First Digital Endocasts of late Oligocene Notohippidae (Notoungulata): Implications for Endemic South American Ungulates Brain Evolution

DOZO, MARÍA TERESA; MARTÍNEZ, GASTÓN

Mendoza

Congreso; 4th International Paleontological Congress. Symposium 10: Evolution of South American endemic ungulates; 2014

IPA-CCT-Mendoza

Notohippidae were middle-sized toxodonts with an early tendency toward hypsodonty and convergence with the Equidae. They have been recorded from the Eocene to the early Miocene. New findings in the Deseadan locality of Cabeza Blanca (Patagonia), the most complete skulls known for Notohippidae, have motivated new studies. We provide the first description of the cranial endocasts of Rhynchippus equinus (MPEF-PV 695) and Eurygenium latirostris (UNPSJB-PV-60) based on three-dimensional reconstructions extracted from high-resolution X-ray computed tomography imagery. We analyze the encephalization quotient (EQ) and infer the location of some functional neocortical areas. Although obvious differences are observed in the morphology of skull, the morphological analysis of the endocasts of Rhynchippus equinus and Eurygenium latirostris indicates that they were similar in size, proportions of the encephalic components, and neocortical design. The digital endocast of Rhynchippus equinus shows the rhinal fissure, other cerebral sulci and casts of arteries, and the dorsal sinus system, which is also distinguishable on the artificial endocast of the same specimen. The neuromorphology of the notohippids Rhynchippus and Eurygenium is similar to the morphology of the endocasts of Proadinotherium, Adinotherium and Nesodon from the Miocene.However these toxodontids show a relatively larger frontal lobe and apparently greater development of secondary sulci in both the frontal and temporal lobes. On the other hand, the neuromorphology of notohippids contrasts with the neocortical morphological patterns described for the brains of living ungulates (orders Perissodactyla and Artiodactyla). The brain of most modern ungulates shows a less pronounced forebrain flexion, favoring the presence of longitudinally arranged sulci but without showing a developed sylvian region. This more simplified pattern resembles the brains of Tertiary Artiodactyla and Perissodactyla from the Northern Hemisphere and proterotheriid litopterns from the South American Tertiary. The present study is the first to include an estimate of EQ for Eurygenium latirostris. The EQ estimate (between 0.584-0.633 and 0.613-0.669) is somewhat lower than the EQ of R. equinus (0.670-0.695 and 0.709-0.738), and both are consistent with those observed for other toxodonts such as Adinotherium (EQ = 0.78) and Nesodon (EQ = 0.62). Regarding the location of some functional neocortical areas, the expansion of the frontal lobe in Notohippidae may reflect acquisition of heightened tactile sensitivity in the front of the snout as recorded in the somatic sensory cortex of living ungulates. The bulging temporal lobe may reflect expansion of the auditory cortex, likely related to the marked enlargement of the middle ear chamber.