Online guide to the continental Cretaceous-Tertiary boundary in the Raton basin, Colorado and New Mexico

Footprint of a Giant

Pictured below is a fossilized track of the hind foot of a large dinosaur. The track was discovered in northeastern New Mexico in 1983 by Charles Pillmore, a research geologist with the U.S. Geological Survey in Denver, CO. The track only recently was identified as a footprint made by a Tyrannosaurus rex, and presently is recognized as the only known fossilized track made by this creature. Although several nearly complete fossil skeletons of the large dinosaur have been found, until 1993 no tracks attributable to this creature had been reported.

Figure 17. Photograph of fossilized track of the hind foot of Tyrannosaurus rex in northeastern New Mexico.

Discovery:
Pillmore discovered the track while mapping geology in northeastern New Mexico. He recalls chancing upon the track while he was mapping in North Ponil Canyon on the Philmont Scout Ranch north of Cimmarron: "I noticed a strangely shaped feature on a block of sandstone only a short distance above the creek level. It appeared to be a natural cast that resembled the footprint of a large, three-toed animal, probably a dinosaur. The block appeared to have fallen from a ledge higher up on the slope and rotated as it moved down the hill, so that the bottom of the block now faces upward. I noted the discovery in my field notes and took some samples of the sandstone, but didn't suspect it was anything particularly unusual."

Identification:
In 1989 Pillmore showed pictures of the Philmont track to Dr. Martin Lockley, a dinosaur track specialist at the University of Colorado at Denver. Lockley agreed that it was the footprint of a dinosaur and proposed that the animal that made the track was probably a large hadrosaur. He agreed to accompany Pillmore to the Philmont site and confirm the identification. In late summer of 1993 the two men made a trip to New Mexico to examine and make a mold of the track (pictured below). As they began to clear away the dirt, Lockley speculated that the track was too big for a hadrosaur and that its heel was much larger than any hadrosaur heel he could recall. He then noticed a distinctive shape on the side of the track and proposed that it might have been made by a fourth digit called a hallux. Lockley commented that the size and shape of the track and the presence of the fourth digit were convincing evidence that they were looking at possibly the first Tyrannosaurus rex track ever seen. After sketching an outline of the track on clear plastic the scientists made a latex mold of the track.

Figure 18. Replicas of the dinosaur track; mold (left) and cast (right). The mold mimics the depression made in sediment by the dinosaur, and the fiberglass cast mimics the sandstone filling of the depression that is the preserved track seen in the other photos.

Position in the Rocks:
Dr. Farley Fleming, a USGS fossil pollen specialist helped to establish the relative age and stratigraphic position of the track layer. He determined that the track was probably made 65-70 million years ago, the proper age for a T. rex. He also determined that the environment of the trackway was a vegetated wetland mudflat, dominated by palm trees and ferns.

Significance:
The discovery of the natural cast of the T. rex track provides much important information: (1) the shape of the bottom of the dinosaur's foot contributes to knowledge of the soft tissue and the probable muscle structure of the foot and how it supported the animal's great weight; (2) the position of the hallux on the foot is apparent in the footprint. The track indicates that the hallux was fairly high on the foot and well back on the heel, which could assist in skeletal reconstruction; (3) the presence of claw marks suggests the foot had large claws, which may indicate a predatory way of life; (4) the range of the T. rex is extended south about 250 miles from the nearest known occurrence to the north; and (5) sixty-five million years ago, T. rex roamed across a broad river floodplain in a sub-tropical wetlands environment, as indicated by palm leaves and other plant fossils in rocks nearby and by fossil palm pollen and fern spores noted in samples of the muds he/she walked in.

Preservation:
A number of circumstances occurred that enabled the T. rex track to be preserved. First, the mud that the dinosaur was walking across had to be of a particular consistency and character - firm enough to preserve the shape of the foot, but not so soft or fluid to allow mud to flow back in and destroy the footprint. Second, the footprint had to remain open until the river flooded, allowing sand-laden water to flow across the mudflat, filling the track and depositing a three- to five-foot layer of sand on the mud surface. The flood current had to be gentle enough not to wash away the mud containing the track, yet strong enough to carry the sand sediment that filled the footprint and covered the mudflat. This sand layer was later covered by thousands of feet of other sediment layers that solidified into rock. These layers of rock were eventually eroded to form valleys and ridges, exposing the sandstone bearing the footprint.

Vital Statistics:
The footprint on the rock is 33 inches long by 28 inches wide. The depth of the infilling was about nine inches. Lockley determined from its position on the block that the animal's stride was at least nine feet. He estimated from the size of the footprint and the stride of the animal that it probably was moving at least six to seven miles per hour. When mature, T. rex reached a length of about 60 feet, sttod nearly two stories tall, and weighed approximately 8000-12000 pounds.

Official Name:
In 1994, a paper proposing the name Tyrannosauripas pillmorei for this track was submitted by Lockley and associate Adrian Hunt to honor Pillmore's discovery. The paper was published in Ichnos (Lockley and Hunt, 1994), an international journal for plant and animal traces, assuring that the name will be an official part of the scientific record.

Figure 19. Chuck Pillmore seated by the T. rex track he discovered.

Back to Online Guide Home Page