DISCERNING PATTERNS IN EPIBIONT DISTRIBUTION ACROSS A LATE ORDOVICIAN (CINCINNATIAN) DEPTH GRADIENT

Epizoans may be more sensitive environmental indicators in the rock record than their more ubiquitous shelly substrates. Without accurate re-creation of their environments, this sensitivity cannot be tested. Recent detailed stratigraphic, faunal gradient, and microendolithic boring studies in the Late Ordovician (Cincinnatian), have yielded a detailed paleogeographic analysis of Indiana, Ohio, and Kentucky. Moreover, recent microendolithic studies by K. Vogel have extrapolated a relative depth and light zonation in the Ordovician based on the light sensitivity of modern distinctive boring organisms such as algae, cyanobacteria, and fungi. Three euphotic zonations have been identified in these Cincinnatian deposits; shallow euphotic III, deep euphotic, and dysphotic. This framework allows analysis of epibionts along the Late Ordovician depth gradient.

Preliminary analysis of 15 localities in the Cincinnatian series of OH, KY, and IN indicates that different suites of epibionts dominate at various depths along the gradient. Criterion like occurrence, dominance, diversity, evenness, and percent cover vary predictably with depth. Among encrusters, cornulitids and Sphenothallus holdfasts were most abundant in the deep euphotic deposits, and inarticulate brachiopods were most abundant in shallow euphotic III deposits. Encrusting bryozoans, discerned from one another by morphotypic assignment, were most abundant in all zones, but morphotypic dominance changed with depth. Bryozoan morphology was most variable in the shallowest euphotic deposits, sheet-like encrusting bryozoans with large zooecia were abundant in moderately shallow waters, and paleotubuliporid bryozoans were most abundant in the deep euphotic zone samples. A bryozoan characterized by particularly large zooecia was the only bryozoan seen in the dysphotic zone deposits. Subdividing the epibionts into suites based on Vogel's light zonations, an epibiofacies model for the Late Ordovician is established. Methodology used to discern the epibiofacies seen in the Cincinnatian of the tri-state can be used in other stratigraphic sequences to discover what changes in encrusting life habits have occurred through time.