The History of Early Mesozoic Reef Communities: A Three-Step Process

Abstract

The Triassic to Jurassic records the transition from late Paleozoic to Mesozoic reef communities and can be regarded as the modernization of reefs. This 60-million-year interval was especially crucial for the restructuring of reefs. Mass extinctions and evolutionaty innovations profoundly affected the composition and paleoecologic structure, resulting in especially the loss of Permian-type holdovers and the rise of large skeletal organisms in the framework buildingguild, which in turn resulted in major changes in the early Mesozoic reef ecosystem. Rather than a single step, the evolution of these reefs is viewed as a three-step process-steps in which mass extinction events exerted major influences. The first step took place following the Permo-Triassic extinction, and began with a protracted nonreef interval, represented by the Early Triassic (Scythian stage). When conditions on shallow shelves improved in Middle Triassic time it once again permitted warm-water carbonate deposition to occur. Survivors of the Permian reefs then returned to establish essentially Permian-type reef communities in the Anisian and Ladinian stages of the Middle Triassic. These holdover communities, dominated by Tubiphytes, calcisponges, bryozoans, and calcareous algae, were long-lived, lasting some 17 million years. In the second step, the ecologic structure and dominance of reef ecosystems were changed by a late Triassic (Karnian) reorganization event. This was due to a smaller scale mass extinction eventfollowed by rapid turnover of the reeffaunas. The succeedingLate Triassic (Norian) reef ecosystem was longer lived (19-27 million years' duration). It culminated in the latest Triassic when high-growing scleractinian corals in reefs of the Tethys region increased dramatically in importance. They eventually became significant on the reef crest, contributing substantial framework, thus allowing for scleractinians the first approach to organic framework-produced reef rims. It is argued that the importance of corals in the latest Triassic arose from a subtle ecologic event: the onset of coral-zooxanthellae symbiosis. Following this innovation, a third extinction, the end-Triassic event, once again decimated corals and reefs. After another hiatus in reefs during the Early Jurassic (Hettangian-Early Sinemurian stages) lasting 4-10 million years, the third step in the evolution of reefs began in the early Jurassic (Pliensbachian Stage) when carbonate deposition resumed. By the Middle Jurassic, coral-algal-dominated framework communities once again thrived and most Permo-Triassic holdovers were gone. Reef activity primarily centered in the tropical to subtropical Tethys seaway, where diverse communities flourished within different paleogeographic settings. It was there in the relatively uniform, warm-water settings that coral-algal symbiosis most likely began. Other regions of reef community development, at near-tropical latitudes in the ancestral Pacific (Panthalassa Ocean), existed on volcanic seamounts and subsea plateaus as well as fringing volcanic islands. Such oceanic settings may have served as refugia during times when environmental perturbations adversely affected the Tethys. Moved by seafloor spreading over time, the organisms inhabiting such oceanic settings may have traveled far, experiencing various degrees of isolation. Displaced terranes now lodged along the Pacific Rim contain the remains of hese reef communities. Their detailed paleontologic study is elping explain and reconstruct evolution and paleobiogeography before the opening of the Atlantic-Pacific marine corridor in Middle Jurassic time, which resulted in free faunal exchange between the Tethys and the eastern Pacific.