PERMO-CARBONIFEROUS VERTEBRATE FOSSILS FROM THE HALGAITO SHALE, CUTLER GROUP, SOUTHEASTERN UTAH

An examination of faunas spanning the Permo-Carboniferous boundary (approximately 270-290 million years before present) provides an invaluable glimpse into the radiation of early amniotes and their adaptations to a completely terrestrial habitat. Formations containing fossils of this age are best known from North America, especially from Oklahoma, north-central Texas, and the Four Corners region of the southwestern United States; however, recent work in Germany has greatly expanded our understanding of contemporary faunas occurring at a similar paleolatitude within Pangea.

The Early Permian Cutler Group, spanning northeastern Arizona to southeastern Utah, west of Comb Ridge, San Juan County, Utah, has been divided into four formations. From oldest to youngest these formations are the Halgaito Shale, the Cedar Mesa Sandstone, the Organ Rock Shale, and the DeChelly Sandstone. The Halgaito Shale rests unconformably on the Late Pennsylvanian Honaker Trail Formation of the Hermosa Group. Although there are local unconformities between the Cutler and Hermosa groups, recent biostratigraphic studies have suggested that the basal portion of the Halgaito Shale in the area of Glen Canyon National Monument (GCNM) spans the Permo-Carboniferous boundary.

Recent finds from GCNM represent the first recorded locality from the Halgaito Shale inside the monument. Here, the Halgaito is characterized by fine grained red sands, typically in aeolian depositional structures. Fossils are primarily recovered from small channels of limestone, pebble to cobble conglomerates. As such, fossils occur as isolated bones and fragments; diagnostic material is extremely rare. The GCNM locality has produced remains of both freshwater aquatic and terrestrial vertebrates. These fossils along with others collected from adjacent valleys (including John’s Canyon and the Valley of the Gods) are currently under study at CSUSB.