During the late Miocene and early Pliocene about 5.7 to 4.75 million years ago, 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 horse species, Pseudhipparion simpsoni and Cormohipparion emsliei, are also typically found. Here we compare horses from four Florida fossil sites of this age, including three from the Bone Valley mines and a fourth from the recently discovered Montbrook site. Two of these sites have all six predicted species, one has five species, and one has only four species present. To explain these differences, we used species occurrences from research databases to better understand the relative abundances, species richness, and occurrences of these horses from these four sites. One site (Palmetto Mine Agrico), with five equid species, appears to lack the sixth species owing to ecological reasons. This is different from Montbrook, the site with only four of the six species. Results indicate that Montbrook is likely missing the two horse species for a couple of reasons: sampling bias and biological/ecological causes. Our results demonstrate that fossil sampling biases can account for observed horse species occurrences when the overall abundance of certain species is low. Nevertheless, other factors, including ecology and with sufficient resolution, perhaps also time, may also explain the distribution and occurrences of individual horse species at these and other fossil sites.

During the late Miocene and early Pliocene (latest Hemphillian, Hh4 interval, 5.7 to 4.75 Ma) 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 also 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 sites have all six expected 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 clarify 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 owing to ecological reasons. This is different from Montbrook, the site with only four of the six species. Results indicate that Montbrook is likely lacking two missing equid species for a couple of reasons: sampling bias and biological/ecological causes. 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. In a broader perspective, analyses such as this example provide an opportunity to address a persistent challenge in paleontology, that is, how do we explain absences of extinct taxa from the fossil record?