A receptaculitid–echinoderm pioneer community in a Middle Ordovician reef

Abstract

The Elk River Reef of Tennessee has an assemblage of incrusting bryozoans, ‘incrusting’ echinoderms, and receptaculitids (calathids) concentrated near its base. This assemblage is thought to have initially inhabited small local sites in a substrate of pelmatozoan sand containing various amounts of carbonate mud. The gregarious style of growth of these organisms and their close interrelationships stabilized the substrate and produced a biological hardground, later occupied by colonial corals and stromatoporoids to form the main reef body. The anastomosing root structures of the echinoderms helped stabilize the substrate and hindered sediment movement. In an effort to support themselves, these echinoderms achieved additional baffling effects of their incrusting and engulfing growth abilities. They particularly attached themselves to calathids. Calathids are the most noticeable taxon in the community. They differ from other calathids presently known in that they have a ‘porous' convoluted outer wall very much like a sponge in appearance. The Elk River Reef and its contained receptaculitids fall within 10–15° of the presently accepted position of the Ordovician equator. Summary A unique assemblage of incrusting bryozoans, incrusting echinoderms, and upright branching calathids (algae?) occur in the lower parts of the Elk River Reef in south-central Tennessee. Because of their numbers and incrusting and gregarious growth characteristics these taxa stabilized the substrate by limiting the movement of carbonate mud and skeletal debris. The incrusting bryozoans and echinoderms grew over and around each other and each incrusted calathids, eventually forming a biologic hardground. The echinoderms had the peculiar ability to ‘invade’ the living chambers of dead organisms and completely ‘engulf’ entire fragments. The calathids described herein have a thick, commonly amorphous outer layer formed by the expansion of the heads of laterals which extend outward in all directions from the central axis. The fusion of these irregularly shaped heads forms a ‘porous’ sponge-like layer a feature which has not been previously described in calathids. The local sites stabilized by this assemblage were later occupied by stromatoporoids and colonial corals forming the main body of the reef (for a discussion of the upper parts of these reefs see Alberstadt et al. 1974).