The masticatory apparatus and paleobiology of Early Miocene native South American ungulates

CASSINI, GUILLERMO HERNÁN

Mendoza

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

International Palaeontological Association

The exposures of the Atlantic coast of the Santa Cruz Province provide a large number of well preserved specimens constituting an excellent sample for paleobiological studies. Three orders of South American extinct native ungulates are among the most representative mammals of the Early Miocene Santacrucian Age (Santa Cruz Formation). They are represented by the Notoungulata (Adinotherium, Nesodon, Interatherium, Protypotherium, Hegetotherium and Pachyrukhos), the Litopterna (Theosodon, Anisolophus, Tetramerorhinus, Diadiaphorus and Thoatherium) and the Astrapotheria (Astrapotherium). This coexistence of many ungulates from different lineages suggests a marked resource partitioning of vegetation. To evaluate this hypothesis, the masticatory system was studied in the theoretical framework of functional morphology and ecomorphology with focuses on body size, habitat use, diet and digestive physiology. Body mass estimates were obtained using quantile regressions equations based on centroid size. Habitat preference and diet types were assessed based on the relationship between craniomandibular traits and environmental attributes using a broader reference sample composed of 141 Artiodactyla, 16 Perissodactyla, and 5 Hyracoidea species in a geometric morphometrics approach (allometric scaling, principal component analyses and phylogenetic generalized estimating equations). Digestive physiology was evaluated by analyzing the relationship between the occlusal surface area of the cheek teeth and body mass. The results allow grouping of santacrucian ungulates into three body size categories: (i) small sized (1-10 kg); (ii) medium-sized (10-100 kg) and (iii) large-sized ungulates (100-1000 kg). The notoungulates were characterized as open habitat dwellers, with some taxa foraging on grass (Protypotherium, Interatherium), and others on grass and leaves (Hegetotherium, Pachyrukhos, and Adinotherium. Nesodon may have dwelled in mixed habitats with a mixed feeding behavior, while small proterotheriids (Anisolophus and Thoatherium) may have fed predominantly on dicotyledonous plants. The remaining litopterns (Tetramerorhinus, Diadiaphorus, and Theosodon) and Astrapotherium may have foraged in closed habitats and fed on dicotyledonous plants. The relationship between dental occlusal surface and body mass indicates that litopterns, especially proterotheriids, carried out extensive intraoral food processing rather than having relied on a specialized digestive physiology. Conversely, notoungulates, which have lower occlusal surface per unit mass, would have had a better digestive capacity than litopterns. Finally, a paleoecological generalization was made based on the paleobiological reconstructions; niche partitioning by herbivores ungulates was based on the differential use of habitats (open, mixed and closed) and the differentiation of feeding behavior, and is mainly reflected by three biological attributes: (1) body size, (2) shape and function of cranio-dental features, and (3) energy requirements.