Earliest bison dispersal in Western Palearctic : insights from the Eobison record from Pietrafitta (Early Pleistocene, central Italy)

The late Villafranchian is one of the pivotal timespans in the succession of Pleistocene European faunal assemblages, setting the bases for the major faunal renewal that characterized the continent during the Epivillafranchian. Bison is one of the most important and successful large mammals to spread in Europe at the latest stages of the Early Pleistocene. Here we describe the remains of a large bovid from the late Villafranchian site of Pietrafitta (ca. 1.5 Ma), previously attributed to Leptobos. Our analyses allow to refer this sample to the genus Bison. The primitive characters featured by this sample suggest the attribution to the subgenus Eobison, a long-debated taxon which includes all the earliest forms of Bison. At Pietrafitta, thus, one of the largest and most complete record of primitive Bison is recorded. The vague diagnosis and confused taxonomic history of Eobison called for a reappraisal of its status. We present a re-definition of the diagnostic characters and a review of the Eurasian record of Eobison with a focus on the late Villafranchian samples from the Mediterranean area. We recognize at least three valid species of Eobison remarking, however, the extreme morphological variability of this group. The comparative analysis of Eobison and its closest relatives (i.e., Leptobos and Bison s.s.), confirms that both the leptobovine and bisontine clades underwent to an increase of stoutness of the appendicular skeleton in response to the shrinking of forested habitats and the onset of colder, arid climate during the Pleistocene.