The Equidae have a long evolutionary history that has interested palaeontologists for a long time. Their morphology-based taxonomy, however, is a matter of controversy. Since most equid species are now extinct, the phylogenetic tree based on genetic data can be established only imperfectly via deduction of present day genomes and little is known about the past genetic diversity of these species. Recent studies of ancient DNA preserved in fossil bones have led to a simplification of the phylogenetic tree and the classification system. The situation is still particularly unclear for the wild asses whose geographical distribution in the Pleistocene and the early Holocene stretched from Northern Africa to Eurasia before they became endangered or extinct. Therefore, we performed a phylogeographic study of bone remains of wild asses covering their former geographic range over the past 100,000 years based on the analysis of ancient mitochondrial DNA. Here, we will not show but rather discuss our results calling the morphology-based classification into question and indicating that morphological criteria alone can be an unreliable index in inferring various equid species. Indeed, the diversity of mitochondrial lineages in populations with similar morphology along with genetic signatures shared between morphologically distinct animals reveal a significant morphological plasticity among Equus species. The classification of palaeontological species based on morphological and genetic criteria will be discussed.
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