A nonmarine pelecypod assemblage in the Pennsylvanian of Arizona and its correlation with a horizon in Pennsylvania

Abstract

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.

Succession, palaeoecology, evolution, and speciation of Pennsylvanian non‐marine bivalves, Northern Appalachian Basin, USA

“…This site occurs 5 km southeast of Bentleyville (Eagar 1975, p. 21 and figure 6). A well-preserved fauna attributed to the natural species Anthraconaia protracta was recognized here and earlier in Arizona (Eagar and Peirce 1993) and, remarkably, near Olso, Norway (Eagar 1994a), where the range of variation is wide but distinctive. Logarithms of standard shell dimensions pennsylvanian non-marine bivalves, usa 131 confirm the identity of the bands in these three now distant areas, which were formerly much closer.…”

“…It is distinguished from the holotype of the natural species, A. protracta, in lacking the posterior taper of the latter. This is shown by Eagar and Peirce (1993), who contrast the varieties of Figure 7, q, A. extrema sp. nov., and Figure 7, q 0 , A. protracta.…”

“…1F), and also of the A. saravana group (Eagar 1975, plate 1E). Dimensions of shells and the carbonate content of the shale with the Sewickley Limestone were given by Eagar and Peirce (1993, Non-marine bivalves were found in a 12.5 cm thick band of shale that lies immediately above the freshwater Benwood Limestone (Figure 2) and below the younger 'Limestone A', at Bishop, near Cecil, Washington County, Pennsylvania. It is also referred to as the 'Supra-Benwood Limestone Band'.…”

“…The two systems are in fact compatible and clear if pictographs are used, as in this paper. Thus in the pictographs of Eagar and Peirce (1993) showing the comparison of the faunas of Pennsylvanian and Arizona, both systems include the morphological species Anthraconaia extrema sp. nov. (Eagar and Peirce 1993, figure 7, p 0 , p).…”