ATURIA (CEPHALOPODA: NAUTILIDA) IN THE LOWER OLIGOCENE SUWANNEE LIMESTONE OF FLORIDA

Though uncommon, occurrences of the nautiloid Aturia have been reported from Florida’s late Eocene through middle Miocene deposits. Despite their documentation and continued collection, most lack formal description. Here, we focus on presence of the genus in the lower Oligocene Suwannee Limestone, as individuals recovered from these deposits possess unusual preservation and a distribution that is highly restricted.

While Oligocene deposits are common across Florida (except the southern portion) and rich with invertebrate fossils, only recently have nautiloids been reported in these deposits. This may be due in part to under-sampling, but also to composition of the organism, as nautiloids have shells that are aragonitic, and preserve poorly in limestones. Approximately 50 individuals have been collected from the Suwannee Limestone, all of which occur in a single locality along the Suwannee River. At this locality, Aturia preserve as recrystallized phragmocones inside silicified concretions. These individuals are mostly intact (lacking the living chamber) and generally between 20 and 60 millimeters in size.

Particularly noteworthy specimens from these deposits are individuals covered in concentric growths of a colonial organism (suspected to be a hydrozoan). We posit that this encrustation was post mortem, rather than occurring during Aturia’s life cycle. In living nautiloids, epizoans do not have an opportunity to encrust to the extent seen in these specimens, due to movement of the organism in the water column. Furthermore, cephalopod literature reports similar types of encrustation in cephalopod occurrences in the French Miocene.

By conducting an in-depth study of these individuals, we hope to gain better understanding of whether Aturia occurrence in this single locality is due to preservational conditions or just an under-sampling across coeval sites.