SKULL GEOMETRIC MORPHOMETRICS AND PALEOECOLOGY OF SANTACRUCIAN (LATE EARLY MIOCENE; PATAGONIA) NATIVE UNGULATES (ASTRAPOTHERIA, LITOPTERNA, AND NOTOUNGULATA)

Abstract

Three orders of South American extinct native ungulates are recorded from the Santa Cruz Formation along the Atlantic coast of Patagonia: the Notoungulata (Adinotherium, Nesodon, Interatherium, Protypotherium, Hegetotherium and Pachyrukhos), the Litopterna (Theosodon, Anisolophus, Tetramerorhinus, Diadiaphorus and Thoatherium) and the Astrapotheria (Astrapotherium). This work is an ecomorphologic study of these taxa based on geometric morphometrics of the masticatory apparatus. As a reference sample, 618 extant specimens of the orders Artiodactyla, Perissodactyla, Hyracoidea and Diprotodontia were included. Thirty-six cranial and 27 mandibular three-dimensional landmarks were digitized. Allometric scaling, principal component analyses, and phylogenetic generalized estimating equations on the cranium and mandible were preformed separately. The cranial analyses show strong phylogenetic constrains, whereas the mandibular analyses show a functional pattern related to habitat-diet and hypsodonty. The extant brachydont and closed habitat ungulates show a more elongated and narrower mandibular symphysis with a lower mandibular corpus, than hypsodont, open habitat species. The latter have short symphyses with a high, curved mandibular corpus. This general pattern was was also present among Santacrucian ungulates, which permit characterization of the notoungulates mainly as open habitats dwellers, with some foraging on grass (Protypotherium, Interatherium), and others on grass and leaves (Hegetotherium, Pachyrukhos and Adinotherium), depending on the availability. Nesodon may have dwelled in mixed habitats and had 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.