USING OXYGEN ISOTOPES IN TOOTH ENAMEL AND BONE TO DIFFERENTIATE BETWEEN TERRESTRIAL AND AQUATIC ANIMALS

Attempts to identify aquatic mammals in the fossil record have traditionally relied on identification of key morphological characters associated with aquatic adaptation. These features include cranial (e.g., position of nares, orbits) and post-cranial features (e.g., limb structure, body shape and thoracic spine size) that are found in modern semi-aquatic and aquatic species. However, these morphologic characteristics may be missing or ambiguous in fossil specimens, which can make identification of aquatic species tricky. The purpose of this study is to test oxygen isotope values as a morphologically independent method of determining aquatic habits in fossil mammals. The oxygen isotope composition of tooth enamel and bone reflect the isotopic values of the animal's body water, which is influenced by many factors (e.g., drinking water, atmospheric oxygen, oxygen in food, water exchange across skin, etc.). For aquatic and semi-aquatic animals, a greater flux of surface water versus evaporated leaf water may result in lower enamel and bone δ18O values than associated terrestrial mammals, a pattern that has repeatedly been observed for living and fossil hippopotamids in other studies. To evaluate the effectiveness of this proxy, tooth enamel and bone were sampled from a suite of small- to medium-sized terrestrial (white-tailed deer, raccoon) and aquatic (muskrats, mink and beavers) mammals that had been collected from a single field site in Missouri. To avoid the effects of nursing on enamel isotope composition, teeth that formed late in life were selected from each species: canines and second molars were sampled from the carnivores; third molars were sampled from the deer; and incisors and third molars were sampled from the rodents. Observed differences in living species will be used to construct a model for identifying aquatic species from the fossil record based on enamel or bone oxygen isotope values. This model will then be used to test whether morphological interpretations of aquatic habits for anthracotheres and amynodonts from the White River Group are supported by geochemical evidence.