Silurian and Devonian reefs: 80 million years of global greenhouse between two ice ages

The Silurian-Devonian (mid-Paleozoic), a time of periodic, exceptional sea level highstands, vast epicontinental sea lanes, and global greenhouse climates well above Holocene norms, also identifies ihe ma ximal extent of Phanerozoic metazoan reef development and the acme of coral and sponge reef biodiversity Two peaks are identified for reef distribution, a mid-Silurian (Wenlock) maximum and a Mid- to Late Devonian (Eifel-Givet-middle Frasne) maximum, with reefs spread to latitudes as high as 45 lo 60 degrees, and major barrier-platform tropical reef belts stretched over more than 2000 km. Reef abundance broadly matched patterns of rise and fall in generic biodiversity for (he major cneiazoan reef builders (corals and stromatoporoids) and algae during this 80 million year episode. The mid-Paleosoic reef community originated in the Late Ordovician, taking over from the Cambro-Ordovician mud mound-dominated reef spectrum. It was only modestly affected by the glaciation-induced end-Ordovician(Hirnantian) mass extinction events Late Devonian reef extinction losses were initiated al the Givetian-Frasnian boundary by major declines in coral, stromatoporoid, and brachiopod bentliic components Reefs then experienced a second global expansion in the middle Frasnian, but with reduced faunal diversity. Catastrophic reef declines began in the late Frasnian rhenana conodont Zone and continued through the final lingurformis Zone, with events probably lasting ca.l Myr.Thecorat-stromatopofoid reef community was eliminated worldwide, with surviving patch reefs and reefalrnud mounds in ihe Famennian constructed by consortia of catcimicrobes, lifhisrid sponges, green and red algae, and foraminiferans. The surviving Famennian corals were primarily solitary, deep-water forms that played no major reef role. The last stramatoporoids died out within the Strunian praesuicata Zone, at the Devonian -Carboniferous boundary Widespread organic-rich carbonates of Civetian through Famennian age, normally taken as indicative of anoxia, do not show any correlation with local or global reef declines The Frasnian-Famennian extinction events foi reefs and reef faunas appear to have been second only to the end-Permian reef losses in terms of severity Metazoan reef extinction in the Late Devonian best parallels evidence for Famennian glaciarions, loss of reef accommodation space assealevels fell, 3nd increasing oxygenation of the atmosphere via the evolution of the first pteridophyte rainforests, forcing the terminal Famennian icehouse phases.