Traces of Life Found in Stone
Moab’s Rich Trace Fossil Record by Allyson Mathis
Two tracks at the Mill Canyon Dinosaur Tracksite.
Most people think of dinosaur bones, petrified wood, leaf impressions, shark teeth, trilobites, or shells when they hear the word “fossil.” All of these things are indeed fossils—that is, they are the remains or evidence of something that was alive in the geologic past. However, body fossils like the examples above are not the only type of fossils that are present in the geologic record.
An entirely different type of fossils does not consist of the actual remains of an animal, plant, or microorganism that lived earlier in Earth’s history. Instead, these fossils are traces of life were made when an organism was alive and interacting with its environment. Trace fossils (ichnofossils) provide fascinating records of ancient life that often provide information about ancient life that body fossils do not.
Trace fossils consist of any evidence of past life that doesn’t include the direct remains of an organism. Tracks (footprints), trails, burrows, coprolites (fossilized dung), eggs, and nests are all trace fossils that record behaviors, typically are from a moment of an animal’s life, such as a dinosaur walking along the shore of an ocean or a fish’s meal.
Much of the information that can be gleaned from trace fossils cannot be derived from the actual remains of an animal. For example, estimates of how fast dinosaurs ran or walked comes from fossil trackways. The distance between footprints indicate speed. Information about reproductive behaviors like nest-building is from trace fossils, as is insight into whether animals travelled together (e.g., sets of parallel tracks of multiple animals walking across a landscape may indicate herd or group behavior). Trace fossils may be the only record of some organisms that lived in the past, of and especially soft-bodied animals that lack hard parts like bones or shells. For example, burrows of marine worms are typically the only fossils of these organisms, which otherwise would be unknown to science.
Moab has a rich fossil record, especially from the Mesozoic Era (252 to 66 million years ago), also known as the Age of Dinosaurs. This was the time interval during which most rocks exposed in the canyon country around Moab were deposited. Many important dinosaur body fossils have been found near Moab, including on lands that are now Utahraptor State Park. People can see body fossils in situ, e.g., encased in bedrock, at the Mill Canyon Dinosaur Bone Trail.
Part of the coprolite exhibit in the Fossil Butte National Monument. NPS photo.
As significant as Moab’s body fossils are, the area’s trace fossils may be even more impressive. Dinosaur tracks have been found in many rock layers near Moab and are viewable at many interpretive sites. The Mill Canyon Tracksite is not just a great place to see trace fossils, it is one of the most scientifically important tracksites from the Early Cretaceous Period. Therapod (a group of three toed dinosaurs) tracks are also viewable along the Potash Road, at the Dinosaur Stomping Grounds, and at Copper Ridge. The Copper Ridge Tracksite has both therapod and sauropod (large long-necked herbivore dinosaurs) tracks, including the only known trackway of a dinosaur making a right turn.
Bromalites are fossilized remains produced by the digestive system. Coprolites are the most common type of these fossils, although there are no places to see coprolites near Moab. Regurgitates (fossilized vomit or other regurgitated material as in owl pellets) are very rare in the fossil record; but like coprolites, they provide important information about the lives of ancient organisms.
It takes specific geological conditions for a footprint or other trace of an organism’s life to be preserved. Think of the last time you walked on a beach. Your tracks were only distinct on damp (but not too wet) sand, but they were likely washed away with the next wave. Tracks must be buried by other sediments before they are destroyed, and dung must also be buried before it decays. Other interesting facts about trace fossils is that they are found only where the animal lived (sometimes carcasses or logs can be carried by rivers far from where the organisms lived) and a single organism can leave many trace fossils or none at all.
Carmel Formation (Dewey Bridge Member)
The Dewey Bridge Member is a unique subdivision of the Carmel Formation found only in Southeastern Utah. Its most famous exposures are in the Windows Section of Arches National Park. It is dark red to orange in color and is striking because of its contorted bedded formed by soft-sediment deformation. The conditions that caused the Dewey Bridge’s beds to wrinkle while its sediments were still soft (e.g., before they were lithified) are unknown, but the wavy and uneven bedding is pervasive. Even the contact (boundary) with the overlying Entrada Sandstone shows this uneven layering.
Entrada Sandstone (Slick Rock Member)
The Entrada Sandstone has the starring role in Arches National Park. Most of the park’s arches are found in this beautiful reddish-orange layer. It also makes scenic cliffs with shear walls like those in Park Avenue. Other scenic spots like Devils Garden, Delicate Arch, and the Fiery Furnace are predominantly made of the Entrada.
While also eolian (e.g., deposited in sand dunes) in origin like the Navajo, the Entrada Sandstone is also massive, meaning that it lacks pronounced layering within it. Therefore, the Entrada is more uniform than the other layers in Arches, which helps make it a good tableau for the weathering and erosional processes that carve natural arches. The fact that the Entrada is strong enough to hold up vertical cliffs and overhanging slopes while still eroding rather readily is also key.
Curtis Formation (Moab Member)
The Moab Member is another unique subdivision of a rock layer found (you guessed it) in the Moab area. It is also another eolian in origin but is thinner than the other prominent layers in Arches at usually less than 100 feet thick. Besides being off-white and the lightest-colored rock unit in the park, it is also harder (e.g., more resistant to erosion). It forms the hard cap rock on the top of fins in places like Devils Garden and the cap on Delicate Arch.
A fossil burrow in the Navajo Sandstone near Moab that was likely made by a mammal or tritylodontid (a type of mammal-like animal). Body fossils are rare in this layer so burrow fossils provide critical information about the animals that lived when the Navajo deposited about 185 million years ago. Photograph courtesy of BLM Paleontologist Emily Lessner.
The sauropod trackway at the Copper Ridge Dinosaur Tracksite showing the turn to the right. The regular spacing between sauropod tracks, which are usually large round depressions, help indicate that they are trace fossils. Photo by James St. John. CC by 2.0
Regurgitate fossil from the Morrison Formation in southeastern Utah. Researchers think that this concentration of tiny frog and salamander bones was vomited by perhaps bowfin fish living in a pond as it was being pursued by another predator during the Jurassic. Note that the amphibian bones are body fossils, but the overall regurgitate is a trace fossil. Image courtesy of John Foster, Utah Field House of Natural History. Scale bar 5 mm.
A self-described “rock nerd,” Allyson Mathis is a geologist, informal geoscience educator and science writer living in Moab.
To learn more about Moab’s geology, visit the Geology Happenings archive online at https://www.moabhappenings.com/Archives/000archiveindex.htm#geology
A self-described “rock nerd,” Allyson Mathis is a geologist, informal geoscience educator and science writer living in Moab.