Notoungulata is an extinct order of ungulates, endemic from South America. It has two main suborders: Toxodontia, including the large-bodied ungulates, and Typotheria. Researchers from Argentina have described the endocasts from two species of Notohippids, a family from the South American Oligocene that is included in the Toxodontia group. The endocasts from Rynchippus equinus and Eurygenium latirostris were virtually reconstructed from CT scans and compared to other fossil and extant ungulates. Both endocasts were similar in size and in their overall shape, proportions and sulci morphology. They fitted into the general “design” of the Toxodontia endocasts, which have the most complex surface within the Notoungulates, with pronounced telencephalic flexure, a developed Sylvian sulcus, and a bulging temporal lobe. These features are also similar to those displayed by the rabbit-like Typotheria group, although Notohippids had larger frontal region. In contrast, extinct and extant ungulates display a different endocast morphology, without prominent Sylvian and temporal regions. According to the authors, functional interpretations for the expansion of the frontal region and the Sylvian and temporal areas in the Notohippids can suggest an increase in the snout sensitivity and an auditory specialization, respectively.