TAPHONOMY OF NAUTILOID CEPHALOPODS FROM THE MIDDLE SILURIAN MASSIE FORMATION OF SOUTHEASTERN INDIANA: PRELIMINARY RESULTS

Nautiloid cephalopods are a moderately abundant but under-studied faunal component of the middle Silurian (Wenlock: Sheinwoodian) Massie Formation of southeastern Indiana. The mudstone lithofacies of the Massie Formation as exposed at the New Point quarry near Napoleon, northern Ripley County, has yielded a large collection of specimens, a sample of which (N = 100) was studied to assess taphonomic patterns. The cephalopod fauna is of low diversity, consisting of Dawsonoceras annulatum (84%), indeterminate orthoconic forms (12%), and Michelinoceras (4%). Specimens consist of incomplete phragmocones infilled with well-lithified gray biomicrite; 88% of specimens contain at least some original shell material, with the remaining 12% of the collection preserved as internal molds. 81% of specimens display one hemisphere characterized by heavily degraded, missing, or “planed off” shell material (traditionally interpreted as the upward-facing side that was subjected to weathering prior to burial), with most of the others impossible to assess due to embedment in matrix. Semi-quantitative indices were developed to describe the relative extents of encrustation (EI), bioerosion (BI), and corrasion (CI), with 0 indicating a pristine surface and 3 indicating a severely encrusted/bioeroded/corraded surface. In total, 61% of all specimens (71% of non-moldic specimens) are encrusted to some extent, with microconchid tentaculitoid mollusks (39%) and trepostome bryozoans (36%) representing the most common encrusters. However, encrustation was relatively light, with a mean EI of 0.45 for the entire collection and mean EI of 0.74 for encrusted specimens. Bioerosion structures consist exclusively of the ichnogenus Ropalonaria and occur in 40% of specimens (43% of non-moldic specimens). As with encrustation, bioerosion was not intense, with a mean BI value of 0.33 for all specimens and 0.37 for non-moldic specimens. Corrasion values displayed a mean value of 2.02 for all specimens and a mean value of 1.80 for non-moldic specimens. Collectively, these data suggest a paleoenvironmental setting characterized by a slow background sedimentation rate, in which cephalopod shells or upper portions of partially buried shells were exposed on the seafloor for a limited amount of time following death and prior to final burial.