The skull of Spermophilus nogaici (Rodentia: Sciuridae: Xerinae) and the affinities of the earliest Old World ground squirrels

The Old World ground squirrels of the genus Spermophilus are derived marmotine xerines that reached a wide distribution across temperate Eurasia during the latest Neogene and Pleistocene. Their origin and evolution remain unclear. Based primarily on evidence derived from dental morphology, the early representatives of the genus have been attributed to morphologically similar Holarctic ground squirrels of the genus Urocitellus. Here, we provide the first detailed description of the cranial anatomy of one of the earliest Old World ground squirrels, ‘Urocitellus’ nogaici from the Early to Middle Pleistocene of south-eastern Europe. The species has an assortment of primitive and derived cranial characters, most of which are shared with more basal marmotins, whereas other characters (posteriorly elongated nasals, circular infraorbital foramen, distinct median crest of the hard palate, globular auditory bulla with a slightly extended meatal portion and lightly iron-pigmented incisor enamel) are unique to Spermophilus. Results from a phylogenetic analysis of morphological data from a broad sample of marmotins suggest that Spermophilus nogaici, as well as other European ground squirrel species previously attributed to Urocitellus, namely ‘U.’ polonicus and ‘U.’ primigenius, are referred to Spermophilus. They also corroborate the monophylies of Spermophilus and Urocitellus, recently proposed by molecular-based phylogenies, and yield the first explicit morphological synapomorphies for these genera.