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

Seventeen horizons of non‐marine bivalves are described within the Appalachian succession from the base of the Pottsville Group of Westphalian A‐B age to the Uniontown coal of Stephanian C age at the top of the Carboniferous System. A new highly variable fauna of Anthraconaia from the roof shales of the Upper Freeport coal near Kempton, west Maryland, dates from late Westphalian D or very early Cantabrian time, on the evidence of non‐marine shells and megafloras. Below this horizon, the Appalachian sequence reveals zones of Anthraconauta phillipsii and Anthraconauta tenuis in the same order as in Britain, whereas faunas of Anthraconaia of these zones are less common and differ from those of Britain. In all horizons above the Upper Freeport coal all non‐marine bivalve faunas consist of stages in the sequences of two natural species, the groups of Anthraconaia prolifera and Anthraconaia puella‐saravana. The first shows evidence of having lived in well‐oxygenated, probably shallow, fresh water conditions of relatively wide extent. The second group lived preferentially in a plant‐rich environment of relatively stagnant fresh water. Both groups are found in horizons associated with coal seams and may be seen together in the same habitats, but diagrams of variation (pictographs) suggest that there was no interbreeding between the two groups in either the Northern Appalachians or in southern Germany where the species split was first recognized. In the northern Spanish coalfields of Guardo‐Valderrueda and Central Asturia, facies evidence suggests how an initial split may have taken place in the same morphological directions and into the same palaeoenvironments as the later split into two species. Appalachian deposition was generally slow and intermittent with frequent palaeosols. There is also evidence of erosion and of small palaeontological breaks in the sequence, especially near the eastern edge of the Northern Appalachian Basin in western Maryland. The amount of accumulated sediment was less than one‐tenth of that of western Europe when basin centre deposition is compared. We found no evidence of a major palaeontological break representing Westphalian D strata overlain by Stephanian C strata. We figure non‐marine bivalve faunas of Stephanian B age in association with the Pittsburgh and the Little Pittsburgh coals. Two new species of non‐marine bivalves are described: Anthraconaia anthraconautiformis sp. nov. and Anthraconaia extrema sp. nov. Copyright © 2003 John Wiley & Sons, Ltd.