A fortuitous rockfall in the Grand Canyon uncovered the earliest fossil evidence of vertebrate footprints in the region, left by an egg-laying animal hundreds of millions of years ago. The discovery, published in the journal PLOS One, was made by a group of hikers tramping the Grand Canyon National Park in 2016.
When the boulder in question was released by the cliff near the canyon’s Bright Angel Trail, it exposed a stratigraphic cross-section of the Manakacha Formation. The serendipitous series of events gave researchers a rare opportunity to glimpse into this ancient layer of rock that was committed to the ground around 313 million years ago.
Their investigations revealed two sets of cracks that appear to have been laid down by early four-limbed vertebrates, which judging by their narrow gait were either basal reptiles or synapsids. From the drifting pattern seen in the first set of footprints, it seems the animal was moving diagonally up a slope using a lateral-sequence gait that sees the legs on one side of the body move before the legs on the other side. The same type of walking is still seen in modern tetrapods such as dogs and cats.
The vertebrate footprints are the earliest example ever found in the Grand Canyon and reveal new information as to how early this style of locomotion emerged in the evolutionary tree. It’s unclear if the animal’s gait was affected by inclement weather or the steepness of the slope, but it shows that ancient vertebrates were living in the sand dunes 313 million years ago.
The second set shows evidence of claw use, indicating this animal, which was likely the same species as the other set of footprints, made a beeline straight up the slope rather than making a gentler, diagonal approach. From the fossil evidence, it's impossible to say what species left the two sets of tracks, and the researchers state that they could well have been made by an animal not yet known to science.
"These are by far the oldest vertebrate tracks in Grand Canyon, which is known for its abundant fossil tracks," said palaeontologist and lead study author Stephen Rowland, from the University of Nevada Las Vegas, in a statement. "More significantly, they are among the oldest tracks on Earth of shelled-egg-laying animals, such as reptiles, and the earliest evidence of vertebrate animals walking in sand dunes."