Morphological Integration of Native South American Ungulate Mandibles Based on 3D Landmarks

GUILLERMO H, CASSINI; NAHUEL A. MUÑOZ; SANTIAGO E. HERNÁNDEZ DEL PINO; SERGIO F. VIZCAINO

Mendoza

Congreso; 4th International Paleontological Congress; 2014

Grupo de Paleontología, IANIGLA, CCT-CONICET Mendoza

South American native ungulates include several extinct lineages that evolved within the context of the South American Cenozoic geographic isolation. By the late Early Miocene Santacrucian Age, the three orders Notoungulata, Litopterna, and Astrapotheria were clearly differentiated. Recent studies, in an ecomorphological framework, highlighted a complex functional pattern related to habitat/diet and hypsodonty. In order to understand these complex relationships in an evolutionary context we used geometric morphometrics methods to evaluate the morphological integration of two of the primary functional units of the mandible: the alveolar region, and the ascending ramus. Twenty seven mandibular three-dimensional landmarks were digitized. Principal component analyses (PCA), and Partial Least Squared (PLS) were used to explore the patterns of covariation between these two modules. The PCA of tangent space coordinates resulted in the six PCs accounting for nearly 85% with the three first PCs accounting for most of the 70% cumulative variance. The shape changes associated to the first eigenvector goes from a robust curved mandibular corpus and expanded coronoid and angular process on the negative end to a gracile straight mandibular corpus and shrink coronoid and angular process on the positive end. The PLS analyses shows a very high RV coefficient (~0.73) and only the first pair of PLS were significant explaining the 88% of covariation. The shape changes associated to PLS1 was very similar to those of PC1 (5º in angular comparison between these two vectors). These results were consistent when the PLS was repeated on the independent contrast. These results indicate a strong morphological integration in these three orders. In the Astrapotheria and Litorpterna we found a common pattern of covariation, which might be due to similar constraints (developmental, functional and/or biomechanical). Conversely, in the Notoungulata the dimensions displayed a clear distinction between species suggesting a slighty different pattern of covariation in the different families of the order. The PLS inside the litopterns and notoungulates indicate that only the PLS1 were significant explaining a ~93% and 87% of covariation and showing a higher RV coefficient 0.91 and 0.66 respectively. The fact that the three orders differs in their hypsodonty could shaped the strength of integration between the two modules. The lower morphological integration in the mandible of notoungulates than litopterns, and given that the differences can be explained by the membership to different taxa is also in concordance with both the taxonomical and morphological diversity of the clade, particularly in the typotheres.