FIGURE 1. Late Triassic bird-like footprints from the Santo Domingo Formation.

a, View of part of the lower trampled horizon, showing footprints preserved as natural moulds. Note an increase in footprint density at the upper left of the photo. Scale, 10 cm. b, Detailed view of a track preserved as a natural cast and composed of three tetradactyl footprints (arrowed). Note the high pace angulation and rotation of foot axis toward the midline. c–f Close-up of four footprint preservational variants, as natural moulds (c,d) and casts (e,f). All footprints except e correspond to the left hind limb. I to IV refer to first to fourth digit imprints. c, Deeply imprinted footprint lacking pad impressions (note remains of matrix within the footprint). d, Footprint with well-defined pad impressions and rounded ends of digit prints II to IV. Digit I shows claw mark and a single pad impression. e, Footprint with pad impressions, small rounded sole, distal curvature of lateral and medial claw marks away from the foot axis, and shallow hallux impression. Part of a second footprint is shown in the upper right corner. f, Probable undertrack that exhibits an elliptical sole and a moderately large hallucal impression. Lighting is from the top except for b, illuminated from the bottom.