SHALLOW-WATER VERTEBRATE TRACES ON A CARNIAN MUDSTONE, BUCKS COUNTY, PENNSYLVANIA

Numerous quarried Lockatong Formation (Carnian, Upper Triassic) argillite blocks along the Neshaminy Creek (Bucks County, Pennsylvania) contain ripple marks, mudcracks, and disjunct double and triple-striations recently interpreted as vertebrate swim traces (Characichnos isp.). A newly discovered block correlates contains >20 groove sets, with preferred movement reflected in travel directions clustering within 10° of azimuth. Based on quartz veins along the sides of the boulder, the general orientation of swim traces is consistent with previously studied blocks. Drying pattern of the wetted surface highlights the micro-topography and accentuates a single large tridactyl footprint. The Atreipus-like track contains faint claw impressions and distinct interdigital pressure ridge is tentatively attributed to a reptilian (possibly dinosaurian) origin. Digit length varies from 10.5 cm (II/IV) to 15.5 cm (III), with a total track width (II-IV) of 12.0 cm. No other large tracks are present on the available surface, which is consistent with a large pace (>30 cm) of the wading tracemaker. The lack of mudcracks common on other track-bearing blocks indicates a slightly greater depth and/or more rapid burial of the tracking surface. With limited local exposures of Lockatong Formation, exported track-bearing fragments provide important evidence for tetrapods traversing late Triassic lake margins.