Morphological and morphometric variability in Quaternary rhinoceroses as revealed by a small-sized adult Stephanorhinus etruscus skeleton from Poggio Rosso in Upper Valdarno (Early Pleistocene, Italy)

A nearly complete skeleton of Stephanorhinus etruscus from the Upper Valdarno Basin (Italy) provides a rare opportunity to investigate the morphological and morphometric variability of Early Pleistocene rhinoceroses. The specimen, IGF8212V, is here described and compared with S. etruscus, S. hundsheimensis, and S. jeanvireti from several European localities. A detailed analysis of the postcranial remains shows that several characters previously proposed as diagnostic among these species are more variable than traditionally assumed. Because IGF8212V preserves cranial, dental and postcranial elements belonging to a single adult individual, it provides a rare test case for evaluating the reliability of isolated postcranial characters. The skeleton shows features consistent with an assignment to S. etruscus, but also displays some features previously regarded as typical of S. hundsheimensis or S. jeanvireti. These characters do not form a coherent diagnostic association with either of these taxa, suggesting that part of the morphological overlap among Early Pleistocene Stephanorhinus species reflects intraspecific variability. The hypothesized size-reduction trend in western European S. etruscus is not supported by the data considered here. Instead, the observed variability may reflect a combination of sexual dimorphism, ontogeny, geographic variability and possibly local ecological factors.