines Paedotherium and Tremacyllus are studied in a detailed, mainly qualitative, comparative analysis of 36 specimens. Tooth morphology and the reconstructed muscular configuration of pachyrukhines indicate an important mediolateral component during chewing, and predominant crushing over grinding, as well as anteroposterior movements for the coupling and action of stronger gnawing incisors. These actions are more compatible with hard and brittle or turgid fruit food consumption than specialized folivorous, and particularly grazing, habits. The infraorbital and palatal foramina morphology and other rostral features indicate increased touch sensibility for object recognition and are congruent with the presence of infoldings of the lips protecting the gingiva during gnawing on hard foods. Additionally, there was a morphological gradient between Tremacyllus and P. bonaerense, from high selection of relatively soft and small food items, to specialized hard item consumption and higher resistance for abrasion and masticatory efforts (e.g., in eventual association with digging habits), respectively. Paedotherium typicum presents intermediate characteristics, with incisors designed for better cropping action or poorer selectivity during feeding. This more profound understanding of the feeding habits of pachyrukhines further allows the suggestion of paleoecological factors that could have contributed to niche segregation between these long-term coexisting rodent-like taxa. [ABSTRACT FROM AUTHOR]