Taxonomic diversity dynamics of the Jurassic bivalves in the Caucasus: Regional trends and recognition of global patterns

Abstract

Jurassic bivalves were quite diverse in the Caucasus—a region located on the northern margin of the Tethys Ocean. Stratigraphic ranges of 513 species were compiled. Total species number, already low, declined during the Early Jurassic, reaching its minimum in the Toarcian (27 species). Although this coincided with the Pliensbachian–Toarcian global mass extinction, the latter is not evident for the Caucasian bivalves. Diversity rises occurred in the Aalenian and then in the Bajocian, when 77 species existed. In the Bathonian species number did not change. A diversity explosion took place in the Callovian, when favourable palaeoenvironmental conditions were established. 163 species are known from deposits of this age. This event is recognized globally as generic diversity significantly rose at this time all over the world. When a rimmed shelf was developed in the Late Callovian–Oxfordian, total species number reached its maximum (166 species). But a regional salinity crisis in the Kimmeridgian–Tithonian led to a final diversity drop, when 86 species existed in the Tithonian. Bivalves could not recover at the end-Jurassic, along with other representatives of shelly benthos. Comparison with the other well-known salinity crisis, which occurred in the Messinian in the Mediterranean, suggests that the Late Jurassic event in the Caucasus was much stronger and stressed bivalve assemblages for a longer time. The influence of the end-Jurassic mass extinction has not been fully documented. Throughout the Jurassic benthic assemblages within the Caucasus changed rapidly. This regional diversity dynamics corresponds quite well to the global trends. Both sea-level and marine palaeotemperature changes may have had an indirect influence on regional bivalve diversity, as they fluctuated strongly during the Jurassic.