TRACE FOSSIL OF A WALKING FISH FROM THE POTTSTVILLE FORMATION (LATE CARBONIFEROUS) OF ALABAMA

In the past decade or so, fish trace fossils have become more recognized for their paleontological value, sometimes providing the only evidence of fish in paleoecosystems. Fish trails, caused by fins that interacted with sedimentary surfaces, are particularly important for assessing fish behaviors that might not be as readily interpretable from their body fossils. Fish swimming just above sediment-water interfaces caused most such trails, which were made as incisions by their caudal, anal, pectoral, or pelvic fins. In contrast, fish trace fossils that demonstrate “walking” behaviors, in which a fish used its lateral fins to pull itself forward along a surface without swimming, are quite rare. Thus I am pleased to report a fish “walking” trace fossil from the Pottsville Formation (Late Carboniferous, ~310 mya) in northwestern Alabama. The trace fossil, preserved in a laminated silty shale, was recovered from the Steve Minkin Paleozoic Footprint Site and is stored and curated at the McWane Science Center in Birmingham, Alabama as specimen MSC-27015. The trace fossil is a “finway” (rather than a trackway), in which the fish left alternating linear imprints (probably from its pectoral fins) a central undulating groove from its caudal fin, and a faint ventral body-drag impression. The “finway” is 24-26 mm wide on its exterior and 12 mm on its interior, the latter indicating probable fish body width. Pectoral fin traces were 11.5 +/- 2.4 mm long and 1.2 +/- 0.9 mm wide (n = 17). Caudal trail amplitude is 7.4 +/- 0.2 mm with a wavelength of 17.0 +/- 0.3 mm (n = 3); average “stride” (same-side fin distance) was 17.8 +/- 2.3 mm (n = 16). The fish mostly moved straightforward, but turned abruptly to its right toward the end of the preserved trail. This change of direction was likely an obstacle-avoidance reaction to buried plant debris preserved 3-4 cm to its right. Moreover, water depth must have been low enough to have forced this fish to walk instead of swim. The sedimentary environment, which is interpreted as an upper-estuary mudflat, may have been at slack tide then, stranding this little fish so that it had to take a stroll.