A NEW THEROPOD TRACKWAY WITH AN ASSOCIATED TAIL TRACE FROM THE UPPER JURASSIC MORRISON FORMATION, BIGHORN BASIN, WYOMING, USA

A new partial theropod trackway with an associated tail trace is reported from the Upper Jurassic Morrison Formation, Bighorn County, Wyoming, USA. The trackway is associated with other vertebrate and invertebrate ichnofossils preserved in convex hyporelief on the bottoms of sheet sandstones. Four facies are recognized in the study area: moderately to well-developed paleosols, weakly developed paleosols, ribbon sandstones, and sheet sandstones. Ribbon and sheet sandstones are interlayered with weakly developed paleosols. These three facies are collectively called heterolithic intervals. Heterolithic intervals are bounded above and below by moderately to well-developed paleosols. The heterolithic intervals and well-developed paleosols are interpreted as avulsion and distal floodplain deposits, respectively. Excellent preservation of ichnofossils within the sheet sandstones is attributed to high sedimentation rates during channel avulsion.

The trackway consists of one right pes print and the successive left pes print with a slightly sinuous groove confined to the interpedes area. The left pes print displays positive rotation from the trackway midline and preserves digits II and III, hypex II-III, and a separated digit IV; no heel impression is present. The right pes shows negative rotation from the trackway midline and only clearly preserves digits III and IV and hypex III-IV.

The tail trace passes closely to the medial border of the left pes print and intersects the right pes print. It has a maximum width of 1.3 cm and a V-shaped profile with expulsion rims on both edges. The tail trace appears to be continuous, interrupted slightly only by variation in substrate consistency and invertebrate bioturbation. Where best preserved, the tail trace has a narrow 0.2-cm-wide-ventral keel impression.

Based on the track length:width ratio, V-shaped track outline, elongate nature of digit III, narrow digits, and wide divarication of digits, the trackway is attributed to a theropod dinosaur.