LATE CRETACEOUS RUDIST BIVALVES AND CARBONATE PLATFORMS: RATES OF EVOLUTION AND TIMING OF EVENTS CORRELATED BY STRONTIUM-ISOTOPE STRATIGRAPHY

Due to the imprecise biostratigraphy of Late Cretaceous platform-carbonate deposits, the correlation with data from the Boreal realm and the deep-sea, and thus the evaluation of their history in relation to global environmental change, has been impossible. Rudist bivalves evolved rapidly into a large variety of shell structures and morphologies, but a precise zonation based on these abundant and characteristic fossils has not yet been developed.

Numerical ages for more than 50 Tethyan localities of rudist bivalves from the Caribbean to the Middle East have been derived from Sr-isotope stratigraphy. Pristine biological calcite of rudist shells is the preferred material analyzed, and the study focuses on species-rich associations and/or type localities of widely distributed species. The results have several implications for the evolution of both, rudist bivalves and carbonate platforms. The age and ranges of several taxa and lineages is significantly revised. Species-rich rudist formations persisted into the latest Maastrichtian in the Caribbean as well as in the Mediterranean Tethys. Heterochronic evolution was demonstrated for several lineages and tied to chronostratigraphy. The rate of morphological change decreased significantly after the Santonian, and no significant morphological innovations are evident after the early Campanian.

Two important extinction events occurred at the Santonian-Campanian boundary and in the late Campanian. Correlation with Boreal events based on Sr-isotope stratigraphy indicates that these two extinction events correspond to sea level lowstands, carbon isotope excursions, and in case of the Late Campanian event, to a significant cooling both in equatorial and higher latitudes. Ongoing work aims at an improved calibration of the Sr-isotope curve for the Campanian based on belemnites, and on the pattern of demise of the characteristic rudist-dominated ecosystems at the end of the Cretaceous.