MORPHOLOGICAL INTEGRATION OF NATIVE SOUTH AMERICAN UNGULATE MANDIBLES. A TRIBUTE TO D’ARCY THOMPSON IN THE CENTENNIAL OF 'ON GROWTH AND FORM'

South American native ungulates include several extinct lineages that evolved within the context of South American Cenozoic geographic isolation. By the late early Miocene Santacrucian Age, the orders Notoungulata, Litopterna, and Astrapotheria were differentiated. Recent ecomorphological studies highlighted complex functional patterns between their habitat/diet and hypsodonty. In order to understand these relationships in an evolutionary context we used geometric morphometrics methods (27 3-D landmarks) to evaluate the morphological integration of two of the primary functional units of the mandible: the alveolar region and the ascending ramus. Principal component analyses (PCA) and two blocks Partial Least Squares analyses (PLS) were used to explore the patterns of variation and covariation between these two modules. The main variation in shape is related to the robusticity and curvature of the mandibular corpus, and the relative size of the angular and coronoid process. The PLS analyses show a very strong morphological integration within the Litopterna, but a pattern of modularity in the Notoungulata. There is a common pattern of covariation in Astrapotheria and Litopterna, which might be due to similar constraints. The different covariation patterns in notoungulates with respect to litopterns are in concordance with both the taxonomic and morphological diversity of the clades, particularly in the typotheres. The modularity present in notoungulates could have allowed specializations in each module of the mandible, related to crown height or muscle attachments, without a high morphological constrain. This could have resulted in their high morphological diversity, in contrast with the more conservative morphology present in litopterns.