A Middle or Lower Devonian Psilophyte Flora from Central Arizona and its Paleogeographic Significance

Abstract

An assemblage of fossil plants has been discovered about 16 feet above the base of the Devonian section in central Arizona. This section, at the mouth of Flying V Canyon, is rediscribed, and rocks of the lower portion are reported in detail. All Devonian deposits in this region have previously been assigned to the Upper Devonian series, but the plant fossils reported here indicate either Middle or Early Devonian age for the basal part. The plant fossils are at the base of the lower dolomitic member of the Martin formation as denned by Huddle and Dobrovolny (1952). This member is the stratigraphic equivalent of the lower part of the Jerome formation of Stoyanow (1936). Thus it is evident that sedimentation began no later than in Middle Devonian time over a wide area in central Arizona. The plant assemblage consists of branching axes similar to Hostimella and Aphyllopteris; of fertile structures comparable in organization to Cooksonia, Hicklingia Rhynia, Dawsonites, and Hedeia; and of isolated megaspores and meiospores evidently representing several kinds of terrestrial plants. A singular type of fungus-like microfossils, resembling Tasmanites and Leiosphaera in its composition, is also present. The character of the assemblage is evident, but several genera probably are new, and a later taxonomic report is planned. The plant fossils and lithologie facies suggest that the environment of deposition was essentially estuarine or lagoonal. Sedimentation seems to have been continuous between the time of deposition of the plant-bearing bed and the deposition of the marine beds above that contain Late Devonian fossils. A further study may indicate the position of the boundary between the older and younger Devonian series.