The longest delay: Re-emergence of coral reef ecosystems after the Late Devonian extinctions

Reefs are an excellent tool for tracking marine-ecosystem changes, especially through mass extinction transitions. Although metazoan reefs proliferated during the Phanerozoic, prolonged metazoan reef-recovery intervals often occurred after extinction events. Here, we document and review the reef-recovery interval following the Late Devonian Frasnian-Famennian (Kellwasser) and end-Famennian (Hangenberg) mass extinctions, which eliminated the largest area of metazoan (stromatoporoid-coral) reefs of the Phanerozoic. Previous reports of the late Visean coral bioconstructions from western Palaeotethys Ocean, may mark the first metazoan reef proliferation after the Hangenberg extinction. In this study, abundant coral reefs, coral frameworks and coral biostromes were described in detail for the first time from the late Visean strata on the South China Block (eastern Palaeotethys Ocean). The occurrence of these coral bioconstructions further suggests that the late Visean coral reef recovery may have been a widespread phenomenon. Based on the high-resolution reef database constructed in this study, three sub-intervals of the Mississippian metazoan reef recovery were distinguished, which are (1) metazoan “reef gap” phase (MRG) without metazoan reefs during the Tournaisian; (2) metazoan reef re-establishment phase (MRR) containing a few metazoan reefs from early Visean to early part of the late Visean; and (3) metazoan reef proliferation phase (MRP) with global coral reef flourishment during the middle part of the late Visean (late Asbian to early Brigantian substages). Hence, coral reef ecosystems proliferated and became dominant in marine ecosystems during the late Asbian to early Brigantian, indicating a prolonged metazoan reef recovery of about 12 Ma and 23 Ma until the MRR and MRP, respectively. Coral reef proliferation at this time shows that the Mississippian was not solely a period dominated by microbial reefs. Late Visean coral reef development coincided with increased nektonic and benthic diversity, showing that metazoan reef recovery closely tracked overall marine ecosystem evolution. Even compared with other slow reef-recovery intervals, such as the middle-late Cambrian and Early-Middle Triassic with the intervals until the MRR and MRP of 5 Ma and 2 Ma, and 15 Ma and 9 Ma respectively, the Mississippian metazoan reef recovery was the longest in reef history. Harsh climatic and oceanic conditions were present during the Mississippian, mainly including the widespread marine anoxia during the middle part of Tournaisian and the following recurrent glacial and interglacial climatic episodes with frequent changes in sea level, sedimentary facies and sea-water surface temperature, which may have stymied metazoan reef recovery during this time. During the late Visean, marine communities flourished during a phase of relative warm conditions and high sea level, and coincided with the long-delayed re-emergence of coral reef ecosystems after the Late Devonian extinctions.