Skeleton of Early Miocene Bathyergoides neotertiarius Stromer, 1923 (Rodentia, Mammalia) from Namibia: behavioural implication

ABSTRACT Essentially known from dental remains, the species Bathyergoides neotertiarius Stromer, 1923 (Bathyergidae, Rodentia) is widely recorded in Lower Miocene sites, notably in the localities of Elisabethfeld, Grillental and Langental (Namib Desert, Namibia). On the basis of dental morphology, this species belongs to the Bathyergidae, an extant family represented by the mole rats, which developed a fossorial, predominantly subterranean lifestyle. The discovery of a skeleton in connexion at the locality of GT Carrière (Grillental) permits description for the first time of the postcranials of Bathyergoides neotertiarius. Comparisons with extant rodent species suggest a similar behaviour to the Miocene ones, showing burrowing adaptations using principally the skull/mandible/incisor complex, evidenced by an accentuated robustness of these structures. The results of the postcranial analysis confirm the previous hypothesis, showing a robust anterior limb, probably used for the extraction of material during digging. However, a difference is seen in the robustness of the hind limb, being gracile in GT 50'06, but showing a prominent m. popliteus process, which suggests an important stabilization of the knee joint and an internal rotation of the lower limb allowing postural control in the burrows. The presence of a long tail distinguishes the Lower Miocene species from the extant subterranean rodents which exhibit short tails and recalls the morphology seen in modern semi-fossorial scratch-diggers such as Cricetomys gambianus Waterhouse, 1840. Thus, the specimen GT 50'06 shows clear fossorial adaptations, with a morphology close to the chisel-tooth diggers but exhibiting characteristics useful for a scratch-digging strategy.