DIETARY INFERENCE FOR SOUTH AMERICAN NOTOUNGULATES FROM THE SANTA CRUZ FORMATION

Dietary interpretations for extinct South American notoungulates have primarily been based on gross dental morphology. In living mammals, both hypsodont (high-crowned) and hypselodont (ever-growing) molars are generally associated with grazing diets and/or feeding in open habitats. Since many notoungulates have hypsodont or hypselodont teeth, it has long been assumed that they were grazers.

We used low-magnification enamel microwear analysis to test the utility of hypsodonty for inferring diet in three hypselodont notoungulate: Nesodon, Adinotherium, and Protypotherium. Microwear data were taken from both the enamel and dentin and compared to an enamel microwear database of 39 modern ungulates of known diet. A discriminant model based on modern taxa was built using eight microwear variables and correctly classified 83.3% of the modern ungulate species by diet. The enamel data classified the notoungulates as browsers with relatively high conditional probabilities and with small Mahalanobis distances to the browser centroid.

Among modern ungulates, browsers typically exhibit low-crowned molars, thus the classification of these notoungulates as browsers is unexpected. The enamel microwear data are robust, however: the number and kinds of features seen in the notoungulates fit the ungulate browser profile well and the sample size of fossil taxa is large enough to accommodate individual variation.

We also evaluated microwear features on dentin, a major component of the occlusal surface in notoungulates. The dietary signal of dentin has yet to be fully assessed, but one might expect a dietary signal similar to that seen in enamel. Using the enamel microwear disciminant model, the dentin data classified the notoungulates as mixed feeders, with very low conditional probabilities and large Mahalanobis distances to the mixed feeder centroid. The dentin results suggest that the functional properties of dentin and osteodentin need to be further explored in order to understand the value of dentin microwear for inferring diet. To do that, a comparative dataset of modern ungulate dentin microwear is needed.