Cranial variability in Stephanorhinus hemitoechus (Mammalia, Rhinocerotidae): A revision of Azzaroli’s subspecies and the systematics of Middle-Late Pleistocene European rhinoceroses

Western European rhinoceroses have been the subject of several investigations in recent decades, but only a few studies have taken into account the broad morphological variability in extant rhinoceroses. The latter has given rise to doubts concerning the validity and variation of fossil species. Among the Pleistocene fossil rhinoceroses, Stephanorhinus hemitoechus represents one of the most recorded and described species in Western Europe, being documented by well-preserved cranial remains. Moreover, in addition to the nominotypical subspecies, three further subspecies of S. hemitoechus were erected based on differences in cranial features, namely Rhinoceros hemitoechus falconeri Azzaroli, 1962, Rhinoceros hemitoechus aretinus Azzaroli, 1962 and Dicerorhinus hemitoechus intermedius Azzaroli in Anfossi & Cantaluppi, 1987. The validity of these subspecies and the cranial variability of S. hemitoechus are herein revised after consideration of the available morphological traits of the lectotype of the species, comparison with fossil cranial remains assigned to S. hemitoechus and the morphological variability in extant rhinoceroses. In both extant and fossil rhinoceroses, morphological features such as the shape and size of the occipital face and the position of the nasal notch and of the orbit are affected by ontogenetic patterns, revealing marked changes during the life cycle of an individual. Some other features of the cranium instead, such as the development of the nuchal crest, may have been affected by changes in feeding habitus or sexual dimorphism. Accordingly, the occurrences of subspecies of S. hemitoechus are herein rejected, both from a chronostratigraphic and geographic perspective. Finally, we suggest herein that an estimation of the age at death of the studied material and a more comprehensive comparison with the extant species be provided in future studies in order to arrive at a better understanding of fossil rhinoceroses and their taxonomic assignment.