Reefs Through Time: An Evolutionary View

Abstract

Although reef-like structures formed in the Neoproterozoic, reefs built by metazoans did not appear until the early Paleozoic. From then until the Recent, reefs diversified, underwent extinctions many times and then diversified again. Reef-inhabiting organisms included many different groups from algae to vertebrates as well as enigmatic, extinct suprageneric taxa. Evolution of these groups continued unabated and sometimes resulted in significant changes in the communities making up reefs. These reef groups varied over geologic time with extinction events commonly marking dramatic changes in the biotas. Paleozoic reefs consisted of sponges, corals, foraminifera, algae, bryozoans, and brachiopods, among others. The major extinction event at the end of the Paleozoic eliminated these forms as reef constituents and new groups (e.g., the first scleractinian corals) appeared in the Triassic. The Mesozoic was dominated by sponges, corals, rudist bivalves, and algae, most of which were eliminated in the end-Cretaceous extinction event. The Cenozoic reef biotas included red algae, foraminifera, sponges, corals, various invertebrates, and fish. Throughout the Phanerozoic, these biotas were eliminated by extinction events of differing magnitude. Each event corresponded to warming due to rising greenhouse gases (CO2 and CH4), and ocean acidification caused by lowered pH and anoxia of shallow waters that took severe tolls on reef organisms. These extinction events caused the decline of reef organisms and the reefs they built, resulting in decreased diversity and slower carbonate deposition. Photosymbiotic reef ecosystems failed during extinctions and these failures may have been driven, at least in part, by either the demise of the symbiosis or the extinction of symbionts. Reefs were generally absent in post-extinction times due to different ecologies. The ancestors of the next radiation of reef organisms must have been present somewhere—perhaps in deeper water, remote seamounts or isolated shallow seas. These ancestral faunas gave rise to radiations of reef organisms following several million years of depauperate and unusual biotas. Once underway, these radiations were relatively rapid and were responses to an amelioration of the extinction conditions and an increase in ecological opportunities. They did not restore the same taxa; rather new organisms at the familial or generic levels commonly diversified in most groups. The extinction events of the past do not bode well for the survival of modern reefs because of impending anthropogenic changes. Although humans have caused reef destruction through pollution, sedimentation, nutrient influx, and physical damage, increasing global warming and ocean acidification caused by CO2 and CH4 emissions are the principal threats to modern reef ecosystems with severe degradation or even extinction possibilities. Scientific and political will to change these inputs soon are essential to the survival of reefs, as well as other aspects of modern civilization.