Abstract
A collection of over 150,000 molluscs from the Cretaceous through Recent of the Gulf Coast is analyzed to determine major changes in the abundances, diversities, and taxonomic compositions of microgastropods with a size limit of 5 mm. Five major microgastropod groups are recognized based on the relative abundances of high-order taxa of superfamilial rank and feeding types. First, an early Cretaceous assemblage dominated by Turritella, Nerinea and Nerita. Second, a Campanian Trochoidea-dominated deposit and herbivore feeding type assemblage. Third, a Maestrichtian Philinoideadominated carnivore-feeding type assemblage. Fourth, a Paleogene assemblage that is increasingly dominated by carnivores that continues until the end of the Oligocene. Fifth, a more evenly distributed late Neogene assemblage with less dominance by any feeding type. The decline of Trochoidea dominance is closely associated with the ocean thermal minimum at the beginning of the Maestrichtian. The drop in relative microgastropods abundance in the early Paleocene likely refl ects paleoenvironmental changes following the KT boundary event. The changes in composition of the carnivoredominated microgastropod populations in the Oligocene and Miocene appears similarly coeval with the ocean minimum at the end of the Oligocene. This study reveals that the relative abundances and composition of microgastropods are much more sensitive to drops in temperature than for the larger gastropods, and that these temperature drops could be a major contributing factor in the evolution of Gulf Coast microgastropod biotas.
