THE FOSSIL INVERTEBRATE MARINE FAUNA OF THE GAIMAN FORMATION (LOWER MIOCENE) AT BRYN GWYN (CHUBUT, PATAGONIA, ARGENTINA)

The Bryn Gwyn (Chubut Province, Patagonia, Argentina) is a classic fossiliferous site, long recognised for its Cenozoic vertebrate record. However, its invertebrate content remains poorly documented. The Lower Miocene Gaiman Formation, representing the “Patagonian Transgression”, has yielded abundant vertebrates, whereas invertebrate faunas were historically mentioned only briefly, mostly as molds. New material was collected following the most recent stratigraphical framework at the Bryn Gwyn locality. Fossiliferous levels were identified, and several specimens were described and studied using updated systematic arrangements. The Bryn Gwyn fossil invertebrate assemblage previously comprised only 11 taxa and a few higher-level groups. Our new study documents 27 genera or species-level taxa, nearly doubling the known diversity. A new species, namely Buccinanops halleri sp. nov., is also described. Several groups previously reported without precise determinations (e.g., bryozoans, gastropods, irregular echinoids) are now taxonomically resolved, and two additional higher-level groups, Brachiopoda and Scaphopoda, are recorded for the first time in the region. Comparisons with other Patagonian Cenozoic assemblages reveal strong faunal affinities with the time-equivalent Chenque and Monte León formations. These results substantially refine the taxonomic composition of the Bryn Gwyn invertebrate fossil fauna and provide a stronger basis for its inclusion in regional biostratigraphic correlations and paleoenvironmental reconstructions of the “Patagonian Transgression”.