TRACKWAY OF A SIDEWAYS-WALKING PELYCOSAUR IN THE CARBONIFEROUS MANAKACHA FORMATION IN GRAND CANYON NATIONAL PARK

A recent rockfall along the Bright Angel Trail in Grand Canyon National Park produced a large block of medium-grained sandstone from the Carboniferous (Atokan) Manakacha Formation (second oldest of the four-formation Supai Group). This block contains a conspicuous vertebrate trackway consisting of 28 tracks preserved as impressions, occurring in a single trackway that extends about 1.0 m across the width of the fallen block. Individual tracks are plantigrade, with round sole impressions and three forward-directed digits. Pes and manus tracks are approximately the same size, about 5 cm long. We tentatively assign this trackway to the ichnogenus Chelichnus, which is well known from the Permian Coconino Sandstone but has never before been reported from the Carboniferous. We infer the trackmaker to have been a pelycosaurian-grade reptile.

This trackway is unusual for two reasons. First, it is the first reported vertebrate trackway in the Manakacha Formation and the oldest in Grand Canyon. Vertebrate trackways are common in the Coconino Sandstone, and several have also been reported from the Wescogame Formation (third oldest of the Supai Group), but no vertebrate tracks have previously been reported from the Manakacha Formation. Second, the animal that produced this trackway had an unusual sideways-walking gait; individual tracks are pointed in a direction that differs by 40° from the trend of the trackway itself. The result is an unusual trackway consisting of a series of rows of tracks; each row consists of four tracks, and each row is offset to the right approximately 20 cm, in comparison with the previous row. We interpret this sideways-walking gait to record the influence of a strong current, probably wind, that was pushing this animal toward the right while it struggled to move forward.

Because this important trackway lies directly adjacent to “the world’s most famous footpath” and is passed daily by hundreds of hikers, it presents a management challenge to the Park Service. Management options include: 1) no action, 2) creating a trailside interpretive display, including a protective cover on the trackway surface, 3) removing the rock and placing it in an interpretive exhibit in the Geology Museum at the South Rim.