Grallator Hitchcock, 1858

Ichnogenus Grallator Hitchcock, 1858

Referred specimens: UWGM 7435, a partial right pes trace preserved in concave epirelief was collected in the upper Jelm Formation along the Red Wall, Natrona County, Wyoming; UWGM ‘Jelm Dino Track’ site ( Fig. 5 View Figure 5 ).

Description: UWGM 7435 is a small (9.0 × 5.6 cm) partial pes impression preserving digits II–IV and, potentially, a very faint manus impression. The isolated slab was found ex situ, though confidentlyconstrainedto 1–2 mofstratigraphicthickness (topographic constraint) ~ 15 m below the top of the Jelm Formation. The track is located on a 21.5 × 12-cm slab of fluvial sandstone with broad-wavelength, ripple-marked bedding; the preserved trace is digitigrade and tridactyl, mesaxonic where digit III is longest and potentially quadrupedal ( Fig. 5 View Figure 5 ). The pes digits are relatively straight, long, and slender with small acuminate claw impressions. Pads are observable, but not sharply defined. Digit IV is closely aligned to III with a 7.5° divarication. Digit III to II has just over double the divarication (17°). The impression of digit IV is c. 8 mm wide with little variance, while digits III and II are c. 13 and 11 mm, respectively. Although lacking morphological details, a faint impression anterolateral to pes digits II–III is interpreted to be a manus with three anterolaterally projected digits. Because of this ambiguity in manus identification, we consider this trace within the Atreipus – Grallator plexus, but cannot confidently assign it to either Atreipus or Grallator . Considering the ecological implications of a Grallator (i.e. dinosaurian) tracemaker, we conservatively assign this trace to Atreipus .