Biodiversity of Malacofauna in the Caspian–Manych–Pont System in the Last Interglacial Era

The conditions for the formation of biodiversity and their comparative analysis at individual stages of development of the Ponto-Caspian basins in the MIS 5 era have been identified. This work was carried out based on the results of studying materials from many years of field research in the Caspian Sea and Sea of Azov–Black Sea regions and the Manych Depression. The biodiversity of malacofauna in the Caspian–Manych–Pont system under the climatic conditions of the MIS 5 era is inextricably linked with the history of the development of the basins. In the Caspian Sea, two transgressive basins have been reconstructed: the Late Khazarian and Hyrcanian. The Late Khazarian malacofauna of the northern part of the paleo-Caspian Sea is represented by 26 species, the main part of which are Caspian endemics belonging to the Cardiidae family. The index species is Didacna surachanica Andrusov, 1910. The Hyrcanian malacofauna includes 22 species, the main part of which are also Caspian endemics. Characteristic types are Didacna subcatillus Andrusov, 1910 and Didacna cristata Bogachev, 1932. The composition of the malacofauna was determined by the main factors: the degree of inheritance from the previous basin, the salinity and temperature of the aquatic environment, and the introduction of freshwater fauna with river runoff. The influence of “random” catastrophic factors, such as the breakthrough of a periglacial lake and the flow of its waters into the Caspian Sea, which sharply changed the biodiversity of the Hyrcanian basin compared to the Late Khazarian basin, is significant. In the Pontian basin, the Karangatian transgression developed in three stages. The malacofaunal composition in the northeastern part of the Pontian basin is represented by 37 species, the main part of which are marine Mediterranean species, both euryhaline and stenohaline. Each stage is characterized by its own complex of malacofauna with varying degrees of development of euryhaline or stenohaline elements in them. The second complex, corresponding to the maximum stage of transgression, was characterized by the greatest diversity and the presence of the most halophilic elements. An important feature of the third complex is the presence of Caspian species characteristic of the Hyrcanian transgression of the Caspian Sea. In the Manych Depression, events closely related to the development of the Caspian and Pontian basins were reconstructed: the ingression gulf of the Karangatian transgression of the Pontian basin in the second stage of its development; reduction in the length of the bay, with the simultaneous advance of waters of the Hyrcanian transgression and their discharge into the bay; complete release of the depression from the Karangatian waters and the discharge of the waters of the Hyrcanian transgression into the Karangatian basin in the third stage of development. In the Pontian basin, species diversity was determined by the composition of the malacofauna that migrated to it from the Mediterranean Sea. At the final stages of the development of the Karangatian transgression, its faunal composition was influenced by Caspian invasive species that penetrated into the Pontian basin with the waters of the Hyrcanian transgression through the Manych Strait. In the Caspian Sea, the formation of the malacofauna composition was predominantly evolutionary in nature. There was no evolutionary component in the development of the Black Sea malacofauna; its formation was of a migration nature.