Some 313 million years ago, a large lizard-like creature crawled up a coastal sand dune in what is now the Grand Canyon. Some time later, a light dew wetted the tracks cementing them in place and then a wind-blown sand buried them, preserving the animal's clawed footprints for eons.
The paleontologists who studied the trackway say they are the oldest recorded vertebrate tracks in Grand Canyon National Park. Tetrapods, or four-legged beasts, left this set of tracks, along with another set imprinted a little later in time. The second set of footprints was laid down after some sand had accumulated in the first set, and the researchers said these prints could belong to the same species.
These ancestors of modern reptiles lived in the sand 250 million years before T. rex, and they would have walked using a highly evolved gait.
Allan Krill, a Norwegian geology professor, initially discovered the imprinted tracks in 2016 while leading his students along Bright Angel Trail on an annual field trip to the Grand Canyon. He noticed the fossilized footprints etched into a fallen boulder at the base of a canyon on the trail. Krill took photos of the prints and sent them to Steve Rowland, a geologist who often accompanied the Norwegian group on their trips.
Rowland and his team determined that the track-bearing boulder fell from a nearby cliff-exposure of the Manakacha Formation. "We were able to determine that it was from that area because we studied the nature of the rock, features such as its color and grain size," he said. Knowing the origin of the rock also allowed researchers to date the tracks.
The size of the tracks suggest the creatures would have been about the size of a modern-day chuckwalla, 15 to 30 inches (or 40 to 80 centimeters) long, the researchers said.
A surprising gait
When Rowland's team examined the tracks, they found two surprising features."The first is that they are the oldest known amniotes living in sand dunes," said study co-author Steve Rowland, Emeritus Professor of Geology at the University of Nevada Las Vegas. Amniotes are animals like birds and reptiles whose eggs can survive outside of water. Earlier-evolved species such as fish and amphibians need to lay their eggs in water. "Finding these amniote tracks in what was a coastal dune means that these ancestors of modern reptiles adapted to land almost as soon as they evolved," Rowland said.
"The second surprising thing is the arrangement of the footprints -– these tracks revealed a lateral-sequence gait," Rowland told Live Science. If you watch your pet dog or cat, especially when they are moving slowly, you might see it use a lateral-sequence gait in which they move the right-rear leg followed by the right-front leg, and then the left-rear followed by the left-front. This sequence is more stable than the other known type of gait, called the diagonal-sequence gait, Rowland said. Humans use this diagonal type of movement, swinging our left arm forward synchronously with our right leg, and vice versa. Fish also use diagonal sequence when moving their fins.
Animals with four legs can use both types of gait. "Until we studied these tracks," Rowland said, "no one knew how early in the history of animals the lateral-sequence gait came into use. Now, we know that it was used very early in the history of amniotes, 313 million years ago."
Spencer Lucas, a paleontologist at the New Mexico Museum of Natural History who was not involved in this study, applauded the work. The current research, he said,"documents an important discovery: the oldest record of the footprints of tetrapod vertebrates in an eolian (wind-formed) rock layer. It establishes that vertebrates were living in deserts millions of years earlier than was previously known."
The findings were published Aug. 19 in the journal PLOS One.
Originally published on Live Science.