Paleoenvironmental insights from the early Miocene oysters in the shallow-marine deposits of the Fırat formation in Diyarbakır, SE Turkey

In the shallow-marine deposits of the Early Miocene (Aquitanian – Burdigalian) Fırat Formation exposed in the Eğil district of Diyarbakır, SE Turkey, three oyster species such as Ostrea edulis (Linnaeus, 1758), Magallana gryphoides (Schlotheim, 1820), and Hyotissa hyotis (Linnaeus, 1758) were recorded for the first time. The specimens collected include four individuals of O. edulis and three individuals of M. gryphoides from the Sağlam village area, and one individual of H. hyotis from the Sarıca village area in Eğil. These specimens were found in the yellow-limestone facies and beige-white marl facies, which represent the thickest and largest sections of the Fırat Formation. For the Sağlam village area, O. edulis suggests a littoral environment with excessive detrital inputs, ranging from medium to shallow depths, and a warm subtropical climate. The O. edulis display a slender, small, and comparatively regular morphology, suggesting a preference for stable habitats commonly found in coastal regions with minimal environmental variability. Similarly, the presence of the fossil species indicates littoral environments characterized by nearshore shallow depths, low-energy conditions, normal salinity, and warm subtropical climatic conditions for M. gryphoides. For the Sarıca village area, H. hyotis is linked to reef and peri-reef environments, along with shallow tropical-subtropical marine paleoenvironments at depths of up to 50 m. Overall, this study presents novel information about the paleoecological distribution of Early Miocene oysters and their first records in the Eğil district.