Reefs during the multiple crises towards the Ordovician-Silurian boundary: Anticosti Island, eastern Canada, and worldwide

Multiple latest Ordovician (Rawtheyan–Hirnantian) glaciations in central Africa, with concomitant global sea-level lowstands and cooler, restricted, equatorial carbonate shelves and ramps, interrupted by warmer interstadial highstands, had a dramatic global impact on the tropical shallow-water reef ecosystem and carbonate production. With the Ordovician-Silurian boundary strata on Anticosti Island as a global standard for a carbonate shelf-ramp setting, the latest Ordovician and earliest Silurian reveal three reef phases, ended by three extinctions. The first extinction, towards the end of the Rawtheyan, affected the last "Richmondian"-type reefs (Vaureal Formation, Mill Bay Member). The second extinction was less pronounced, ending with reefs at the base of the Prinsta Member (Ellis Bay Formation), interpreted as the top of the Normalograptus extraordinarius graptolite Subzone. The third and most severe extinction phase capped the Laframboise patch reef complex (Ellis Bay Formation), at the top of the Normalograptus persculptus Zone. In the paleotropics, the Hirnantian interglacials showed higher biodiversity than either the preceding Rawtheyan or following Rhuddanian (early Llandovery) warm intervals, a feature perhaps achieved by high innovation rates via introduction of "Silurian" reef biotas during the Hirnantian. The Anticosti reef succession is compared with latest Ordovician reefs from northwestern Europe (Baltic Basin and U.K.), the northwestern margins of Gondwana (Spain and Austria), the Urals, Siberia, Kazakhstan, northeast Russia, and China. Reefs show a global decline from the late Caradoc through late Ashgill, marked by hiatuses towards the O–S boundary. A protracted 3–4 million-year recovery phase for Early Silurian tropical marine biotas, generally without reefs, marked the succeeding Rhuddanian; full reef recovery was delayed until the mid-Aeronian