Paleoneurology of Litopterna: Digital and Natural Endocranial Casts of Macraucheniidae

Abstract

The digital endocasts of Cramauchenia normalis, Theosodon cf. T. gracilis, and Macrauchenia patachonica described herein provide an interesting perspective to begin understanding litoptern brain evolution, particularly to improve our under¬standing of evolution within the Macraucheniidae.The endocasts of Cramauchenia normalis and Theosodon cf. T. gracilis, with elongated cerebral hemispheres that are rounded in outline, resemble more closely those of Huayqueriana cf. H. cristata (IANIGLA-PV 29, Forasiepi et al. 2016) and differ from the more spherical form exhibited by the cerebral hemispheres of Macrauchenia patachonica. When compared with Proterotheriidae, the most nota¬ble difference is the slightly (but present) development of the Sylvian region.The endocasts of macraucheniids and proteroteriids (without pronounced telen¬cephalic flexure, lacking a prominent Sylvian or suprasylvian sulcus, and lacking bulging temporal lobes) contrast to the neocortical morphological pattern described for notoungulates (Toxodontia and Typotheria).Also, the three-dimensional reconstructions show different morphologies of the dorsal pneumatic sinuses (not only restricted to the frontals but also invading the surrounding bones) between Cramauchenia normalis, Theosodon cf. T. gracilis, and Macrauchenia patachonica. Differences are mainly attributed to the retraction of the nasal aperture and the relocation of the nasal bones, particularly in Macrauchenia and, to a lesser degree, in Theosodon.Regarding the relative brain size, the EQ values obtained for the Oligocene Cramauchenia normalis are higher than those of the coeval notoungulates ?notohip¬pids? Rhynchippus equinus, Eurygenium latirostris, Mendozahippus fierensis, and the leontinid Gualta cuyana.When compared to similar-sized Holarctic ungulates from the late Oligocene, the EQ of Cramauchenia normalis is below that of the perissodactyl Mesohippus and around the values obtained for the artiodactyl Poebrotherium.The EQ values of Macruachenia patachonica could indicate the highest point in the encephalization process among macraucheniids and coincides with a notable skull specialization (Lobo 2020). How strongly related are encephalization (represented by the ?EQ jump? hypothesis) and general skull specialization in M. patachonica is an interesting, and still pending, question.Finally, a concretion recovered from the Deseadan levels of the Sarmiento Formation that outcrops in Cabeza Blanca locality (Chubut), was interpreted as a partial natural endocranial cast. This natural endocast represents the first natural braincast assigned to a litoptern (potentially referable to Macraucheniidae).