Endocranial Morphology and Paleoneurology in Notoungulates: Braincast, Auditory Region and Adjacent Intracranial Spaces

Abstract

Notoungulates are the most diverse group of South American native ungulates (SANUs). A review of the endocranial morphology of notoungulates is here provided, concerning the braincast, the auditory region and other intracranial spaces associated to the caudal vasculature (e.g. internal carotid artery and dural sinuses), whose study have been notably stimulated by the CT scanning techniques. A bulged temporal lobe, the presence of an oblique sulcus (suprasylvian sulcus), and the position of the rhinal fissure roughly differentiate the notoungulate braincast from that of other SANUs. Within the order, braincasts range from almost lissencephalic and comparatively reduced in height (e.g. Notostylops murinus), to convoluted, anteriorly wide and dorsoventrally more developed (e.g. later diverging toxodonts). Concerning the relative brain size, and pending a precise assessment of the brain size/body size allometry for notoungulates, no evidence of marked increase during the late Eocene-Pleistocene lapse emerges from the data. Additionally, inferences on sensory capabilities are discussed based on the size of the olfactory bulbs, piriform and temporal lobes, and dimensions of the osseous labyrinth. Despite improvements in knowledge during the last two decades on most of the above-mentioned topics, there is still much work to be done on such an iconic group of SANUs.