VERTEBRATE SWIM TRACES ON A LATE TRIASSIC LACUSTRINE PALEO-SURFACE, BUCKS COUNTY, PENNSYLVANIA

Bedding planes of numerous quarried mudstone blocks of Lockatong Formation (Carnian, Late Triassic) in Northampton Township, Pennsylvania, exhibit a variety of lake-margin physical structures (ripples, mudcracks) and invertebrate trace fossils. One boulder surface is covered by ~50 sets of slightly curved, sub-parallel striations that are interpreted as bottom contact marks produced by a buoyed organism. Triple (>70%) and double grooves exhibit occasional cross-cutting and juxtaposition. Although directionality, straddle, and limb type cannot be determined, the traces are designated as probable digits II, III, and IV of a vertebrate tracemaker. The sets are 12.5-22.1 mm in total (II-IV) width (with up to 20% asymmetry of II-III/III-IV distances), with lengths varying from 9.3-34.9 mm. Their geometry, 1-3 mm depth, and gradually sloping interdigital surfaces are indicative of claw marks. Thin sections reveal soft-sediment deformation of black clay and light gray silt laminae below each striation. Slightly asymmetric cross-sections and progressive lateral deepening within most 3-digit sets imply a horizontal force component. More than 80% of trace orientations range within 15° of azimuth, consistent with a (uni)directional movement. Although fish were abundant in lacustrine settings of the Newark Basin, they are excluded as tracemakers due to low sinuosity and discontinuity of striation sets. Therefore, we interpret these marks as tetrapod swim traces (Characichnos isp.), with examples of various sizes described in many Mesozoic sequences. Tetrapod ichnogenera of comparable dimensions previously described from the Lockatong and Passaic Formations include Gwyneddichnium (attributed to aquatic tanystropheid archosauromorphs) and Rhynchosauroides (lepidosauromorph/primitive archosauromorph), with Batrachopus (crocodylomorph) typically found in younger (post-Carnian) successions. These reptiles are represented by skeletal remains in lacustrine deposits and are likely responsible for the swim traces. With pentadactyl feet used for propulsion, only two or three longest toes likely contacted the soft substrate. The shallow-water traverse by a group of animals was followed by lake desiccation, as evidenced by large mudcracks and uniform surface oxidation post-dating the traces.