Diversity dynamics of ammonoids during the latest Bajocian and Bathonian (Middle Jurassic) in the epicratonic Polish Basin

Abstract

Results of the analysis of Middle Jurassic (latest Bajocian–Bathonian) ammonoid diversity patterns in the Polish Basin are reported. The data used in this study are based on a large number of ammonoid specimens, collected bed-by-bed, from the Polish Jura in south-central Poland, as well as on existing literature. The ammonoid diversities, both at the genus and species levels, have been calculated for particular ammonite zones and subzones and compared with the regional transgressive–regressive cycles for the Polish Lowlands and Hallam’s global sea-level curve. The patterns of ammonoid diversity dynamics seem to be well correlated with global sea-level fluctuations. Particular diversity peaks correspond with major transgressive episodes. Three main regional bio-events related to transgressions have been distinguished for the Polish Basin: (1) the Latest Bajocian (Bomfordi Subchron) bio-event is related to a short-lasting immigration of Tethyan ammonoids; (2) a late Early Bathonian (Tenuiplicatus Chron) bio-event corresponds to a proliferation of Asphinctites tenuiplicatus (Brauns), most probably as a result of transgression-driven eutrophication of a shallow-marine environment; during this time, the immigration (passive dispersion?) of some single Tethyan species is also observed; and (3) a Late Bathonian (Hodsoni and Orbis chrons) bio-event corresponds to the highest ammonoid species diversity peaks and most probably is related to the major transgression during the Bathonian, which allowed easy migration of several species via several newly opened sea-ways.