Cranial anatomy of an Eocene notoungulate mammal from northwestern Argentina with special reference on the ear region

A detailed anatomical analysis is here presented focused on a notoungulate skull recovered from sediments of the lower part of the Quebrada de los Colorados Formation (LC I; late middle Eocene), cropping out in Salta Province, Argentina. The material was identified as a Toxodontia, although it does not exhibit information useful enough for its generic or even familiar assignment. The description was mainly focused on the basicranium and the auditory region (especially the petrosal and auditory ossicles) given the better preservation of these parts in the specimen. Additionally, a phylogenetic analysis was conducted based on previous studies dealing with a wide sample of South American Native Ungulates. This specimen exhibits some traits traditionally mentioned for the suborder Toxodontia, such as the horizontal bullar septum, and others recently regarded as synapomorphies of Notoungulata, including the lateral location of the tensor tympani fossa and the expanded medial margin of the petrosal. However, it also shows some unexpected features for that suborder of notoungulates, such as the presence of a strongly curved promontorium. The study of this skull increases our knowledge of the auditory anatomy of Eocene Toxodontia and yields a good opportunity to test current phylogenetic hypothesis, mostly based on Neogene representatives.