First skeleton of the notoungulate mammal Notostylops murinus and palaeobiology of Eocene Notostylopidae

Here, we describe the first skeletal remains of Notostylops recovered from middle Eocene levels of the Sarmiento Formation, Patagonia, Argentina. The remains include two teeth of Notostylops murinus, the axis, vertebral bodies, a rib, a left humerus, both radii, two metapodials, two phalanges, the pelvis, a right femur, a right calcaneus and several broken bones. Radial bones are not fused to ulnas, and are shorter than the humerus, very generalized, with an oval head, a marked neck and a radial tubercle. The humerus and the femur show pronounced insertion structures. Our analysis suggests that the appendicular skeleton of Notostylops is too generalized and shares several features with that of terrestrial rodents as Sciuridae. Unlike the appendicular skeletons of cursorial or saltatorial mammals, which restrict mobility, the skeleton of Notostylops indicates the ability to make a variety of different movements, as would be expected for terrestrial, fossorial or arboreal mammals. This skeleton gives new information about the locomotor behaviour of notoungulates, particularly in their basal forms. The results will also allow the identification of isolated notoungulate bones and raise questions about the previous taxonomic assignment of postcrania to Pleurostylodon.