Biostratigraphy, Biotopes and Biogeography in the Lower Devonian (Upper Lochkovian, Lower Pragian) of Nevada

The lower and upper subzones of the Quadrithyris Assemblage Zone are both recognized in Nevada, in allochthonous units derived from the regressive, Lochkovian age reef and bank complex. Quadrithyris Zone assemblages occur as mixtures of two or more communities within the Old World faunal realm. The superjacent, Pragian age fauna of the Spinoplasia Assemblage Zone also comprises mixed communities, whose main constituents can each be identified by deduction because an autochthonous fauna, of the Trematospira Assemblage Zone, is known. These two Pragian age assemblages are from a transgressive sequence laid down on a depositional ramp and they belong to the Eastern Americas Realm. Acrospirifer split from Howellella and the two genera coexisted in adjacent communities in Nevada during Spinoplasia Zone time. The boundary between the Eastern Americas Realm and the Old World Realm in Nevada is a boundary between cratonic and extra-cratonic brachiopod biofacies respectively, a biogeographic framework similar to that of the Cambrian trilobites. Analysis of the Eastern Americas Realm in other regions suggests that it is essentially a cratonic biofacies. The Malvinokaffric Realm of the Devonian is also a cratonic biofacies, but it lacks an extra-cratonic biofacies such as adjoins the Rhenish and, in places, the Appalachian provincial units.