Distal effects of glacially-forced Late Ordovician mass extinctions on the tropical carbonate platform of Laurentia: stromatoporoid losses and recovery at a time of stress, Anticosti Island, Eastern Canada

The Anticosti carbonate platform-ramp that developed from Katian and Hirnantian through Llandovery (Early Silurian) time was situated in a tropical setting at ca. 25º latitude south of the equator, on the southern side and eastern flanks of Laurentia. This was separated from glaciated Gondwana to the south and subtropical Baltica to the east by relatively wide, 2500 km+ oceans. Sediments were deposited in a mid to outer shelf monsoonal environment at, or periodically below, typhoon wave base. During the late Katian (Richmondian), the columnar, ‘tree trunk-like’ aulaceratids (order Labechiida) were the volumetrically dominant stromatoporoids on Anticosti, some 3-4 m long and up to ca. 40 cm in diameter, anchored or rooted in the carbonate sediment by early cemented ‘fence-posting’ strategies, but usually found as storm-redeposited, broken fragments (Cameron and Copper, 1994). These are now assigned to the genus Aulacera Plummer 1843, who identified them as nautiloid conchs, but were a decade later described by Billings (1857) as a new genus, Beatricea, naming two species from the Vaureal Fm, and pigeon-holed taxonomically as ‘Plantae’. These Aulacera, with large central, sometimes off-set megacysts, and layers of laterally concentric microcysts penetrated by pillars, are typical of the upper Vaureal Fm. They vanished in a first extinction wave of Katian benthic faunas, below the base of the Ellis Bay Fm, named by Schuchert and Twenhofel (1910) for strata of post-Richmondian (post-Katian) and pre- Silurian age sediments of Anticosti, and identified as their new ‘Gamachian’ Stage. The only tabular to domal stromatoporoids of the Vaureal Fm belong to the rare labechiid genus Pseudostylodictyon, found in reefs of the upper Katian Mill Bay Mbr, ca. 50 m below the top of the formation: it also disappeared in the first end-Richmondian extinction wave.