Pleistocene Small Mammals from Kozarnika Cave (Bulgaria): Morphology, Systematics, and Paleoecology

Kozarnika Cave revealed a lengthy archaeological sequence covering the Lower, Middle, and Upper Paleolithic periods. The site is a significant milestone in understanding the various stages of early human colonization and migration into Europe. The available paleomagnetic data indicate that the lower boundary of the deposits corresponds to the Brunhes–Matuyama reversal. During the archaeological excavations, an exceptionally rich assemblage of small mammal remains was recovered. Based on small mammals, two biozones have been delineated—lower (Biozone B) and upper (Biozone A)—which correspond to the Late Biharian and Toringian, respectively. In the present study, a total of 75 species of terrestrial small mammals (Eulipotyphla, Lagomorpha, Rodentia) are analyzed in detail. One new species, Nannospalax sirakovi sp. n., is described. Based on geometric analysis of the mass material of voles (Microtina and Lagurus), the controversial taxonomic positions of some species have been clarified. It has been proven that Microtus arvalinus is a distinct species. Rapid evolution within the species Lagurus transylvanicus towards the modern species L. lagurus has been demonstrated. In all layers of Biozone B, two important guide fossils, Mimomys savini and M. blanci, occur. They are indicative of the second stage of the Early Biharian, which is considered earlier than the Brunhes/Matuyama reversal. Besides this, multiple species characteristic of the Early Pleistocene (Villanyian and the Early Biharian) have been identified (Borsodia newtoni, B. arankoides, Prolagurus pannonicus, Ungaromys nanus, Villanyia cf. exilis, Allophaiomys deucalion, A. cf. pliocaenicus, A. cf. burgondiae, Mimomys pitymyoides, M. cf. tornensis, Lagurus arankae). Although very scarce, they occur regularly in the stratigraphic assemblages of Biozone B, which is dominated by typical Late Biharian species, such as Allophaiomys gregaloides, Microtus arvalidens, M. nivalinus, M. nivaloides, and L. transylvanicus. The occurrence of Mesocricetus newtoni and Spermophilus citellus in Biozone B marks the earliest known occurrence of these species. The faunal turnover within the lower part of the deposits has been analyzed in detail. A reorganization of the fauna has resulted from the gradual disappearance of the Early Pleistocene species. Paleoecological reconstructions based on stratigraphic assemblages of small mammals suggest that the faunal turnover recorded in Biozone B is associated with paleoclimatic changes during the MIS19–MIS17 interval and reflects a reorganization of the biota related to the mid-Pleistocene transition. This period was marked by significant changes in Earth’s climate, shifting from low-amplitude 41,000-year obliquity-driven cycles to high-amplitude, roughly 100,000-year fluctuations that define the later Pleistocene and Holocene.