Diversity dynamics of Bajocian (Middle Jurassic) ammonites and transgressions/regressions in the Caucasian Sea (northern Neo-Tethys Ocean): A case high-resolution test of probable dependence

Abstract

Ammonites commonly occur in the Bajocian deposits of the Greater Caucasus Basin, from where 180 species and 52 genera are reported. This back-arc basin was located on the northern Neo-Tethyan margin embraced by the Caucasian Sea. An evaluation of diversity patterns for 7 Bajocian ammonite zones documents high diversity in the beginning, middle, and end of the stage with a maximum in the Humphriesianum Zone and two diversity minima. The mid-Early Bajocian impoverishment of ammonite assemblages was caused by both an increase in the rate of disappearance and a decrease in the rate of appearance, whereas the only latter was responsible for the mid-Late Bajocian diversity drop. An analysis of dominating facies types in the Greater Caucasus Basin documents that the total diversity rose during regressions and fell during transgressions. Shoreline shifts corresponded better with changes in the number of appearances and worse with those of disappearances. A comparison of results from this study with those obtained in the Betic Cordillera does not clarify the observed dependence. Changes in the exterior connections of the Caucasian Sea, its depth, and oxygen depletion of its waters are unlikely explanations of the regionally documented coincidence of ammonite diversity dynamics and transgressions/regressions.