EARLY MESOZOIC REEFS AND THE RISE OF SCLERACTINIANS

Following the Permo-Triassic mass extinction, metazoan reef building did not resume until Middle Triassic time and was centered in the Tethys. During this period the principle players were calcified sponges, Tubiphytes, and calcareous algae which, at high taxonomic levels, were not unlike Permian players. The first scleractinians, evolving from soft anemone, appeared in the Anisian but did not initially take part in reef construction. Middle to Late Triassic reefs of the tropical Tethys expanded in the Ladinian to early Carnian interval. Ecosystems composed of calcareous sponges, microproblematica (including Tubiphyes), algae, and corals increased in importance during this time. During the Late Triassic, the Carnian-Norian reef ecosystem contained similar players but diversity increased markedly with a rise in algae, spongiomorphs and sphinctozoid sponges. It was characterized by a single or multiple small-scale extinction(s), resulting in a major taxonomic turnover among the biotas. The Norian-Rhaetian interval of the Late Triassic was a reef ecosystem turning point. It signaled a major reef expansion and diversification in the Tethys and the rise if scleractinian corals as reefbuilders–an event likely linked with the acquisition of algal symbiosis. Reefs in western North America were isolated from those of the Tethys, of lower diversity, different in composition of the microbiota and much smaller in size, mostly rimming carbonate platforms around volcanically active island arcs. The end-Triassic extinction was severe, resulting in an early Jurassic reef eclipse but was followed by a Middle Jurassic expansion of corals, sponges and hydrozoans. Reefs on isolated island arcs during this eclipse interval, may have provided refuges during post-extinction time, promoting preferential survival of some coral taxa which subsequently repopulated reef ecosystems of the Tethys.