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.