Miocene to present turnover of molluscan assemblages: insights into coastal-marine ecosystem evolution along the Peruvian Margin

Over the past 10 million years, coastal-marine settings along the Peruvian Margin have undergone profound geographic and oceanographic transformations, resulting in extensive changes in coastal-marine communities. While mollusk taxonomy research is slowly being integrated into ecosystem-wide analyses, which have historically centered on vertebrates, a long-term chronostratigraphically controlled analysis of molluscan diversity and compositional changes has not been undertaken for this region. We compiled a database covering 152 species, 97 genera, and 51 families of mollusk fossils from the Peruvian Margin (13–16°S) to assess long-term diversification patterns and faunal turnover from the late Miocene to the present. We identified two distinctive molluscan assemblages. The first, dating to the late Miocene (10–6 Ma), underwent a substantial shift during the Mio-Pliocene transition (6–4 Ma), culminating in a second assemblage more akin to modern counterparts. This shift resulted in an increase in diversity, with the younger assemblage (6–0 Ma) exhibiting greater genus richness than the former late Miocene assemblage. The turnover at 6–4 Ma was driven by peaks in bivalve origination (6–5 Ma) along with elevated extinction rates for gastropods (6–5 Ma) and bivalves (5–4 Ma). Ecological analyses revealed that no single ecological trait consistently changed during this interval, indicating that the turnover resulted from a broad reorganization of ecological strategies. We propose that the major molluscan turnover during the late Miocene–early Pliocene is associated with geomorphological changes related to the Andean uplift, the disappearance of semi-embayments, and a sea-level rise.