Paleo News: UGS Paleontologists Find a Bonanza of Fossil Footprints in Canyonlands National Park
by Don DeBlieux
The Utah Geological Survey’s (UGS) partnership with the National Park Service to inventory paleontological resources is still going strong (see Survey Notes, v. 55, no. 1)! Our most recent collaboration has taken place over the last several years in the spectacular rocks of Canyonlands National Park. Canyonlands is made up of three districts— Island in the Sky, the Needles, and the Maze—all of which preserve a late Paleozoic- and early Mesozoic-age rock record. We have completed surveys in all three districts, but have spent the most time in the Island in the Sky District because of the large volume of accessible fossil-bearing, Triassic-age strata.
Initially, we were most excited about exploring rocks of the Late Triassic-age Chinle Formation because it is one of the most fossiliferous rock units in Utah and has yielded many significant scientific discoveries. We also examined the Early-Middle Triassic-age rocks of the Moenkopi Formation since we would have to traverse these rocks to access the Chinle strata, even though fossils are generally quite rare in the Moenkopi. As expected, we discovered fossil sites in the Chinle Formation, but it seems that fossils are not as common here as in some other regions. The Moenkopi Formation, however, turned out to be loaded with exceptional fossil sites. These sites did not have fossil bones but rather tracks and traces of ancient life.
The Moenkopi Formation was deposited in a nearshore coastal environment about 240 million years ago. In Utah and Arizona, the formation is well-known for preserving a diverse array of reptile tracks that predate dinosaurs. Rocks of the Torrey Member of the Moenkopi Formation, which is made up of primarily sandstone and siltstone deposited in a tidally influenced deltaic setting, crop out extensively in Canyonlands and are known to preserve reptile tracks as well as many other trace fossils. One of the most common types of tracks are swim tracks. These tracks were not made by animals walking on land but by animals moving in or under the water. So instead of preserving a footprint, they preserve scratches made by the animal’s fingers and toes as they contacted the sediment at the bottom of a water body. The Torrey Member of the Moenkopi in Canyonlands preserves what is likely the largest concentration of swim tracks ever found in the Moenkopi Formation and the number of tracksites rivals those of any other region of Moenkopi Formation outcrop on the Colorado Plateau. Not a single Moenkopi tracksite was known from Canyonlands when we began our survey, and now we have documented over 50 track localities! Most of these sites preserve swim tracks and some have hundreds or even thousands of traces. The swim tracks are thought to have been made by reptiles closely related to the ancestors of dinosaurs and crocodiles informally called “chirotheres” for the ichnotaxon (track name) Chirotherium. In addition to the swim track sites, we also found several important sites that preserve tracks made by animals walking on land.
One interesting feature of the tracks in the Torrey Member is that most of them are preserved as natural casts. So instead of being preserved as a true footprint like you might make when walking on the beach, these are preserved by sediment filling in the track. In other words, they are “outies” instead of “innies.” So, instead of the track being on the surface of a rock layer, they are on the underside. To search for these traces, you must either find a block of rock that has fallen and flipped over or look on the undersides of rock overhangs. Luckily in Canyonlands there are plenty of rocks that have flipped over and even more overhangs to look at!
Because of the vast number of swim tracks and traces found in the Torrey Member we suspect that if we had X-ray vision and could see all the tracks buried in the rock, they would show innumerable trackways that crisscrossed the ancient waterbody bottoms. This begs the question of what these animals were doing when they left so many underwater tracks. Some swim tracks were made by animals that were floating in water at a depth that they could touch the bottom and push themselves along. The St. George Dinosaur Discovery Site in southern Utah preserves an array of tracks made by Early Jurassic-age dinosaurs floating on the surface of a shallow lake and using their feet to push off the bottom (Survey Notes, v. 34, no. 3). Because it is doubtful that the water depth during deposition of the Torrey Member was always the right depth for these animals to be floating on the surface and pushing off the bottom, we hypothesize that these animals could sink down and essentially walk along the bottom pushing off with some combination of their hands and feet. The scientific literature calls this sub-aqueous walking or punting. Because they were semi-buoyant, they did not leave complete footprints, but scratch marks made by the fingers and toes. This kind of behavior is seen today in animals such as crocodiles, hippos, and capybaras.
The many tracksites we have documented in Canyonlands will allow us to investigate these ideas in more detail and will help park managers to protect these important resources. We plan to continue to study these sites to learn more about these fascinating traces and the animals and behaviors that made them.