IT WAS A DARK AND STORMY NIGHT: THE ICHNOSTRATIGRAPHIC RECORD OF A BRIEF EVENT IN THE PENNSYLVANIAN OF NORTHWESTERN ALABAMA

The Steven C. Minkin Paleozoic Footprint Site (Union Chapel Mine) is celebrated for its abundance of vertebrate and invertebrate trackways. Its invertebrate burrows are less well known, but even more numerous. The highwall beds are not accessible, but have yielded abundant tailings that allow study of its trace fossils ex situ. Detailed study of trace fossils that are restricted to the base of what seems to be a single, laminated sandstone bed, probably in alternating sandstone and shale, reveals the response of organisms to a storm event. The bed is situated within the 30 m of strata overlying the Cincosaurus beds that yield vertebrate trackways. The bed is recognizable by its reddish color, which is unusual for the section, and by its fairly consistent thickness of 5-8 cm. The hypichnial traces are small (8.4-38.5 mm long), shallow, bilaterally symmetrical, and subquadrate, generally consisting of series of bracket-shaped (}}}) pads of sediment; several morphotypes can be distinguished. Some may have axial shafts. Based on the traces’ subquadrate outline, bracket-like pads, and embedded resting traces, the most likely makers are arthropods 6-12 mm long and 5-10 mm wide. They do not show a strongly preferred orientation, but some are clustered significantly. At first glance, they appear to be typical resting traces, but on further investigation they prove to be elongate, extending upward into the sandstone bed at both ends. It seems likely that the complete trace is more or less U-shaped, suggesting that their makers dug into the freshly deposited sand to reach the underlying clay, which shows signs of having been eroded. Presumably, the sand was deposited as a powerful storm died down; this was immediately followed by deposition of an unknown thickness of mud. By the time the sand was deposited, storm currents would already have been abating. At some point – possibly during the storm itself – the tracemakers dug into the bed, probably to access the organic-rich mud below. Later, the animals evidently dug their way back out of the bed. Probably the arthropods were not unusual members of the local fauna, but their work was recorded only because their traces were made at a depth of sediment greater than most species preferred.