VERTEBRATE TRACKS OF THE EARLY TRIASSIC RED PEAK FORMATION, CENTRAL WYOMING, USA

       The Red Peak Formation, the basal member of the Chugwater Group, frequently outcrops along the flanks of Laramide structures throughout Wyoming.  Previous studies have suggested a wide range of depositional environments for the Red Peak Fm including, outer and inner marine shelf, tidal flat, estuary, and fully terrestrial fluvial, and eolian settings.  Such contradictory environmental interpretations have motivated an ongoing investigation seeking to clarify the depositional history of these enigmatic Triassic rocks. 

       An integrative approach to sedimentology is used to constrain the depositional history of the Red Peak Fm.  Initial field observations demonstrate an abundance of vertebrate tracks and traces within the top 30 m of the sections along the Southern Bighorn and Northern Owl Creek Mountains.  Traces include abundant Rhynchosauroides sp. and Procolophonichnium sp., rare Emydhipus sp., and a single track of c.f. Brachichirotherium.  The Rhynchosauroides tracks are found on thin (2-8 cm) very fine grained sandstone containing slightly asymmetrical ripples and parallel laminations.  Emydhipus tracks and traces are most commonly found as casts on the bases of small (10 m) cut and fill structures and larger (50 m) fluvial sand bodies.  These tracks exhibit a wide trackway (16-18 cm), low pace angle (60-70°), inline manual and pedal tracks, and elongate scratch marks.  This track morphology and behavior is consistent with recent actualistic icnhology studies of terrestrial turtles.  Vertebrate ichnology in the Red Peak Fm suggests deposition in a fully terrestrial environment and extends the possible stratigraphic range of tutles 20 myr beyond the oldest body fossils of turtles (e.g. Odontochelys; 220 mya).  Furthermore, the presence of Emydhipus tracks supports a terrestrial origin for turtles and agrees with the proposed hypothesis (by others) of a secondary loss of the dermal carapace in the marine turtle Odontochelys.