True lemmings (Lemmini, Arvicolinae, Rodentia) in the Eopleistocene (Calabrian) in the North of western Siberia

Based on zoological collections and published information, morphological characteristics of the teeth and mandibles of Lemmus cf. sibiricus from the Eopleistocene Skorodumian faunal complex of western Siberia are compared with those of modern and fossil Lemmini from Europe and Asia so as to clarify the taxonomic identity and determine the place of the fossil form under consideration in the evolutionary history of the Palaearctic Lemmini. Comparative morphological approaches based on qualitative traits and linear measurements of cheek teeth, traditionally used for taxonomic diagnostics of representatives of the tribe, are used. according to the combination of apomorphic and plesiomorphic features, lemmings from the Skorodumian faunal complex of the Eopleistocene of western Siberia are shown to occupy a position intermediate between the more ancient L. kowalskii and the modern species Lemmus sibiricus. According to this intermediate position between the ancestral widespread form and the modern regional taxa of the genus Lemmus, the West Siberian Eopleistocene lemmings correspond to the East Siberian Lemmus sheri Abramson, 1992. The differences between L. kowalskii and the Eopleistocene forms are clearly recognizable and reflect the evolutionary changes in the cheek teeth associated with the transition of the Eopleistocene forms to a higher level of specialization in bryophagy. At the same time, the morphological similarity of the western and eastern Siberian Eopleistocene lemmings to each other is no higher than that of each of the regional forms to modern Lemmus inhabiting the same regions. Based on this, within the Eurasian range of the Eopleistocene Lemmus, differentiation comparable to the intrageneric differentiation within modern Lemmus could presumably have been observed. The combination of similarities in some features and differences in others most likely reflects mosaic morphological variations across the Eopleistocene range of one form, possibly a polytypic taxon, which by right of priority should be termed as Lemmus sheri Abramson, 1992. Taking into account the presence of regional features, it seems appropriate to consider the Eopleistocene lemmings of the Olerian Formation of eastern Siberia as L. sheri sensu stricto, and the lemmings from the Skorodumian faunal complex of western Siberia as the polytypic taxon L. sheri sensu lato.