The Base of the Devonian and an Early Devonian Graptolite Succession in Central Nevada

Abstract

A consensus has developed among stratigraphers interested in the Silurian-Devonian boundary that the Monograptus uniformis graptohte zone should be the base of the Devonian. Use of that zone as the base of the Devonian makes correlation with it relatively easy as significant evolutionary developments m several fossil groups have been recognized close to the base of the zone. If the base of the Monograptus uniformis zone is used as the base of the Devonian, then three, and possibly four, graptolite zones may be recognized in Lower Devonian rocks of central Nevada. The zones are characterized by evolution within the Monograptus hercynicus group as well as the appearance of Monograptus microdon, Linograptus posthumus, and Abiesgraptus. The Nevada zonal associations may be recognized in both carbonate rocks and in thrust-faulted argillaceous rocks. Most of the Nevada species appear to be slightly (probably subspecifically) different from their European relatives.