New data on the neuroanatomy of Adinotherium ovinum (Notoungulata, Toxodontidae, Nesodontinae) from virtual cranial endocast

HERNANDEZ DEL PINO, SANTIAGO; CASSINI, GUILLERMO HERNÁN; CERDEÑO, MARÍA ESPERANZA; VIZCAINO, SERGIO FABIÁN

Mendoza

Congreso; 4th International Palaeontological Congress The history of life: A view from the Southern Hemisphere; 2014

International Palaeontological Association

Toxodontids were endemic South American ungulates, characteristic of the Cenozoic faunas. Although variable in size, their skulls present a conservative morphology. Among them, Adinotherium ovinum is a rather small basal taxon characteristic of the early Miocene Santacrucian Age (Santa Cruz Province, Argentina). The primary objective of this contribution is to provide a detailed anatomical description of the endocranial morphology of A. ovinum, an approximation of the brain and associated soft-tissue structures, and considerations about the relative brain size in this species. For the present study we reconstructed a virtual cranial endocast of an adult specimen (MPM PV-3532) based on CT scans and estimated the encephalization quotient (EQ=brain mass/0.055 × body mass0.74) for a sample of eight genera of Santacrucian South American native ungulates. The obtained virtual endocast is almost complete, showing only a slight lateral deformation due to lithostatic pressure, allowing the description from all views and obtaining linear and volumetric measurements. The general morphology is in agreement with previous anatomical data on the endocranial cavity of Adinotherium documented by Patterson in the 1930s (i.e., the high, arched and upwardly inclined cerebrum, the dorsal exposure of the cerebellum sloping downward, and the enormous hypophysis) and also shows a more complex pattern of sulci for the neopallium than that registered in previous works. The EQ (from an endocast volume of 116 mm3 and estimated body mass of 56.35 kg) is 0.61 and represents a lower value than that previously reported by Radinsky (EQ=0.78) for Adinotherium in his work about brain evolution in South American native ungulates, who also estimated a lower body mass (40.31 kg). The EQ obtained in our estimation is around the mid values when compared with EQs obtained for other Santacrucian ungulates: the interatheriid Protypotherium (0.86; the highest value) and the toxodontid Nesodon (0.34; the lowest one) among Notoungulates, and the astrapothere Astrapotherium (0.43). Our results indicate that A. ovinum has a higher relative brain size than the larger well-known and coeval nesodontine toxodontid Nesodon (body mass of 308.93 kg). This agrees with previous ontogenetic studies on nesodontine skulls, which revealed that cranial measurements related to the neurocranium show higher allometric coefficients in A. ovinum than in the larger Nesodon, resulting in a proportionally greater brain in the former. This kind of description constitutes a base for broader studies, with a larger sample, on the ontogenetic development of the cranial cavity and the brain in both nesodontine genera.