NEW INSIGHTS INTO DIETARY PARTITIONING IN WASATCHIAN (EARLY EOCENE) MAMMALIAN FAUNAS: MICROWEAR, TOOTH MORPHOLOGY, AND STABLE ISOTOPE ANALYSIS

The appearance of modern lineages such as the perissodactyls and artiodactyls at the Paleocene-Eocene boundary in North America fundamentally changed local ecosystems. Microwear and tooth morphometric analyses show that many of the new immigrants were high-fiber herbivores (leaves and other structural parts), a dietary strategy that was rare among North American mammals during the Paleocene. The Eocene saw the first instances of faunas including large proportions of high-fiber herbivores, a pattern that persists to the present day. Stable isotopes can provide information about diet, an independent line of evidence sourcing from the same tooth specimens used for microwear and morphological evaluations. Stable isotope values from both phosphate (d¹⁸O) and carbonate (d¹⁸O and d¹³C) portions of tooth enamel were evaluated. Resulting d¹⁸O and d¹³C values reveal a spread of 6‰ and 4‰, respectively, a wide range of values for an ecosystem composed entirely of C3 plants. The ¹⁸O variability stems from differential reliance on ¹⁸O-enriched leaf water as a proportion of water intake (high-fiber herbivores= more leaf water, frugivores= more meteoric water), while the ¹³C variability likely stems from feeding on plants growing in different microhabitats. The dietary information available from microwear and morphology allows the sources of stable isotopic variability to be addressed, providing a powerful tool to the interpretation of isotopic values in both fossil and living systems.