BIOTIC CHANGES IN EARLY MESOZOIC REEFS IN TRIASSIC TO JURASSIC TIME

The restructuring of Mesozoic reefs did not happen simply as recovery of previous Permian reefs but rather occurred in discrete steps controlled by reef collapse, evolution of new reef players, and the advent of new paleoecological themes. A post-extinction metazoan gap in the Early Triassic to early Middle Triassic (Anisian) was followed by a recovery phase in the remaining Middle Triassic to Carnian time. There ensued a smaller mass extinction, followed by a post-extinction biotic turnover. This led to a robust Late Triassic (Norian-Rhaetian) reef episode characterized by calcareous algae, microbial crusts, microproblematica, stromatoporoid-like sponges, hydrozoans, sphinctozooid sponges and scleractinian corals. Important biotic events in the Triassic include the survival of some key Permian taxa, the evolution of new higher-level groups, including scleractinians which evolved from soft-bodied, Paleozoic ancestors, and the evolution of key reef bioeroders which facilitated excavation of the reef substrate. Zooxanthellate photosymbiosis evolved among scleractinians which rose to dominance as reef builders in the latest Triassic, alongside sphinctozooid sponges. Paleogeography also was an important consideration. While most research focuses on the shallow Tethys reef diversity, American examples offer new insight into survival dynamics among far-flung tropical islands of the eastern Pacific which are postulated to have functioned as refugia. Paleogeography helps explain anomalous appearances of Permian holdovers in the Latest Triassic Tethys. The end-Triassic reef crisis restructured reefs at lower taxonomic levels and recovery was complete by the end of the Early Jurassic. Although the Late Triassic contained the first zooxanthellate-like corals, some aspects of their paleoecology sets them apart from later Cenozoic to Recent examples.