LARVAL SHELLS AND GASTROPOD PHYLOGENY OF THE MAASTRICHTIAN COON CREEK TONGUE IN MISSISSIPPI AND TENNESSEE

The Coon Creek Tongue of the Ripley Formation in Mississippi and Tennessee contains a diverse and well-preserved Maastrichtian molluscan assemblage with some 388 published species, including 257 gastropods, 111 bivalves, 16 cephalopods, and 4 scaphopods. Many elements of this fauna became extinct at the Cretaceous-Tertiary boundary, an extinction so great that the systematic placement of many Cretaceous gastropod genera is problematic. The soft-body anatomy of Cretaceous gastropods is unknown, and the shell form and sculpture are often convergent among groups. Protoconchs and larval shells preserved in the Coon Creek Tongue provide valuable information as to the phylogenetic relationship of extinct groups and their connection to extant forms. Bandel (1991) demonstrated that the shell morphology of living gastropod larvae was often sufficient to place them into one of four subclasses, the Archaeogastropoda, Neritimorpha, Caenogastropoda, or Heterostropha. The authors have found this to hold true for certain exceptionally-preserved Campanian and Maastrichtian faunas of the north-central Gulf Coastal Plain and have used SEM studies of gastropod protoconchs and larval shells to revise the gastropod systematics of the Coon Creek fauna.