Fossil Trackway Discovered At Grand Canyon Traced To Early Reptiles

Friday, May 17, 2019

Artwork depicting the Coconino desert environment and two primitive tetrapods, based on the occurrence of Ichniotherium from Grand Canyon National Park/Voltaire Paes Neto illustrator

Nearly 300 million years ago, in a sandy desert now preserved as Coconino Sandstone at Grand Canyon National Park, some creatures walked across an area left moist very likely by an oasis. Those tracks, revealed today in a remote area of the national park, likely point to the transition of amphibians to reptiles and reflect another of the wonders held within the National Park System.

The discovery of the trackway, which was found a couple years ago, dates back 280 million years, just about to the arrival of the Permian Period and before dinosaurs walked the Earth.

"It's a pretty spectacular trackway," said Vince Santucci, the National Park Service's senior paleontologist. "It's called an 'ichnogenus.' The term ichno refers to trace fossil. This ichnogenus is really well represented on this track block. It is known from Permian age rocks in Europe and South America, and so it's only recently been confirmed and this block contributes to the occurrence of that Permian age ichno form in North America at Grand Canyon National Park.

"There have been some similar finds in northern Arizona, but this particular block is the most well-preserved long trackway, several long trackways," he added during a phone call from his Washington, D.C., office.

Map of Arizona (southwestern USA), indicating the main localities mentioned in the text. The Grand Canyon National Park area is shaded dark brown (left). Stratigraphic section of the Pennsylvanian and Permian rocks exposed in the Grand Canyon area (right). Credit: Modified from Blakey and Knepp 1989.

Brazilian paleontologist Dr. Heitor Francischini, from the Laboratory of Vertebrate Paleontology, Federal University of Rio Grande do Sul, and Dr. Spencer Lucas, curator of paleontology at the New Mexico Museum of Natural History & Science in Albuquerque, New Mexico, first visited the Grand Canyon fossil track site in 2017. The paleontologists recognized the tracks as having been produced by a long-extinct relative of very early reptiles, a release from the museum said. 

"This new discovery at Grand Canyon is the first occurrence of Ichniotherium from the Coconino Sandstone and from a desert environment," the release added. "In addition, these tracks represent the geologically youngest record of this fossil track type from anywhere in the world."

Santucci said the creatures that left the tracks likely were reptiles due to the surrounding desertscape environment. Though they were not far removed from amphibians, he added.

"It's an enigmatic invertebrate, a tetrapod, four-legged animal, that was somewhere between amphibian and reptiles," the paleontologist said. "It's probably closer to the reptilian stock, but when you're only looking at the tracks, they don't know for absolutely sure. There's a group called the diadectids that are this enigmatic group that are transitional between amphibians and reptiles. A lot of people tend to think that they're probably more reptilian because of their occurrence within these arid paleo environments. Biologically, in particular we know this from modern amphibians, modern amphibians don't have the same protective skin and the egg casing that allows them to survive successfully in arid environments. They need moisture. They lay their eggs in water. Their reproductive cycle is dependent upon it."

"Although the actual track maker for the Grand Canyon footprints may never be known for certain," the museum's press release said, "the Grand Canyon trackways preserve the travel of a very early terrestrial vertebrate. The measurable characteristics of the tracks and trackways indicate a primitive animal with short legs and a massive body. The creature walked on all four legs and each foot possessed five clawless digits."

Closeup view of ichniotherium trackway/NPS

The Park Service paleontologist said the discovery will further "debate and discussion among paleontologists who are trying to figure out what this transitional diadectid animal was all about. ... It tends to lean the support towards a more reptilian stock as opposed to an amphibian."

The trackway is about 10 feet in length, and tilted from having slipped down, said Santucci. It's located below the park's South Rim, well off established hiking trails. It was found by a researcher who headed off trail to relieve herself.

The paleontologist said the tracks likely lend little additional knowledge to what is known about the environment that laid down the Coconino Sandstone, as it is well known. 

"Closer to the (Grand Canyon's) rim you get into the Coconino Sandstone fairly easily, and it's this very distinquishable, identifiable crossbedded sandstone, and those crossbeds represent dune-set facies of giant sand dunes," he said. "It was that time period where we had this really strange diversification of these early tetrapods that went all sorts of directions. Most of them were very short-lived, most of them were deadends. They didn't give rise to anything that survived into the recent, and so it's a really enigmatic group. There's enough evidence that has generated a lot of curiosity about them, but it's still a big debate as to whether these tetrapods, these four-legged creatures, were amphibian or reptile. What's important about this trackway is because it's found in clearly a dry, arid desert type of paleo environment, that it lends itself more with this particular group of organisms being more reptilian in character."