Boom and Bust Cycles In Paleozoic Reefs: Comparing Late Ordovician and Late Devonian Mass Extinctions and Subsequent Recoveries

Coral and stromatoporoid sponge reefs, with calcimicrobe binders, dominated Late Ordovician through Late Devonian shallow water tropical shelf ecosystems. Such reefs expanded at exceptional sealevel highstands, flooding continent interiors, a ‘supergreenhouse' lacking polar glaciation, and atmospheric CO2 from 10x to 20x modern concentrations. Most of these reefs displayed a decidedly ‘modern' aspect, utilizing nearly all the techniques to build complex skeletons, skeletal growth rates, building barrier complexes vastly exceeding the size of the modern Great Barrier Reef, and structured by ecologic zonation comparable to the Recent. Different is that the corals were precipitated in calcite oceans (with calcite skeletons), in contrast to the aragonite of today. Yet this equatorial reef ecosystem suffered two setbacks, (1) at the Ordovician-Silurian mass extinction (ME) boundary, from which reefs took some 3-4 myr to recover, and (2) the Late Devonian (Frasnian-Famennian) ME events with recovery requiring ca. 12+ myr. ∂18O data confirm that both declines were due to global oceanic cooling and sealevel drawdowns: the O/S events due to Saharan-central African glaciation, and the F/F events to glaciation centered in Brazil. The three late Katian-Hirnantian episodes eliminated ca. 30% of the reef biota, with surprising evolutionary innovations during the extinctions that brought in ‘Silurian' faunas, and reef recovery retaining the same coral and stromatoporoid sponge families and genera. The F/F, extinctions eliminated ca. 70% of the reef and peri-reef genera, with severe losses at the ordinal and family level (e.g. complete loss of the stromatoporoids except one family persisting to the late Famennian), loss of all colonial rugosans (only deep water solitaries survived), and most tabulate coral genera. Associated peri-reefal and reefal shelly taxa were equally affected.