Evolutionary transformations of the malacofaunas in the Neogene basins of paratethys as an example of development of the ecosystems of insular type

: Evolution of the bivalve and gastropod mollusks is considered under conditions of the biochore isolation, ecosystem impoverishment, and competition weakened in the Cenozoic marine half-closed and nearly closed brackish basins during the last 30 million years (Oligocene to Pliocene). In these environments, the mollusks have first undergone drastic impoverishment of the taxonomic composition at expense of stenobionts, followed by a rapid diversification of few survived eurybiotic taxa accompanied by formation of numerous endemic species, genera, and occasionally families. Diversification rates used to rise drastically with the stability disturbance in the benthic ecosystems. Evolutionary transformations of mollusks under such circumstances surpassed frequently ranges of the ecological space of the respective taxa in their original marine basins (extralimital specialization). The morphological changes resulted often display features of fetalization, that is, retarded development with final stages lost. Constraints of morphogenetic potentials may become apparent as morphological similarity (homeomorphism) of closely or, occasionally, distantly related groups under both similar and dissimilar environments. Homeomorphism often poses problems in classification of the affected taxa and sometimes makes necessary a compromise between the pure morphological and pure genealogical approaches.