Callipterid peltasperms of the Dunkard Group, Central Appalachian Basin

Abstract

Abstract The Dunkard Group is the youngest late Paleozoic rock unit in the Central Appalachian Basin. Its age, however, remains controversial. In its southern and western two-thirds the Dunkard is comprised largely of red beds, sandstone and siltstone channel deposits and paleosols. In its thickest, most northerly exposures, in southwestern Pennsylvania, northern West Virginia, and east-central Ohio, much of the lower part of the unit is composed of coals, non-marine limestones and gray, often calcareous, paleosols. Age dating is confounded by the non-marine nature of the deposit and by the lack of dateable volcanic ash beds. Dunkard fossils include plants, vertebrates, and both aquatic and terrestrial invertebrates. Most of the fossil groups point to an age very close to, if not including, the Pennsylvanian-Permian boundary, though the exact position of that boundary is uncertain. Callipterids make their first appearance in the Dunkard flora in the middle of the Washington Formation and continue into the Greene Formation, but in different beds from those containing wetland floral elements. Publication of these plants in the "Permian Flora" of Fontaine and White (1880) created an immediate controversy about the age of the unit because Callipteris conferta (now Autunia conferta) was, at the time, considered to be an index fossil for the base of the Permian. Subsequent collecting has revealed these callipterds to comprise four species: Autunia conferta, Autunia naumannii, Lodevia oxydata and Rhachiphyllum schenkii. Callipterids and the conifers with which they are sometimes associated are typically found in seasonally dry equatorial environments and most likely constitute an environmentally controlled biofacies. This biofacies is not well known, resulting in limited biostratigraphic utility.