Roger J. Cuffey, Maria J. Di Nardo-Magilton, and Bryan J. Herzing
Department of Geosciences, 412 Deike Building
Pennsylvania State University, University Park, PA 16802
Dinosaur footprints have long been known from early Mesozoic sandstones across the Colorado Plateau, frequently mentioned, but seldom fully documented. An early photograph was provided by Gregory (1917), a partial listing of later finds by Stokes (1978).
Until recently, fossils of this kind had not been reported from Pipe Spring National Monument. Then, Stokes (1988) published a photograph showing a detached or float cast of a footprint on display there. That picture attracted attention (Santucci, 1991), and R. J. Cuffey visited Pipe Spring in 1995. Park Service personnel on duty told him of a couple of footprints along the hiking trail. Cuffey located those, side by side, and found a third out in front of them; a preliminary note announced these footprints (Cuffey, Di Nardo, and Herzing, 1997), which are illustrated in the present paper.
Pipe Spring National Monument is primarily a memorial to late-19th-century cattle ranching, but also includes significant geologic resources, including the newly discovered dinosaur footprints.
Pipe Spring National Monument is located in Mohave County, on the Kaibab-Paiute Indian Reservation, 13.4 mi (21.4 km) west of Fredonia. Its visitor center is about a third of a mile north of the east-west paved highway (Arizona 389). The footprints are along the foot trail up onto the mesa's top, at 5080 feet elevation, above and behind the visitor center, 0.2 mi (0.4 km) N35°W from that center. The footprints are in the NW1/4 SE1/4 SE1/4 sec. 17, T.40 N., R.4 W., Pipe Spring 7.5¢ quadrangle.
The southern face (Figure 1) of the mesa west of the visitor center consists of a lower red covered slope (presumably Moenave), a thin middle dark red sandstone cliff (Kayenta), and a high-standing thick cap of light orange to tan sandstone (Navajo). These sandstones are part of the widespread Glen Canyon Group. The low flat plain on which the highway runs is apparently underlain by the Chinle (Upper Triassic).
Figure 1South face of the mesa at Pipe Spring National Monument, west of the visitor center. The dinosaur footprints are on top of the low bench extending eastward from the high mass of the mesa. Upper half of the mesa is light-colored Navajo Sandstone, the dark-colored thinner sandstone cliff in the middle of the mesa is the Kayenta, and the covered slope forming the lower half of the mesa buries various lower stratigraphic units (particularly within the Moenave).
Figure 2Overview of the footprint site on the Pipe Spring mesa; the pair of footprints are on the gently right-dipping bedrock surface immediately to the left of the solitary bush in the center of the photograph; facing north from the steps and bend in the trail as it comes up on top of the mesa (see text).
The footprints are preserved in orange-red or light orange, concave-upward cross-bedded, medium to coarse, quartz sandstone, 6 ft (2 m) above the base of the Navajo Sandstone, of which roughly 500 ft (160 m) more is exposed higher on the mesa to the west. The Navajo is a classic eolian dune or erg sandstone, and previous investigations nearby have dated it as comparatively high in the Lower Jurassic, specifically Toarcian or about 180-185 million years old.
The base of the Navajo is easily recognized by the color contrast with the underlying dark red or red-brown, flat or horizontal-bedded, medium to coarse, quartz sandstone, of which only about 10 ft (3 m) is exposed along the trail but up to 100 ft (30 m) is visible in the mesa face to the west.
Originally thought (as shown on one of the trailside plaques) to be part of the Chinle Formation, this dark red sandstone is instead the Kayenta. Carefully considering the observed dips and flexures in the mesa face, coupled with published thicknesses and regional stratigraphic variations, permitted clarification of the Pipe Spring units. Particularly helpful were papers by Blakey (1994), Clark and Fastovsky (1986), Peterson (1994), and Wilson (1967).
The foot trail switchbacks up the steep slope forming the south face of the mesa behind the visitors' center and the 19th-century buildings. It ascends through dark red sandstones (topmost Kayenta), and then climbs, via several rock-cut steps, up a small vertical cliff in light orange sandstones (basal Navajo), comes up onto the mesa's upper surface, and turns sharply left or west. The footprints lie directly ahead or north of those steps, 10-12 ft (3-4 m), off on the side of the trail's turn (Figure 2).
Two of the footprints are side by side (Figure 3A), about half a meter apart, with the heel of the left one (Figure 3C) even with the tip of the middle toe of the right one (Figure 3D). The third (Figure 3B) is isolated, 8 ft (2.7 m) ahead, and approximately on line enough to be part of the same trackway.
All are shallow depressions on the exposed bedding-plane surfaces in the sandstone bedrock. The right footprint (Figure 3D) is the best preserved of the three, is three-toed (tridactyl), 30 cm long from toe-tip to heel, and as wide between the two lateral toes' tips. More details would be desirable, but are not preserved clearly.
The float cast specimen (Stokes, 1988) appears much the same, from his published photograph.
As evident in the accompanying photographs, the exact outlines of the footprints are obscure, difficult to determine, and thus prevent precise identification. Moreover, previous ichnotaxonomists have finely split footprint species elsewhere, with only the most subtle differences discriminating them.
Several species (possibly all synonymous) of Eubrontes known from Early Jurassic parts of the Newark Supergroup on the East Coast (Haubold, 1986) could be reasonable possibilities for the Pipe Spring footprints: E. approximatus, E. divaricatus, E. giganteus, E. platypus, or E. tuberatus. And, Eubrontes has now been reported from the Colorado Plateau sandstones generally (Lockley and Hunt, 1994).
Eubrontes was the track of a moderately large theropod dinosaur, a bipedal carnivore, probably a carnosaur and perhaps a megalosaurid more specifically, although other taxinomic affinities might be suggested. It is known from fossilized footprints only (i.e., is an ichnogenus), rather than skeletal remains.
Figure 3Dinosaur footprints (probably a species of Eubrontes) in place in Pipe Spring National Monument (see text for exact location); pen is 15 cm long. A, left and right footprints together. B, isolated footprint. C, left footprint. D, right footprint.
Several steps should be taken to protect the Pipe Spring footprints and enhance their value to the visiting public. An appropriate exhibit could be constructed in the visitor center, including the detached specimen photographed by Stokes (1988), and featuring photographs of the in-place footprints up along the hiking trail, as well as drawings of the dinosaurs reconstructed as in life. The actual footprints can be marked with a suitable plaque and surrounded by a protective fence or railing, which would permit viewing but prevent trampling. A one-page handout or map might be passed out at the center to help guide hikers to the footprints themselves. The trailside plaque en route up should be corrected to read "Kayenta" instead of "Chinle" formation terminology. Finally, the bedrock bedding-plane surfaces around the three in-place footprints should be carefully searched for additional dinosaur tracks, which if any more are found should be incorporated into the site as well.
Several seasonal and volunteer personnel at Pipe Spring National Monument, supervised overall by Andrea Bornemeier, greatly aided R. J. Cuffey's field examination of the dinosaur footprints.
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