HIGH-ACTION LAKE FLOOR: A VERTEBRATE ICHNOASSEMBLAGE ON A MOBILE SUBSTRATE, LOCKATONG FORMATION, SE PENNSYLVANIA

A quarried argillite block of the Lockatong Formation (Carnian, Upper Triassic) reveals a rich assemblage of biogenic structures dominated by vertebrate traces. Undulating microtopography, where small ripples (wavelength: 4.5-6.0 mm) dominate the depressions, is traversed by five sinuous grooves with a preferred orientation of 40-80° to the ripple crests. The regularity of the traces (wavelength: 30.6-40.1 mm; average amplitude: 7.1 mm) and negligible bifurcation, indicate that they were produced by the anal or caudal fin of a fish (Undichna isp.). These traces suggest permanent buoyancy and some degree of orientation with respect to the current. At least six tetrapod footprints with differences in size and the number of digits likely belong to two tracemakers. Of the three available tetrapod tracks on the sole of the removed fragment (positive hyporelief), one is five-toed and two are four-toed. The pentadactyl track (total length: 17.3 mm; mean interdigital width: 1.7 mm) is interpreted as Gwynnedichnium isp. Tetradactyl prints, with additional examples as true negative epirelief on the remaining paleo-surface, have an average length of 8.0 mm and interdigital width of 4.2 mm. These may be examples of an incomplete preservation of the fifth digit or manus traces. Unidentified 3-5-mm-wide traces interrupt ripple crests and may represent either invertebrate trails or drag marks. The naturally weathered paleo-surface provides a unique window into a shallow Triassic lacustrine ecosystem and helps constrain the mechanical properties of the bottom sediment during tracemaking activity.