EQUUS STEHLINI AZZAROLI, 1964 (PERISSODACTYLA, EQUIDAE). A REVISION OF THE MOST ENIGMATIC HORSE FROM THE EARLY PLEISTOCENE OF EUROPE, WITH NEW INSIGHTS ON THE EVOLUTIONARY HISTORY OF EUROPEAN MEDIUM- AND SMALL-SIZED HORSES

The present work provides a revision of the complete fossil collection of Equus stehlini from the Italian Early Pleistocene (late Villafranchian) of the Upper Valdarno Basin (Tuscany, Italy). The species was originally described in the second half of the last century, even if recent studies questioned its distinct species status, considering it a subspecies of Equus senezensis (i.e., E. senezensis stehlini). Over the succeeding decades the debate about the taxonomic status of E. stehlini extended to its possible evolutionary origin from two different Equus species from the European Early Pleistocene, Equus stenonis or E. senezensis. The revision undertaken here of E. stehlini, by means of morphological, morphometric and statistical comparisons, provides new insights into the taxonomic status and the evolutionary history of this enigmatic species. It is concluded that E. stehlini is a distinct species derived from the medium-sized E. senezensis. This work further highlights new insights and perspectives in the evolutionary framework and in the paleoecological understanding of the medium to small-sized Equus species from the early to middle Early Pleistocene of Europe.