ANALYSIS OF DISARTICULATED PELMATOZOAN COLUMNS FROM THE SILURIAN OF INDIANA: A KEY TO ASSESSMENT OF BIODIVERSITY, TAPHONOMIC BIASES, AND PALEOECOLOGY
Fossil crinoids and blastozoans (pelmatozoans) are dominated by disarticulated columnals and pluricolumnals; however, because of the difficulties of association of these parts with calyx-based taxa, this material has rarely been utilized in studies of paleoecology or calculations of biodiversity. The present case study highlights the value of disarticulated stems in improving interpretations of morphological (and potentially taxonomic) diversity, relative abundance patterns, environmental distribution, and preferences of infesting parasites using Silurian crinoid and blastozoan stem material. Bulk samples of argillaceous limestone, weathered shale, and microbioherm-flank sediment were collected from the Wenlock (Sheinwoodian) Massie Formation exposed at the Napoleon quarry of southeastern Indiana, USA. These samples yielded fewer than 15 pelmatozoan calyx plates or thecae, but produced more than 3000 columnals, pluricolumnals, and holdfasts representing at least 20 distinct species. Columnals were sorted morphologically, assigned, where possible, to calyx-based taxa, based on rare complete specimens, and weighed as a measure of relative biomass. The most abundant taxa, identified from calyces (Eucalyptocrinites and Caryocrinites) also produced the largest masses of stem material. However, more fragile crinoids, e.g., dimerocrinitids and Periechocrinus, are strongly under-represented by calyx material relative to abundant pluricolumnals and attachment structures. Frequencies of encrustation and parasitic embedment structures indicate strong preferences for particular pelmatozoan species. Notably, small pentameric stems attributed to dendrocrinid cladids were disproportionally afflicted with parasitic pits (Tremichnus ispp.). Future paleoecological and biodiversity studies will benefit from continued and increased attention to disarticulated pelmatozoan columnals, pluricolumnals, and attachment structures, especially if these can be linked to calyx-based taxa.