Palaeontological description of Nizhneirginsky Grotto Upper Holocene sediments (Ufa Plateau, Fore-Urals) with taphonomic and palaeoenvironmental remarks based on bird and small-mammal assemblages

摘要

Mammal and bird bone remains from Upper Holocene sediments at the Nizhneirginsky Grotto were studied. Nizhneirginsky Grotto is located in the northern part of the Ufa Plateau in the Middle Fore-Urals. The sequence of deposits contains three layers which have been accumulating during the last 4150 years. Identified mammal elements include 9839 bone remains of 29 species belonging to five orders, i.e., Eulipotyphla (435), Chiroptera (1608), Lagomorpha (52), Rodentia (7688), and Carnivora (56). Recovered bird remains include 1946 bones of 38 species. Most identified species currently inhabit the Nizhneirginsky Grotto area. The analysis of alterations in the first lower molars of arvicoline rodent species caused by the digestion showed that eagle owl (Bubo bubo, Linnaeus, 1758) is the most possible agent responsible for the accumulation of small mammal assemblages in the grotto sediments. An unusually high number of birch mouse fossils was noted over the entire sediment section of the grotto (MNI on average by layers is 8.1-15.4%). This phenomenon, probably, has a taphonomic origin and is associated with the predation activity of mustelids. Palaeoenvironmental analysis of small mammal assemblages based on the habitat weighting method showed the predominance of open landscapes and woodlands around the grotto during the Late Holocene. This landscape did not change in the last 4150 years. The connection of the history of the yellow-necked mouse in the northeast of its modern range with the palaeogeographic events of the second half of the Holocene was established. The material from the sediments of the Nizhneirginsky Grotto shows that the species was common in the northern part of the Ufa Plateau, in the Irgina River valley, in the period of 4150-3100 cal BP, which coincides with the maximum distribution of broad-leaved trees in the Fore-Urals during the mid-Subboreal thermal maximum (SB-2; 4200-3200 BP). Pika's (Ochotona, perhaps O. pusilla, Pallas, 1769) sinister mandible was found among osteological material from the upper part of layer 2 which dates to 710 +/- 20 cal BP (1240 +/- 20 cal AD). This is the first find of the pika's remains of such late age in the Middle Urals and in adjacent territories. The species was common in the northern part of the Ufa Plateau during Late Pleistocene and Early Holocene. Steppe pika is now disjunct from the Nizhneirginsky Grotto by approximately 200-300 km to the south and to the southwest. O. pusilla probably inhabited the valley of the Irgina River as a relic of the Late Pleistocene and Early Holocene faunas as early as 12th and 13th centuries AD.