Postnatal ontogenetic scaling of Nesodontine (Notoungulata, Toxodontidae) cranial morphology

Toxodontidae is a clade of endemic South American ungulates that comprises medium to very large animals, including strict megammamals, i.e., 1000kg or more. Adinotherium at approximately 120kg and Nesodon at 550kg are, respectively, the smallest and the largest Nesodontinae of Santacrucian age (early Miocene). The large number of specimens recorded and the quality of preservation (including very young animals) permit a morphometric study of cranial ontogeny. We measured 17 cranial variables on an ontogenetic series of 23 specimens of Adinotherium ovinum and 11 of Nesodon imbricatus. Bivariate analysis (standardized major axis) was performed to obtain the coefficients of allometry using skull length as the independent variable. Results indicate that eight of 16 variables show an isometric trend, seven exhibit positive allometry, and only the height of the orbit in N. imbricatus exhibits negative allometry. Contrary to expectation, neurocranial variables are positively allometric or isometric. With respect to the splanchnocranium, most variables related to the rostrum, palate, and masticatory muscles show positive allometry, suggesting a strengthening of masticatory system in adults of both taxa. The splanchnocranial allometric trends fail to support previous inferences of specialized herbivory, suggesting generalized herbivory in nesodontines.Fil: Cassini, Guillermo Hernán. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata; Argentina. Universidad Nacional de La Plata. Facultad de Ciencias Naturales y Museo. División Paleontología Vertebrados; ArgentinaFil: Flores, David Alfredo. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Museo Argentino de Ciencias Naturales “Bernardino Rivadavia”; ArgentinaFil: Vizcaíno, Sergio Fabián. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata; Argentina. Universidad Nacional de La Plata. Facultad de Ciencias Naturales y Museo. División Paleontología Vertebrados; Argentin