Species occurrences of Mio–Pliocene horses (Equidae) from Florida: sampling, ecology, or both?

During the late Miocene and early Pliocene (latest Hemphillian, Hh4 interval, 5.7 to 4.75 Myr) a distinctive suite of four species of extinct horses (Family Equidae) were widespread in North America. This includes Nannippus aztecus, Neohipparion eurystyle, Astrohippus stocki, and Dinohippus mexicanus. In Florida, two additional equid species, Pseudhipparion simpsoni and Cormohipparion emsliei are typically found at Hh4 localities. Here we compare horses from four Hh4 Florida fossil sites, including three from the Bone Valley mines, and a fourth from the recently discovered Montbrook site. Two of these have all six predicted species, one has five species, and one has only four species present. To explain these differences, we used species counts from research databases and rarefaction simulation to better understand the relative abundances, species richness, and occurrences of these horses from these four sites. The Palmetto Mine (Agrico) site, with five equid species, appears to lack the sixth species because of ecological reasons. This is different from Montbrook, the site with only four of the six species. Results indicate that Montbrook is likely lacking the two missing equid species for multiple reasons: one because of sampling bias and the other because of biological/ecological reasons. Our results demonstrate that sampling biases can account for observed equid species richness when the overall abundance of certain equid species is low. Nevertheless, other factors, including ecology and with sufficient resolution, perhaps also time, may also explain the distribution and occurrences of individual species at these and other fossil sites.