Fossil Trackway, Moenkopi Formation – Flagstaff, Arizona
Jeri Y. Ben-Horin, Ph.D – Arizona Geological Survey
David Gillette, Ph.D – Museum of Northern Arizona, emeritus
An invited blog post by Jeri Young Ben-Horin Ph.D, and David Gillette, Ph.D. Jeri is a research scientist at AZGS; David, now retired, served until recently as the Colbert Curator of Vertebrate Paleontology at the Museum of Northern Arizona.
Two-hundred-forty million years ago, a large reptile left its tracks on a mud flat, perhaps in search of food, or water. The tracks were found in November of 2020 by heavy equipment operator, Spencer Phillip during excavation for the Starpoint Fort Valley Apartments. The tracks were preserved in red sandstone from the Moenkopi Formation, which is estimated to be early to middle Triassic in age (252 to 235million years). The Moenkopi Formation contains layers from environments that include ancient sand dunes, and rivers and deltaic settings. The tracks found at this locality are set within a sandstone with preserved mudcracks (Figure 1). In addition to fossilized mudcracks, the Moenkopi also contains other features created from flowing water and blowing sand, such as “ripple marks, and high-angle cross-bedding”.
The Arizona Geological Survey, working with Straightline Construction, contacted the Museum of Northern Arizona to see if the tracks were significant. Dr. Dave Gillette (far right, Figure 2 - see cover photo) agreed to come out to the site and preliminarily examine the trackway. According to Dr. Gillette, the tracks are high quality and seem to preserve a pace sequence of right-to-left. The best hind limb track is about 20 cm wide, 15 cm long (back to front) with the best front limb track measuring about 9 cm x 5 cm. Pace distance is about 0.7m.
Dr. Gillette tentatively identified the trackway as Chirotherium rex (“hand beast king”) (Figure 3). Similar tracks have been found on the Colorado Plateau, but only in a few places. The technical name Chirotherium rex (KYE-row-THEER-e-um REX) is the name applied just to the tracks as an “ichnospecies.” The specific track-maker is unknown from actual skeletons, but its general shape and size can be inferred from the tracks. The illustration in Figure 3 shows an artist’s interpretation of a related reptile from the same age found in Germany, with a slab containing similar tracks in the background. Total length is roughly 1.5 meters.
Given that the tracks are indeed “footprints” and not fossilized bone, interpretation as to what animal made the tracks is difficult. In eastern Arizona, a partial skeleton of a fossil reptile, Arizonasaurus babbitti (Figure 4 below), has been found in the Moenkopi Formation. The remote possibility that these tracks were made by Arizonasaurus cannot be ruled out.
The early and middle part of the Triassic Period was the time when dinosaurs originated but they did not become abundant until the Late Triassic, so these Chirotherium tracks were probably not made by a dinosaur. Instead, the track-maker was probably among the immediate ancestors of dinosaurs. There is considerable scientific interest in tracks like these because the actual ancestry of dinosaurs remains obscure.
Fossil footprints show a record of living animals, a glimpse into their behavior and locomotion. They are highly educational for visitors of all ages. The public remains fascinated by the fact that tracks like these can be so well preserved from so long ago, in this case, around 240 million years.
The footprints found at this site are particularly important since they represent tracks made by reptiles that predate dinosaurs, and provide a new fossil locality important in paleontological research. The closest other localities with similar tracks are on the other side of the San Francisco Peaks in Wupatki National Monument and near Cameron, Arizona.