An advanced form of <i>Microtus nivaloides</i> Forsyth Major, 1902 (Arvicolinae, Rodentia) in the late Middle Pleistocene of West Siberia: facts and hypotheses

Grey voles <i>Microtus</i> sensu lato represent one of the youngest and most successful radiations of rodents in the Holarctic that gave rise to several genera and over 60 living species. Despite abundant fossil record encompassing up to one million years, the early stages of <i>Microtus</i>’ evolution remain poorly understood. We describe <i>Microtus</i> voles from fluvial deposits (Tobolsky Horizon, the Late Middle Pleistocene) found in 2019–2020 in Gornopravdinsk 2, West Siberia. Based on linear measurements, m1 morphotypes, and configuration of mandibular masseteric crests, we identify the sample as a new taxon <i>M. nivaloides lidiae</i> ssp. nov. It appears more advanced than type specimens of <i>M. nivaloides</i> sensu stricto from West Runton Freshwater Bed (the Early Middle Pleistocene, Great Britain), and falls within the range of m1 variability of <i>M. nivaloides</i> sensu lato from the Middle Pleistocene of Eastern Europe. <i>M. n. lidiae</i> is found in a non-analogue faunal assemblage with <i>Lemmus sibiricus, Dicrostonyx simplicior, Alexandromys oeconomus, Lasiopodomys gregalis</i>, rare <i>Clethrionomys</i> cf. <i>glareolus, Alexandromys</i> aut <i>Microtus</i> cf. <i>malei, Lagurus</i> sp. We hypothesise that <i>M. n. lidiae</i> was a meadow-steppe element in the Middle Pleistocene communities and a terminal taxon in the ‘<i>nivaloides</i>’ clade that became extinct with no descendants. urn:lsid:zoobank.org:act:CB35BDE6-F133-4FD2-ADDA-0FBE93DAE5CA