Biogeography of Ordovician bryozoans
Abstract The following biogeographic patterns have been determined from a quantitative analysis of the global distribution of the Ordovician genera: Arenig faunas are of low diversity, with provinces appearing in North America and Baltica. Three provinces appear in the Llanvirn: a Baltic Province, a North American Province and a Siberian Province. Bryozoan provinciality is most pronounced in the Llandeilo and Caradoc, with four provinces appearing: North American, Baltic, Siberian and a Mediterranean Province consisting of faunas from Southern Europe and Northern Africa. A dramatic decrease in provinciality occurred in the Ashgill, as only two provinces are recognizable: a North American-Siberian Province and a Baltic-Mediterranean Province. Subprovincial units, or biomes, existed on the North American, Baltic and Siberian plates, as geosynclinal and platform faunas can be distinguished. Biomes are also recognized in North American Caradoc and Ashgill platform faunas, and correspond to major lithotopes. The Baltic fauna was an open ocean fauna which also inhabited oceanic islands and localities around the North American continental margin. The presence of a Baltic bryozoan fauna in mid to late Ashgill formations of the Missouri-southern Illinois region may indicate the invasion of an oceanic fauna into the North American midcontinent through the subsiding Reelfoot Basin, a precursor to the Mississippi Embayment. Baltic migrants from the Missouri region settled in adjacent midwestern localities, forming the Maquoketa Biome.