SHELL ENCRUSTING EPIBIONT BIOFACIES ALONG DEPTH/PHOTIC ZONE TRANSECTS: COMPARISON OF RECENT AND LATE ORDOVICIAN GRADIENTS

Encrusting organisms have been valuable tools in many studies ranging from life habit patterns to paleobathymetric indicators. In this study, preliminary comparison of epibionts encrusting on ubiquitous brachiopods along an Ordovician depth gradient suggests aspects of guild structure, dominance, richness, and composition may indicate distinctive epizoan community structures. Vogel and colleagues have calibrated modern microendolith borings within marine photic (light-intensity) zones at Lee Stocking Island, Bahamas (LSI) and recognized ~ 5 distinct, light sensitive microendolith assemblages. Extensive epizoan data collected by the Shelf and Slope Taphonomy Initiative (SSETI) further permits excellent calibration of epibiont diversity, guild structure and % coverage with light zonation. Recent work by Vogel & Brett (unpublished data) indicates several distinct microborings diagnostic of particular photic regimes at LSI are also seen in Lt. Ordovician brachiopods from the Cincinnati Arch region of Ohio and Kentucky, thus outlining three separate photic zonations; upper euphotic, lower euphotic, and aphotic/dysphotic. This permits direct comparison of encrusting epibiont diversity, composition and coverage within photic zones between the modern and the Orodovician.

This work compares epizoans on Rafinesquina, Hebertella, Platystrophia, Dalmanella, and Sowerbyella in identified photic zones of the Edenian - early Richmondian to those in modern LSI. Initial data show the relationships of skeletalized encrusters in comparable photic zones. Serpulids and encrusting foraminifera that dominate most modern zones are lacking in Ordovician samples, but bryozoans are abundant in the lower-upper euphotic zones in both cases. As with the modern, Ordovician dysphotic/aphotic zones were occupied by a low- density and -diversity suite of small bryozoans; whereas lower and upper euphotic zones contained a more diverse association dominated by bryozoans and inarticulate brachiopods. Percent encrustation, which attains a peak of about 17% in LSI's upper euphotic, is lower in the Ordovician and dominated by bryozoans. Overall, results suggest endolith associations are more evolutionarily conservative than encrusters, but the latter do show somewhat analogous depth related patterns through time.