Use of vertebrates to solve biostratigraphic problems: Examples from the Lower and Middle Devonian of western North America

Because of the absence of invertebrate faunas, previous correlations of Lower and Middle Devonian nonmarine sedimentary rocks in the western United States have resulted in various age assignments. Although vertebrate fossils are present in these strata, they have not been widely used for biostratigraphic correlation in North America. Recent work indicates that, despite problems of endemism of these faunas during the Early Devonian, vertebrate fossils do provide a tentative basis for correlation. The lower part of the Grassy Flat Member of the Water Canyon Formation and the Beartooth Butte Formation at Beartooth Butte are shown to be Emsian in age, whereas the Beartooth Butte Formation in the Bighorn Mountains is late Lochkovian to early Pragian. An endemic fauna restricted to the Sevy Dolomite and Lippincott Member of the Lost Burro Formation is also considered to be Emsian. The Middle Devonian fauna has a more cosmopolitan distribution; similar assemblages at disparate localities in North America provide a framework for the use of the fauna in placing age constraints on stratigraphic units. For the first time pteraspidids are unequivocally identified from Middle Devonian strata. Together with a characteristic fauna of arthrodires and antiarchs, they are used to correlate the Yahatinda Formation of Alberta with the Spring Mountain channel of the Lemhi Range, Idaho, and the upper part of the Grassy Flat Member of the Water Canyon Formation, all of which can now be dated as late Givetian.