Geographic variation in diets of ancient populations of 5-million-year-old (early Pliocene) horses from southern North America

Abstract

Using tooth morphology and carbon isotopes, geographic differences in diets are presented for four species of early Pliocene horses (late Hemphillian, Hh 4, between 5.8 and 4.8 Ma) from four localities in Florida, Texas, and Mexico. These four species include three-toed Nannippus aztecus, Neohipparion eurystyle, and one-toed Astrohippus stockii, and Dinohippus mexicanus. The amount of dental variation represented in local populations of these extinct species is similar to that seen in extant equid populations (coefficients of variation, Vs, between ~ 4 and 10). As such, the fossil accumulations of individual species from single localities can be treated as paleopopulations. All four of these horses have high-crowned teeth (hypsodonty indices between 2.6 and 3.1) and the question is addressed how these closely-related species partitioned their available resources to minimize competition. Comparisons of tooth morphology indicate negligible geographic differences between paleopopulations of the same species. Therefore local differences in diets evolved within a context of stasis in tooth morphology. Carbon isotopes of fossil equid tooth enamel indicate that most of the populations and species studied here were primarily C4 grazers, although with some exceptions: Nannippus aztecus tends to be more negative, indicating a higher proportion (~ 20%, or sometimes greater) of C3 plants in its diet. A few individuals within the paleopopulations demonstrate relatively negative tooth enamel carbon isotope values (between ~ − 3 and − 10‰), indicating a diet with some, to predominantly, C3 plants. The sources of isotopic variation result from a variety of factors including latitudinal and seasonal differences in C4 and C3 grass availability, local microhabitat differences, and possibly migration patterns. For the most part these four species of early Pliocene horses have similar diets consisting of predominantly C4 grasses. They therefore differentiated their niches through other evolutionary and ecological strategies, possibly including body size and local habitat differences.