FAUNAL CHANGE ACROSS THE ORDOVICIAN-SILURIAN MASS EXTINCTION BOUNDARY ON ANTICOSTI ISLAND, E CANADA

Matching +ve shifts in ?13C and ?180 isotopes in the Rawtheyan-Hirnantian and early Rhuddanian carbonate strata, Anticosti shelly, coral and stromatoporoid faunas show corresponding shifts in community diversity. The Anticosti carbonate platform-ramp sequence, 800m+ thick, identifies progressive shallowing into the reefal Mill Bay Member (Vaureal Fm., late Rawtheyan), with stepdown extinctions defining the disappearance of the classic Richmondian fauna, e.g. Catazyga, 100m+ below the O/S boundary), and replacement by exotic, ? endemic taxa, e.g. the smooth atrypid Xysila. The end-Rawtheyan is the deeper facies, fossil poor Schmitt Creek Member, with remnant Richmondian faunas. The following richly fossiliferous Hirnantian Ellis Bay Mbr, c. 60m thick, is identified by rapid introduction of a ‘ Eurasian' shelly Hindella-Eospirigerina fauna, with ‘Silurian' characters, a feature mimicked in the coral and stromatoporoid faunas. A last gasp of the Richmondian fauna occurs in the highly variable, oscillating facies of the Ellis Bay Fm: eastwards siliciclastics dominate; westwards, carbonates. The final Ordovician phase is the reefal Laframboise member marked by a relatively diverse fauna, and the final appearance of the Ordovician ‘pillar-tree sponges', the aulacerids. The O/S boundary is relatively abrupt, and the cap of the final reef facies, where present some 250 km along strike, commonly shows hardgrounds, cemented surfaces, and storm influence (e.g. broken aulacerids, corals, reef talus). The overlying Fox Point member of the Silurian Becscie Fm, c. 10-15m thick grainstones, is marked by low benthic diversity, lilliput faunas, a general absence of ‘Ordovician' taxa, and presence of some Hirnantian survivors (e.g. Leptaena, Eospirigerina). Stromatoporoids and colonial rugose corals are missing, favositids and heliolitids rare, and small nautiloid conchs of 1-2cm diameters. Reefs do not return to the Silurian sequence until the late Aeronian, some 250m above the boundary.