TETRAPOD FOOTPRINT ASSEMBLAGE FROM THE UPPER TRIASSIC PASSAIC FORMATION, MONTGOMERY COUNTY, PENNSYLVANIA

Northeast of Graterford, Montgomery County, Pennsylvania, in a tributary of Perkiomen Creek, strata of the Upper Triassic (Norian) Graters Member of the Passaic Formation yield an extensive tetrapod footprint ichnoassemblage. The footprints come from an approximately 2-m-thick interval of red-bed siltstones and fine-grained sandstones with mudcracks and some ripple laminations, and occur on multiple bedding planes. Four ichnogenera are present: (1) Atreipus (ornithischian dinosaur): relatively large (12-17 cm long pes) tridactyl pes imprints in which digit III is longest and the thick digits have oval metatarsal-phalangeal pad imprints, associated with much smaller (2-3.5 cm long) tridactyl, blunt-toed manus imprints; because digits II and IV have nearly equal forward projection, and the claw bases of digits II and IV are nearly opposite the crease between the phalangeal pads of digit III, these tracks are assigned to A. milfordensis; (2) Grallator (theropod dinosaur): relatively small (8-10 cm long pes) tridactyl pes tracks with thin, pointed digits, a compact foot and no manus tracks; (3) Rhynchosauroides (small archosaurs): small (1-2 cm long) lacertoid tracks of a quadruped in which manus and pes are of nearly equal size and nearly overstepped, and in which digits increase in size from I to IV, and digit V is much smaller and everted; and (4) Brachychirotherium (probably aetosaur): relatively large (18 cm long pes, 6-7 cm long manus) tracks of a quadruped with a pentadactyl pes and much smaller tridactyl manus with thick digits and small claws. The most extensive track surfaces yield abundant Atreipus, a few Grallator and rare Brachychirotherium. A stratigraphically higher bedding plane yields all of the Rhynchosauroides tracks. The total number of collected tracks numbers in the hundreds, making this one of Pennsylvania's most extensive Late Triassic tracksites. The footprint assemblage from Graterford is characteristically Late Triassic, but the abundance of ornithischian tracks assigned to Atreipus is unusual. It runs counter to recent claims (based on a re-evaluation of the body-fossil record) that there are no Late Triassic ornithischians.