Description: Understanding how bivalves responded to past temperature fluctuations may help us to predict specific responses of complex calcifiers to future climate change. During the late-Early Aptian, aragonite-rich rudist bivalves decreased in abundance in northern Tethyan carbonate platforms, while rudists with a thickened calcitic outer shell layer came to dominate those of Iberia. Seawater cooling and variations in calcium carbonate saturation states may have controlled this faunal turnover. However, our understanding of how rudist lineages responded to changing environmental conditions is constrained by a lack of quantitative data on the evolution of thickness, size, and mineralogy of the shell. This study is based on volumetric measurements of the shell and shows the transition in lineages of the family Polyconitidae from aragonite-rich mineralogy in the earliest Aptian, to low-Mg calcite-dominated mineralogy in the middle Aptian, returning to aragonite-dominated composition in the latest Aptian. The platform biocalcification crisis that occurred at the Early-Late Aptian boundary in the Tethys was marked by a relative increase of calcite and a decrease in skeletal thickness and commissural diameters. The highest calcite/aragonite (Cc/A) ratios in polyconitid rudists accompanied the late Aptian cold episode, and the lowest values were reached during the warmer intervals of the earliest and latest Aptian. These results imply a correlation between Cc/A ratio values and temperature and suggest that some bivalves adapted to less favorable calcification conditions by changing calcite and aragonite proportions of their bimineralic shells and decreasing skeletal thickness, thereby reducing the metabolic cost of shell growth.

Description: Fossilized remnants of parts of the digestive system of wood-boring pholadoidean bivalves are reported from late Oligocene–early Miocene deep-water sediments in western Washington State, USA. They are reconstructed using serial grinding tomography and computer-based 3D visualizations. Two types are distinguished: (1) a U-shaped structure with a groove on its inner side, interpreted as the caecum of xylophagaines; (2) an elongate structure with a central groove and thin tubes running parallel to it on its dorsal and ventral side, interpreted as the caecum and the intestine, respectively, of teredinids. Petrological thin-section observations show that these structures are filled by a mass of fine woody material, suggesting that being filled with woody material facilitated their fossilization. Screening the fossil record for similar structures in Mesozoic pholadoidean fossils can potentially help to clarify feeding strategies, phylogenetic relationships and the evolution of feeding strategies among early pholadoidean bivalves.