Independent water vole (Mimomys savini, Arvicola: Rodentia, Mammalia) lineages in Italy and Central Europe

Water voles are important key fossils of the Quaternary. Given their wide distribution, regional differences were expected to exist in different areas. Early hints on possible independent evolutionary trends of water voles in Italy came from palaeontology and specifically from the comparison of enamel differentiation (SDQ value) of the first lower molars between specimens from Italy and Germany. The data available at that time indicated that in the early Middle Pleistocene there were only minor enamel differences between first lower molars of water voles from these two geographical regions, whereas from the late Middle Pleistocene onwards, two lineages were clearly distinguished. Examination of mitochondrial DNA of extant Arvicola populations from across Europe by Wust-Saucy led to a similar conclusion that Arvicola populations from the Italian Peninsula had undergone independent evolution during the last 0.2 million years. The inclusion of new fossil and extant Arvicola samples from Italy and Central Europe, together with the examination of additional morphological parameters of the occlusal surface (so-called van der Meulen indexes), have provided further support for the proposed evolutionary pattern. The combined analysis of length, SDQ and A/L index reveal a certain degree of intercorrelation and indicate an essentially continuous evolutionary trend. However, variations are discernible, related to the age and geographical origins of the samples, and become more clearly seen at least since the beginning of the Late Pleistocene. Italian samples have a characteristic tendency to grow larger, elongate the anteroconid, and have less derived SDQ. This corroborates the suggestion that Italian water voles underwent an evolutionary history distinct from that of their Central European counterparts. The differences in morphology may be related to a combination of etho-/ecological (aquatic or terrestrial habits) and palaeobiogeographical factors.