A history of islands in the Iapetus Ocean and their destruction is recorded by widespread occurrences of Lower to Upper Ordovician volcanic and epiclastic rocks and some limestone through the axial parts of the Appalachian-Caledonide orogen in North America and northwestern Europe. Assemblages of Arenigian and Llanvirnian benthic shelly fossils, chiefly brachiopods, from these rocks constitute the Celtic biogeographic province that is distinguished by its endemic genera and by some that are older than in other biogeographic provinces, some that are younger than elsewhere, and others that have not hitherto been found together. Evidence from these fossils and from the geology of the rocks pertaining to them suggests that North American terranes that included islands of Arenigian-Llanvirnian age lay at middle to high latitudes, both in mid-ocean and within the Armorican continental margin, whereas coeval rocks in west-central Ireland and Norway that have Toquima–Table Head shelly fossils were originally part of the Laurentian continental margin.
Eradication of the Celtic assemblage and the reduction of Ordovician faunal provinciality in the Appalachian-Caledonide region coincide with Llanvirnian obduction-subduction of hundreds of kilometres of Iapetus Ocean crust. Late Llanvirnian and younger Ordovician rocks in the remnant basin, including some of peri-insular origin, have brachiopods of Scoto-Appalachian affinities. No discrete islands of Ashgillian age have been recognized, although fossiliferous volcaniclastic and polymictic conglomerates indicate intermontane basins in some places. Shelly fossils in these rocks, and those in the siltstones and limestones from the Gaspé Peninsula, are like those of Ashgillian age elsewhere.