PALEONTOLOGICAL AND BEHAVIORAL SIGNIFICANCE OF AMPHIBIAN TRACKS FROM THE POTTSVILLE FORMATION (LOWER PENNSYLVANIAN: WESTPHALIAN A), UNION CHAPEL MINE, ALABAMA

Fossil vertebrate tracks are abundant at the Union Chapel Mine in strata above the Mary Lee coal seam (Black Warrior Basin, northcentral Alabama) of the Pottsville Formation and constitute one of the most important Carboniferous tracksites in the world. These tracks, originally described in 1930, now merit new study as a result of many additional specimens recovered recently by the Birmingham Paleontological Society. Tracks are preserved in a gray shale that likely represents a tidal-flat deposit in the upper reaches of an estuary; all trackways were seemingly made on emergent flats. Tracks are about 1-12 cm wide, and although the majority of tracemakers were relatively small animals, some were nearly as large as adult humans. One trackway that reveals manus and pes outlines confirms an amphibian tracemaker with five digits on each foot, bulbous terminations on digits, and an elongated digit IV to the pes. Mean glenoacetabular distance (an indirect measure of torso length derived from trackways) is 5.3 cm, whereas mean trackway width is 5.6 cm (n=25). Low pace angulations explain the latter value, indicating sprawling postures. One trackway shows 38 sequential strides, providing a detailed record of variations in speed and gait. Turning motions were a frequent behavioral variant, at least one specimen indicates a sideward shuffle, and a few specimens have tail drags. Aldrich and Jones (1930, Alabama Museum of Natural History Museum Paper 9) reported Attenosaurus and Cincosaurus as the two most abundant ichnogenera in the assemblage. However, our preliminary analysis of interdigital angles in specimens of these ichnogenera, combined with interpretations of foot movements, suggest that the two ichnogenera were made by the same species of tracemaker. If this hypothesis is supported by more data, then future studies of the tracks should help with ascertaining better estimates of the ontogeny and life histories of Carboniferous amphibians, which are unrepresented by body fossils in this region.