SMALL CARBONATE BUILDUPS IN THE MIDDLE SILURIAN OF SOUTHERN INDIANA: INFLUENCES ON ECHINODERM AND TRILOBITE TAPHONOMY

Small carbonate buildups, often termed microbioherms, are conspicuous at a major flooding surface within the middle Silurian (Wenlock: Sheinwoodian) Massie Formation throughout much of the greater Cincinnati Arch region. However, most localities where these bodies are present preclude detailed description and analysis with regard to effects on fauna due to pervasive late-diagenetic dolomitization, which obscured primary fabrics and altered preserved fossil content. The New Point Stone quarry near Napoleon, southeastern Indiana, managed to escape significant dolomitization and therefore serves as an important site for studying the influences that the presence of microbioherms had on taphonomic patterns within the Massie Formation. The taphonomy of two faunal groups—pelmatozoan echinoderms and trilobites—is notably affected by these carbonate buildups. Echinoderms, specifically the monobathrid camerate Eucalyptocrinites and the hemicosmitid rhombiferan Caryocrinites, are represented by dendritic attachment structures that densely encrust microbioherms. A long-term occupation is indicated by the presence of radices at multiple horizons within microbioherms. Interestingly, these attachment structures are commonly severely swollen by amorphous secondary stereom, potentially reflecting some form of adverse reaction brought about by encrustation of microbioherms, but enhancing the overall preservation potential, and increasing the likelihood of in situ discovery, of these structures. In addition, large pluricolumnals produced by disarticulation of the long stems of these taxa accumulated in large numbers in areas immediately surrounding buildups. This prevented encrustation of these areas by other pelmatozoans but resulted in increased bioerosion by organisms that bored into the abundant, thick calcareous masses. Irregular masses composed entirely of isolated pygidia and cranidia of bumastine trilobites are found in crevices or underhangs within buildups, potentially reflecting some aspect of molting or unusual hydrodynamic forces in operation. Calymenid trilobites, including the distinctive spathacalymenids, are found articulated in softground muds only in the vicinity of buried or partially buried microbioherms; the reasons for this association are unknown.