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A nonmarine pelecypod occurrence was reported in 1977 within a zone of gray shaly mudstones yielding carbonized plants at Promontory Butte, Mogollon Rim, east-central Arizona. On paleobotanical evidence, the zone appeared then to be either of Late Carboniferous or Early Permian age. Work on 50 measurable shells indicates that the fauna is of Anthraconaia protracta Eagar and has wide variation. Standard dimensions of the Arizona fauna are identical to those of the type assemblage of A. protracta, which forms a thin band immediately above the Benwood Limestone of the Monongahela Group near Pittsburgh, Pennsylvania, 2,700 km to the east. The Arizona shell varieties match 50–60 percent of those from the band in Pennsylvania. Matching falls off rapidly to nil 70 m below this horizon and 100 m above it. Small differences in the trends of variation of the shells from Promontory Butte and Pennsylvania are accompanied by small differences in their respective host sediments, as reflected in their organic carbon and carbonate contents. When these results are compared with earlier conclusions on recurrent, apparently ecophenotypic, changes in the shape of Anthraconaia from other horizons in the Carboniferous and Lower Permian, they provide further evidence that the Arizona and Pennsylvania faunas were approximately contemporary. When the two bands are correlated, the Promontory Butte band is placed in the upper Pennsylvanian and in the upper Virgilian as traditionally accepted.

The geologic features of principal interest at this site are revealed in the rugged walls of Fossil Creek Canyon in central Arizona. Access in most weather conditions is by a partially paved gravel road that intersects Arizona 87 at Strawberry (Pine 7½-minute Quadrangle, and Fig. 1). The most accessible panoramic overlook is where the gravel road crosses the lip of the south wall. The ideal viewing location, however, is about 0.5 mi (0.8 km) to the northeast near a gravel pit that is clearly marked on the Strawberry 7½-minute Quadrangle. Access to this overlook site is either by foot or by vehicle (other than sedans), and is a short walk from the borrow pit to a nearby vantage point. Excellent general access by foot to the canyon itself is provided by an old Jeep trail that branches from the pit road. It, too, is indicated on the Strawberry map. The heart of the canyon is now designated as Wilderness; therefore, vehicles are prohibited.

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