Lower Devonian Gastropod Biogeography of the Western Hemisphere

Abstract Early Devonian biogeography has been based primarily on articulate brachiopods, rugose corals and trilobites. Study of gastropod literature and unpublished collections confirms previously utilized biogeographic units based on other taxa: the Eastern Americas Realm; the Malvinokaffric Realm; and the Old World Realm. Spiny platyceratids, Strophostylus and Crossoceras are restricted to the Eastern Americas Realm which can be informally divided into three paleoclimatic regimes: Hudson Bay Platform — Michigan Basin (tropical to subtropical with abundant evaporites and oolites), Appalachian Basin (subtropical to warm temperate), and the Amazon-Colombian area (moderate to cool temperate with limited to no carbonates). Overall gastropod taxic diversity is highest in Old World Realm faunas, which include more highly ornamented forms such as the oriostomataceans and salpingostomatids, and the genera Kodymites and Euomphalopterus found in Arctic and western North America (Alaska, British Columbia, Yukon and Northwest Territories), Bohemia, and southeastern Australia. Nevada gastropods confirm the shifting biogeographic boundaries recognized previously by other faunal groups. In Lochkovian time this area was part of the Old World Realm. During Pragian-early Emsian time Nevada contained typical Appalachian gastropod genera (Nevadan Subprovince of Eastern Americas Realm). Later in the Emsian (Eurekaspirifer pinyonensis Zone) mixing occurred with elements of both realms co-occurring. By Eifelian time the gastropods were wholly of Old World Realm character. The Malvinokaffric Realm fauna of southern and central South America, Falkland Islands and Antarctica is strongly depauperate, with only four superfamilies represented from the seventeen then extant. Characteristic genera are Plectonotus (Plectonotus) and large species of Tropidodiscus. The reduction in taxonomic diversity from the Old World Realm to the Eastern Americas Realm to the Malvinokaffric Realm is consistent with an interpreted change from warm tropical to warm temperate to the cooler water environment of high paleolatitudes of the southern hemisphere. The distribution of Old World Realm faunas, as well as calcareous algae and oolites favor paleogeographic reconstructions with the Early Devonian equator passing through Alaska and the Canadian Arctic Islands.