A New Exposed Trackway on the Bright Angel Trail in Grand Canyon

Artists rendition of how the new trackway was created from Figure 8 in the paper. Art by Emily Waldman. 

SECOND ADDENDUM 

AUGUST 20, 2020: Today I received from Professor Steve Rowland at UNLV notice of his publication on this amazing trackway along the Bright Angel Trail in Grand Canyon. When I first came upon this newly fallen boulder in December, 2016, I knew it was something special just by looking at the detail encased on its surface. However, the authors have studied the entire trackway and setting in detail and conclude that: 1) these are the oldest tetrapod trackways in the Grand Canyon; 2) two sets of tracks are present on the dislodged block and are separated in time by only hours or days; 3) a light dusting of new sand (windblown) separates the two sets; and 4) the animals walked up a dune slope that was at an angle of 20° and that the animals walked at a speed of about one foot per three seconds. The trackways resemble those of the ichnofauna (trace fossil) Chelichnus, a common form found in younger layers above such as the Coconino Sandstone.

This paper is Open Access and can be downloaded here.

Citation: Rowland SM, Caputo MV, Jensen ZA (2020) Early adaptation to eolian sand dunes by basal amniotes is documented in two Pennsylvanian Grand Canyon trackways. PLoS ONE 15(8): e0237636. https://doi.org/10.1371/ journal.pone.0237636

Editor: David M. Lovelace, University of Wisconsin Madison, UNITED STATES

Received: October 21, 2019 Accepted: July 30, 2020 Published: August 19, 2020

Copyright: © 2020 Rowland et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

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Here is a paleogeographic map of the Southwest during Manakacha Formation time. Courtesy of Ron Blakey, at deeptimemaps.com.

ADDENDUM

February 12, 2018: Since I made this post, Professor Stephen Rowland at UNLV has made a closer examination of the trackway and has determined that this was a single animal walking sideways with the toes pointing in one direction while the trend of the overall trackway is about 40 degrees clockwise from the direction the toes are pointing. He thinks the animal may have been fighting against a current, presumably wind, that was pushing it to the right as it struggled to move forward. 

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ORIGINAL POST 

December 11, 2016: 

Last month Helen and I got to visit Indian Garden located 4.5 miles down the Bright Angel Trail in Grand Canyon. Down in the Pennsylvanian Supai Formation, about 2.5 miles in, we came across a new exposure of a great reptile trackway. I took a few pictures and share them here.

During the summer monsoon, a number rocks were dislodged from the cliff on the left and rolled down onto the trail. Since then, the trail crew has cleared the rocks from the tread and placed them alongside the trail. This is a place where parallel joints are evident in the sandstone. These joints are parallel to the Bright Angel Fault and slivers of rock are always falling at this location.

Blasting was needed to move the rocks off the trail and it is unknown to me if the new trackway was exposed when the rocks fell off the nearby cliff or from subsequent blasting. What is known is that the trail crew was aware of the trackway and placed this boulder in perfect alignment to admire it.

Being closer to the trackway shows some well preserved claw marks in the individual prints (bottom). This must have been a lumbering type of reptile with each track closely spaced but the individual prints about 3 to 4 inches wide.

More claw marks. Two ichnofauna's are described from the Supai - Limnopus and Batrachichnus. A web search did not specify which these might belong to. Another likely candidate for these is Eryops and fossil skeletons that are Pennsylvanian in age have been found in nearby New Mexico. A description of this animal can be found here.

An example of Eryops reconstructed from fossil bones.

A fossil specimen of Eryops on display at the National Museum (Smithsonian, Washington D.C.)

It is possible that this is a set of two individuals walking one after the other. Most descriptions of reptiles from this time period suggest that the back and front tracks are nearly on top of each other and there are four lines of tracks here.

The block is big enough that it should remain in place for a long time.

Thanks to the Grand Canyon trail crew for making this specimen so readily observable.