Role of oysters in biostratigraphy: A case study from the Cretaceous of the Ariyalur area, southern India

The Cretaceous sediments of the Ariyalur area, southern India, are the largest and the most important of all the exposures of that system in southern India. Oyster bivalves are common in these Cretaceous sediments. More than twenty species belonging to fifteen genera are known from the Ariyalur area. They are distributed in almost all the rock formations representing ages ranging from Late Albian to Maastrichtian. The abundance, preservation and restricted ranges of these bivalve species in the Cretaceous of Ariyalur area offer scope for biozonation. They seem to play a role as markers of sedimentary units by their apparent accumulation. This study attempts to interpret the palaeoenvironment of deposition based on size, distribution and carbon isotope study of the shells, the petrology of the associated rocks and inferences based on other groups of fossils including invertebrates, vertebrates and plant fossils.