Paleoneurology, auditory region, and associated soft tissue inference in the late Oligocene notoungulates Mendozahippus fierensis and Gualta cuyana (Toxodontia) from central-western Argentina

Noninvasive computed tomography (CT) scanning techniques performed on the high-quality preserved holotypes of Mendozahippus fierensis (grouped within the paraphyletic ‘Notohippidae’) and Gualta cuyana (Leontiniidae) allowed a thorough description of both endocasts, not only associated with the brain but also with closely related endocranial spaces, which proved to be highly diagnostic among Notoungulata and relevant from a paleobiological perspective. The specimens come from Quebrada Fiera (Mendoza, Argentina), an upper Oligocene locality extensively studied, especially during the last decade. Regarding brain endocast proportions and neocortical complexity, M. fierensis shows morphological affinities to coeval ‘notohippids,’ whereas G. cuyana resembles Leontinia gaudryi. An acute olfaction is inferred from the large olfactory bulbs and piriform lobes. Conversely, evidence concerning the development (and implications) of some necortical areas (i.e., somatosensory cortex and auditory cortex) were inconclusive. The encephalization quotients are around those previously reported for ‘notohippids’ and Toxodontidae Nesodontinae, and slightly lower than in Typotheria (except for Mesotheriidae). Other endocranial traits previously unexplored (such as the transverse diploic communication between the temporal sinuses in M. fierensis and the large temporal sinuses inferred for G. cuyana) show that there is relevant variability within Toxodontia that deserves further research. Extending similar approaches to early diverging taxa will certainly contribute to a better understanding of the morphological evolution of the brain, special sense organs, and associated blood circulation pattern in toxodontians and notoungulates in general.