A bstract A large and abundant columnar stromatoporoid, Quasiaulacera n. gen., from the Ellis Bay Formation, up to 3 m long and 40 cm in diameter, marks the Hirnantian (latest Ordovician) of Anticosti Island. Two species are present: Quasiaulacera stellata n. sp. from the basal Ellis Bay Formation (basal Prinsta Member, lower Hirnantian) along the northeastern coast of the island, and the type species Q. occidua n. sp. from the upper Ellis Bay Formation (Lousy Cove Member, upper Hirnantian) in the western carbonate facies of the island. Quasiaulacera is rare or absent in the reefal Laframboise Member (uppermost Hirnantian) of the formation. The new genus differs from Aulacera in the underlying Vaureal Formation (upper Katian) in having a large central axial zone marked by a single stack of large, convex-up cyst-plates, that is surrounded by a middle layer of small, concentric microcyst-plates, in places denticulate, and an outer layer composed of concentric laminae with dense pillars, in which microcyst-plates are either absent or rare. The outer two layers are defined by longitudinal fluting; there are no branching forms. Both species demonstrate a ball-like holdfast system, some with diameters of 30 to 70 cm, microbially cemented into the substrate. Quasiaulacera "gigantism" in the paleotropical Anticosti Basin evolved at a time of global cooling associated with the Hirnantian glaciation in south polar Gondwana, but terminated in mass extinction of the aulaceratids at the O/S boundary in Laurentia. This supports other evidence that the Hirnantian featured not only generic loss, but also innovation and migration in tropical latitudes.