BIOTIC REORGANIZATIONS AMONG CORALS AND REEF EVOLUTION DURING THE LATE TRIASSIC TO EARLY JURASSIC TIME

Scleractinian corals appeared in the fossil record during the Middle Triassic, about 7-8 million years after the end-Permian mass extinction. Their emergence as reef builders during the Late Triassic coincided with paleoecologic changes and dramatic increases in reef diversity. An episode of late Carnian to early Norian biotic reorganization among reef building corals and sponges of the Late Triassic likely was related to a smaller mass extinction at that time. An intra-Norian event among corals is recognized in the Lacian to Alaunian 1 intervals. Coral diversity during that time reached 50 genera with representatives in the Hexanthiniaria, Volzeioidea, and Coryphylliidae. Paleogeographic isolation and the paleoecologic setting may account for some differences in species composition between the Tethys and more distant reaches of eastern Panthalassa. The biotic turnover leading up to this Norian event has been interpreted as a mass extinction and is recorded not only for corals but also among other reef groups as well. A drop in diversity marks the late Norian-Rhaetian transition when corals exceeded calcified sponges as reef builders. Extinction and habitat reduction took place at that time. Biotic and physio-chemical events at or near the end of the Triassic removed most of the Rhaetian coral diversity and was followed by reef ecosystem collapse with global reduction of carbonate sedimentation. Only a handful of Triassic corals survived into the succeeding Early Jurassic time and these taxa were of low diversity, inhabiting mostly non-reef settings. Taxonomic study is incomplete but among 25 Late Triassic coral families, only 7 survived into the Jurassic and these contained only 1 or 2 genera. Unlike corals of the Late Triassic, those of the Early Jurassic were solitary or of simple corallite integration type. The evolution of photosymbiosis between corals and algal hosts and the severance of that ecologic relationship following mass extinctions, helps explain some of the changes in coral reef diversity, evolution, collapse and episodes of rebuilding.