Spatial heterogeneity of the Early–Middle Toarcian (Jurassic) ammonite diversity and basin geometry in the Northwestern Caucasus (southwestern Russia; northern Neo-Tethys)

Paleontological and geological information from the Northwestern Caucasus, a large region in southwestern Russia, allows to evaluate the spatial heterogeneity of the Early–Middle Toarcian ammonite diversity and its possible controls. The total number of ammonite species and genera is calculated for four time slices, which correspond to the Dactylioceras semicelatum, Harpoceras falciferum, Hildoceras bifrons, and Haugia variabilis zones, in each of the ten areas distinguished within the study region. These areas differ by the ammonite diversity, which indicates heterogeneity. The latter persisted through the entire Early–Middle Toarcian. This heterogeneity was relatively low in the beginning of the Toarcian, when the total regional diversity of ammonites was minimal. Long- and short-term landward shoreline shifts facilitated spatial distribution of ammonites and increase in their taxa number. Shallow-marine paleoenvironments tended to sustain a higher diversity than those deep-marine, and, therefore, changes in the basin depth were also responsible for the observed spatial heterogeneity of the ammonite diversity. Interestingly, a more or less similar diversity dynamics is established in the areas of the Northwestern Caucasus. The distribution of ammonites in the study region indicates that these invertebrates migrated there from the open sea(s) stretched along the northern margin of the Neo-Tethys Ocean. Results of the present analysis also imply that the mass extinction might have been responsible for the low diversity of ammonites (observed in the entire region and its particular areas) in the beginning of the Toarcian.