A receptaculitid–echinoderm pioneer community in a Middle Ordovician reef

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Abstract
The Elk River Reef of Tennessee [USA] 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 much like a sponge in appearance. The Elk River Reef and its contained receptaculitids fall within 10-15.degree. of the presently accepted position of the Ordovician equator.