On the history of forming the steppe malacofauna in the south of the Mid-Russian upland in the late holocene

Basing on the analysis of shells of land snails found in seventeen horizons of the soil profile and modern steppe ecosystems on the territory of the "Yamskaya steppe" and its buffer zone (the nature reserve "Belogorie"), we obtained data on changes in species composition and mollusks' abundance in the steppes in the South of the Mid-Russian Upland from the end of the Subboreal period of the Holocene to the present. In the fossils we detected 2835 shells and their fragments which belong to eight species of land snails. The obtained data showed that there was an increase in species diversity of land snails, the relative number of fragments of shells and their weight with increasing age of the soil horizon which is associated with climate, biotope and biocenosis environmental factors. We marked maximum species diversity for the end of the Subboreal period and associated it with the quality of existing habitats. We showed that the onset of the wet and cold periods of the Holocene resulted in decreased species diversity of xerophilous snails, and dry and warm periods - only to preserve their diversity without the emergence of new species. For the end of the Subboreal period of the Holocene we noted the presence of Pupilla sterrii V. and Vallonia tenuilabris Al., species of the glacial epoch now extinct in the studied area. We proved that under the conditions of a mountainous landscape of the Mid-Russian upland there is an increase in the likelihood of preserving the indigenous fauna and relics of past eras due to the semi-isolated state of its hills and river valleys. We examined modern steppe fauna of mollusks in twelve points of the studied area. Only in the modern steppe ecosystems of the protected area "Yamskaya steppe" and its buffer zone we found nine species of land snails. We considered inter - and intrapopulation variability conchiological traits of xerophilous species Chondrula tridens (Muller, 1774) and Helicopsis striata (Muller, 1774) from modern fauna and 2860±550-year-old soil horizon. Modern populations of H. striata did not significantly differ among themselves according to most conchiological traits, however, are significantly different from the sample of the soil horizon by larger shells and a large number of their whorls. This indicates the existence of more arid conditions at the end of the Subboreal period of the Holocene than at present. In the analysis of morphometric characteristics of Ch. tridens shells all investigated populations split into two clusters, one of which includes samples of morphological form galiciensis, and the other form - albolimbata. We showed the absence of significant gene flow between galiciensis and albolimbata forms. The reproductive isolation between Ch. tridens galiciensis and albolimbata forms is pronounced and requires taxonomic revision of this species with the involvement of molecular-genetic methods.

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