ENCRUSTING ON A CORAL GRAVEYARD: A REWORKED CORAL BED FROM THE UPPER ORDOVICIAN (CINCINNATIAN, RICHMONDIAN) OF CENTRAL KENTUCKY

A new occurrence of edrioasteroid-encrusted corals in the Bardstown Member of the Waynesville Formation (Upper Ordovician, Cincinnatian, Richmondian) in Mt. Washington, Kentucky demonstrates edrioasteroids’ ability to encrust perforated and irregular surfaces. The Bardstown coral bed consists of mud-smothered, reworked specimens of two large colonial corals: Tetradium approximatum, a Chaetetid tabulate with tightly-spaced septae (0.5–1 mm corallite diameter) and Favistella (Columnaria) alveolata, a Cyathophyllid rugose coral with 2–3 cm corallite diameter. Both corals, which are variably reworked and broken, host encrusters; 36% of T. approximatum host encrusters (n=132) and 21% of F. alveolata are encrusted (n=14). Most of the encrusters are cryptostome bryozoans. However, ten of the tabulates are encrusted by Isorophid edrioasteroids and three are encrusted by stromatoporoids, whereas no rugose corals show evidence of encrustation by echinoderms or sponges. These sclerobionts encrust the tops, bottoms, sides, and broken edges of Tetradium, suggesting these corals were encrusted post mortem after being rolled and broken on the seafloor. These biotic clasts served as high-relief hard substrates above the muddy seafloor for encrusters. However, most encrusters targeted Tetradium despite its significantly smaller size (an average diameter of 20 cm compared to 70 cm in Favistella). Although the rugose corals offer greater areas for encrustation, the more regular surface relief of Tetradium (< 0.5 mm surface relief as opposed to 2 mm in Favistella) creates a more favorable substrate, particularly by Isorophids which have a somewhat inflexible basal surface, although these coral heads still exhibit considerable variability.

Only two other occurrences of edrioasteroid-encrusted corals are known, both of which are reported as “Agelacrinites” encrusting Tetradium in the Lower Silurian and Ordovician Maysvillian of the Cincinnati Arch by Foerste (1895) and Williams (1918) respectively. The edrioasteroids of the Bardstown coral bed share affinities with the “Agelacrinites” reported by Foerste and Williams, however this genus designation is under review. These data suggest that there is a genus-specificity for both host and encruster which persists from the Upper Ordovician to the Lower Silurian.