A Revision of the Maastrichtian Exogyrinae Subfamily from the Kallankurichchi Formation, Ariyalur Sub-basin, India — Taxonomy and Palaeobiogeographic Implications

ABSTRACT Oysters are commonly found in marine Jurassic-Cretaceous deposits worldwide, prompting extensive taxonomic studies that have led to inconsistencies in their identification. The present study re-evaluates the Exogyrinae subfamily with an extensive database from the Maastrichtian Kallankurichchi Formation. We identified four species belonging to three genera of the Exogyrinae subfamily-Rhynchostreon cauveryensis sp. nov., Aetostreon sp. indet., Ceratostreon flabellatum and Ceratostreon pliciferum. While the genus Rhynchostreon has previously been reported from the Cenomanian of India, this study reports a new species of the genus Rhynchostreon, R. cauveryensis sp. nov., from the Maastrichtian Kallnakurichchi Formation. Also, this is the first record of the genus Aetostreon in India as well as the first report of Ceratostreon flabellatum from this region. We also present a morphologic revision of the previously described Ceratostreon pliciferum from the Kallankurichchi Formation. Geometric morphometric analysis was employed to quantify and compare the detailed morphology of R. cauveryensis sp. nov. and Ceratostreon pliciferum. In the present study, the paleobiogeographic distributions of the studied taxa were also investigated to trace their potential migration routes. The findings enhance our understanding of the morphology and migration history of the Exogyrinae subfamily during the Late Cretaceous.