RECONSTRUCTING EQUID MOBILITY IN MIOCENE FLORIDA

The Miocene Epoch was characterized by progressive cooling, aridification, and the expansion of grasslands. With the increased prevalence of grasses came the widespread abundance and diversification of large-bodied grazers, such as horses. Equids reached their maximum diversity during the middle Miocene. Dietary adaptations diversified, body sizes generally increased, limbs lengthened, and toe numbers were reduced, presumably in response to an increasingly open landscape and to accommodate the consumption of grasses. However, changes in mobility associated with these environmental and evolutionary changes have not been explored. We used strontium isotope ratios (⁸⁷Sr/⁸⁶Sr) to reconstruct the mobility of horses that once lived in northern Florida. This is an excellent region to test equid mobility; local bedrock is isotopically homogeneous limestone and the Appalachians to the north are geologically heterogenous. Fortuitously, due to elevated sea levels, highly mobile individuals would have only been able to travel north. Accordingly, Sr isotope ratios for relatively sedentary individuals should resemble local Florida bedrock, whereas highly mobile individuals are expected to have more variable Sr ratios, reflecting foraging across a range of geologies. We analyzed two species (Archaeohippus blackbergi and Parahippus leonensis) from Thomas Farm (19 Ma) and six species (Calippus cerasinus, Cormohipparion ingenuum, Cormohipparion plicatile, Nannippus westoni, Neohipparion trampasense and Protohippus gidleyi) from the Love Bone Bed (9 Ma). Species with differing body mass and diets were compared within and between sites. We expected that small equids and browsers would have the lowest and least variable ⁸⁷Sr/⁸⁶Sr, indicating local movement. Larger-bodied species and grazers should have higher and more variable ⁸⁷Sr/⁸⁶Sr, reflecting greater mobility. This is the first study to examine mobility of terrestrial mammals prior to the Pleistocene, and the first to address spatial partitioning among contemporaneous equids with differing body masses and diets. Similar approaches could be used to investigate changes in mobility for other fauna in the Miocene or other periods characterized by major environmental shifts.