Investigating equid mobility in Miocene Florida, USA using strontium isotope ratios

Abstract

Despite extensive research on the evolutionary history and ecology of horses, surprisingly little is known about the daily and seasonal movements (i.e. mobility) of extinct species. We used strontium isotope ratios (87Sr/86Sr) in tooth enamel to estimate mobility patterns of equids from two Miocene fossil sites in northern Florida, USA: Thomas Farm (ca. 18 Ma) and Love Bone Bed (ca. 9.5–9 Ma). The species included in our sample have a range of body sizes (45–252 kg) and reconstructed diets. Gomphotheres and tapirs from Love Bone Bed were also included to represent browsers. Based on modern species' behavior, we predicted small-bodied and browsing taxa living in closed habitats had limited mobility and 87Sr/86Sr similar to local northern Florida limestone. We anticipated larger-bodied and grazing taxa living in open environments had increased mobility, with higher and more variable 87Sr/86Sr reflecting larger movements across differing geologies. Contrary to expectations, the majority of taxa at both sites have higher 87Sr/86Sr than expected for local Eocene bedrock and instead are similar to contemporary Late Oligocene to mid-Miocene seawater. The most plausible explanation for these findings is that most individuals foraged close to the paleo-coastline, where vegetation would have been influenced by marine-derived strontium via sea spray or precipitation. Tapirs have considerably higher and more variable 87Sr/86Sr than horses or gomphotheres at Love Bone Bed, which may reflect foraging on aquatic vegetation in the river channel that is preserved at the site. Our results suggest that horses were relatively sedentary in Florida, even during their peak radiation.