Hi, Christina and Bruce –
Thanks for your e-mails! The question Christina poses is actually a bit thorny. The short answer is that some North American Pleistocene fossils that have been assigned morphologically to Equus lambei are genetically indistinguishable from E. ferus (that’s the name for the wild horse; E. caballus applies to domestic & feral horses only). Strictly technically, one would need to successfully extract DNA from the holotype of E. lambei itself to confirm or refute whether the species is a junior synonym of E. ferus; even if other specimens referred to that species turn out to be E. ferus, the name lambei would still technically remain valid for the holotype. But if one steps away from technicalities like that, it is nevertheless clear that Pleistocene horses genetically similar if not identical to E. ferus were present in late Pleistocene North America.
So, if you’re asking whether or not Equus lambei is a junior synonym of E. ferus, the answer in a taxonomic sense would be “probably”. But if you’re asking whether or not E. ferus (again, the wild form of E. caballus) was present in late Pleistocene North America, the answer is “Yes!” Does that help?
This is not to say, of course, that this means Kirkpatrick & Fazio are necessarily correct about treating living feral Equus caballus as native North American wildlife. But that’s a separate question.
Associate Curator for Paleontology
Dr. John D. Cooper Archaeological and Paleontological Center
Santa Ana, California 92701