What do we learn from this work? The authors Orlando et al. examined ancient DNA from 22 horse specimens and found (as before) that the genus Equus is not monophyletic. Fossils previously identified as Hippidion yield DNA that reveal a close relationship with the modern horse Equus caballus. Hippidion is a South American, pony-sized, monodactyl horse with a skull distinct from any extant equids. It was previously thought to be only a distant relative of Equus, but now we know different.
See? Not very earth shattering in light of the larger story of Hyracotherium to Equus horse evolution, but still very interesting. My favorite part is from their conclusion:
This pattern of taxonomic oversplitting does not appear to be restricted to equids but is widespread amongst other Quaternary megafauna [e.g., Late Pleistocene bison; Holarctic cave lions; New World brown bears, and ratite moas]. Together, these findings suggest that the morphological plasticity of large terrestrial vertebrates across space and time has generally been underestimated, opening the way to detailed studies of the environmental, ecological, and epigenetic factors involved.(See the original paper for citations). When they say "morphological plasticity," I'm pretty sure they mean morphological variation rather than phenotypic plasticity proper. Nevertheless, it's very interesting that what appear to be diverse forms in the fossil record are turning out to be genetically closely-related. It will be very interesting to see how future studies expand or alter this observation.
Orlando et al. 2009. Revising the recent evolutionary history of equids using ancient DNA. PNAS 10.1073/pnas.0903672106.