RUDIST BIVALVES AS ENVIRONMENTAL ARCHIVES

Bivalve shells biomineralization is influenced by various biological (physiological) and surrounded environmental processes, expressed by different growth rates, incremental banding, and life-strategies, which are ultimately reflected by their shell morphology and chemical composition. This renders them valuable for sclerochronological and geochemical approaches to reconstruct paleoenvironmental conditions and several paleophysiological aspects of these organisms. Rudist bivalves became increasingly dominant in the Tethys Ocean during the Late Jurassic and flourished throughout the Cretaceous. Well-preserved rudist (Macgillavryia sp., Vaccinites sp., Torreites sanchezi) and oyster (Oscillopha figari) specimens were collected in life-position from the Campanian Samhan Formation in central Oman. Measurements of growth increments as well as geochemical analyses (δ¹³C and δ¹⁸O) were employed to constrain growth rates and longevity. Petrographic thinsection images under translucent and fluorescence light were used to visualize and accurately count the fine internal increments. Shell mineralogy and composition were inspected via Micro-Raman Spectrometry and EDX. The linearly correlated δ¹³C and δ¹⁸O values was assigned previously to photosymbiotic dinoflagellates in T. sanchezi, and we use this relationship to verify eventual photosymbiosis in all four species. Combined with individual growth performance we discuss the role of symbionts for enhanced calcification. The rudists have external annual growth banding with finer internal increments, but with different numbers in each species. Internal lines in Macgillavryia sp. are characterized by two sequential bundles of finer lines and comparatively thicker ones. The inner fine banded dark-light layers in Vaccinites sp. and Torreites sanchezi were counted, resulting in >400 fine internal increments of extremely regular growth. These are likely daily bands, which would imply a rapid growth of ~5.8 cm/year in the presence of symbionts. Isotopic patterns lacked seasonality but showed ~7 day cycles. Reconstructed temperatures from δ¹⁸O range from 35 to 40°C.