COMPARING SCLEROBIONT COVERAGE OF RAFINESQUINA ALTERNATA IN HARDGROUND AND SOFT-BOTTOM SUBSTRATE SETTINGS IN THE CINCINNATI ARCH REGION (CINCINNATIAN, UPPER ORDOVICIAN)

Sclerobionts are more sensitive indicators of paleoenvironmental changes than host shell substrates upon which they encrust (Lescinsky, 1995). Recent work has established sclerobiofacies on the host brachiopod Rafinesquina as an independent bathymetric proxy in soft bottom substrates in the Lt. Ordovician Cincinnati Arch region (Smrecak, 2008; Brett, et al., 2012). Metrics used to establish the sclerobiofacies were applied to bionts on deployed shells off the coast of Lee Stocking Island, Bahamas (Brett, et al., 2011) with high agreement, suggesting that aspects of sclerobiont coverage may vary predictably with depth through geologic time. However, as sessile, filter feeding organisms, sclerobionts are also highly susceptible to the influx of sedimentation (Mistiaen, et al., 2011). This work compares sclerobiont suites encrusting on Rafinesquina cemented in a hardground (Meyer, 1990, Shroat-Lewis, 2011) with those found on the same hosts in shallow euphotic zone soft bottom samples in the Maysvillian (Grant Lake Fm., Bellevue and Mt. Auburn Mbrs.), and similar environments in the Richmondian (see Vogel and Brett, 2009). Hardgrounds are formed by in situ substrate lithification during times of low sedimentation, frequently during widespread marine transgressions (Brett, et al., 2011; Cornell, et al., 2004), while soft-bottom substrates reflect a more consistent influx of sediment. Metrics including richness, areal coverage, occurrence, and encrustation frequency are used to evaluate sclerobiont suites in hardground and soft bottom samples to discern how sedimentation rate impacts sclerobiont encrustation. Preliminary analysis suggests low sedimentation rate dampens the areal coverage of host shells significantly (from an average of 18%/shell (STD 9.36) in soft-bottom substrates to 7% in the hardground setting). Sclerobiont richness in the hardground sample was higher than soft bottom substrate samples, with 3 taxa, including edrioasteroids, documented only on hardground hosts. Hardground sample occurrence and encrustation frequency fall within the expected range for shallow euphotic zone sclerobiont suites, but richness is significantly higher than expected. These data suggest that suites or sclerobiont taxa may be effective indicators of sedimentation in addition to paleodepth.