Fossilized digestive systems in 23 million-year-old wood-boring bivalves

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.