UTILIZING THE FOSSIL RECORD OF RUGOSE AND SCLERACTINIAN CORALS AS INDICATORS FOR MODERN REEF EXTINCTIONS

At various times through the Phanerozoic, corals have been a component of the reef ecosystem. In the Paleozoic, rugose corals composed a large portion of the reef biota before the Frasnian/Famennian mass extinction. In the Mesozoic, rugosans had been replaced by scleractinians, which later became a dominant reef builder in the Cenozoic. Modern scleractinian corals compose a major part of reef biodiversity. However, reefs are facing global changes at both geologic and human time scales. The fossil record of reefs provides a history of changes in biodiversity that cross mass-extinction events and occurred previous to human influences on modern reefs. Therefore, this record was used to assess the hypothesis that major decreases in coral diversity indicate a mass extinction. The hypothesis was tested by comparing changes in generic diversity of rugose and scleractinian corals across two mass-extinction boundaries (Frasnian/Famennian and Triassic/Jurassic, respectively) and two smaller-magnitude extinction boundaries (Moscovian/Kasimovian and Cenomanian/Turonian, respectively). Previously compiled data were analyzed at the stage level with the diversity metrics of total number of genera, number of originations and extinctions, origination and extinction rates, and turnover rate. Results of these analyses indicate that rugose and scleractinian corals responded similarly to mass extinction events by a decrease in generic diversity, but for smaller-magnitude extinctions rugosan diversity decreased and scleractinian diversity increased. When coral diversity was compared to reef abundance, rugosan diversity declined during intervals of higher reef abundance and increased during intervals of higher mud mound abundance, except during Moscovian–Kasimovian stages. In contrast, scleractinian diversity increased whether reefs or mud mounds were higher in abundance. It is inappropriate to assume that a decrease in modern coral diversity necessarily indicates a coming mass extinction. However, changes in coral diversity can be used as an indication of global change, which ultimately may lead to a mass extinction. Understanding changes in coral diversity and their patterns in the fossil record can be applied to the modern to help understand processes and rates of global change.