PALEOBIOLOGIC AND PALEOENVIRONMENTAL CONTEXT OF CORAL-BEARING EARLY CAMBRIAN REEFS - IMPLICATIONS FOR PHANEROZOIC REEF DEVELOPMENT

Early Cambrian corals from South Australia have been found within foss il reefs of unusual biological and paleoecological composition. The fr amework of these reefs is composed of a diverse assemblage of calcareo us sponges (e.g., archaeocyaths and sphinctozoans), calcimicrobes, and at least two species of coral-like organisms, one of which is first r eported herein and bears similarities to younger Paleozoic tabulate co rals. Complex growth interactions occur among these organisms, suggest ing that space was a limiting factor in Early Cambrian reef ecosystems , as it is today in modern scleractinian reefs. In striking contrast t o most Early Cambrian archaeocyath-calcimicrobe reefs, these South Aus tralian reefs flourished within energetic, mixed siliciclastic-carbona te shallow-marine environments along the margins of arid, coarse-grain ed, sea-marginal alluvial fans. The implications of these coral-bearin g reefs are multifold. First, their existence not only extends the ran ge of tabulatelike corals to the Botomian (middle Early Cambrian), but it adds an additional clade of participants to the Early Cambrian met azoan radiation event. Second, the existence of Botomian-aged skeleton ized colonial cnidarians necessitates an earliest Cambrian or Neoprote rozoic ancestor for the group. Third, the presence of tabulatelike cor als and their involvement in reef budding prior to the Toyonian extinc tion (late Early Cambrian) challenges hypotheses (e.g., lack of a suit able reef builder after the extinction of archaeocyaths until the Ordo vician) used to explain the paucity of Middle and Late Cambrian reefs worldwide. The presence of these corals on sea-marginal fans contradic ts the perception that early reefs were restricted to low-energy, pred ominantly carbonate subtidal environments.