A Jumping Rodent Fossil Trackway from Colorado


A fossil trackway made by a jumping rodent (UCM 20392).
Just as jumping rodents live in Colorado today, they also lived here millions of years ago. A recently described rodent trackway from the paleontological collections of the University of Colorado Museum of Natural History provides unusual evidence about the lives of these ancient rodents.
Martin Lockley, a fossil track specialist from the University of Colorado at Denver, found one track specimen (A) to be especially interesting while visiting the collections in 2007. The specimen of interest was donated to the collections in the 1950s and superficially resembled tracks of a small camel. However, upon closer examination, Lockley confirmed that there was a continuous sinuous line between each of the lobes of the tracks. The tracks were those of a jumping rodent, and the sinuous line was the track of the tail that jumping rodents often use to support themselves when hopping on their hind feet!
Scientists have estimated that the trackway is 9 to 11 million years old. At the time the fossil tracks were described, they were only one of two known examples attributable to jumping rodents and the first known Miocene (5 to 23 million years ago) vertebrate tracks from Colorado.
Jumping rodents can travel using all four feet (quadrupedal) or just two feet (bipedal). When one hops quickly, it does not use its front feet, which is why there are no front-foot tracks on the specimen. The heel print is narrow and the splayed toes are wider on the hind feet, which can help to tell us the direction that the rodent was traveling (B)
Tracings of track outlines (B, C, D) help to identify details such as spider or insect tracks that may not have been initially noticed. The small semi-uniform marks shown on the tracing are tracks of insects with six legs—three on each side.
The location where the trackway was found had not been revisited since the 1950s. In 2007, Lockley and a team of paleontologists returned to the site. During this expedition, 35 more specimens with over 60 trackways of mammals, spiders, and insects were excavated. There are dozens of Miocene track sites in North America but no others are known to contain such an abundance of arthropod (insect and spider) and rodent tracks.
Image C and D are tracings from two of the trackway specimens found in 2007. They show the detail of a jumping rodent's hind feet, as well as the front feet.
The species of jumping rodent that made the trackways has not yet been identified. However, comparison studies of the size, number of digits and habitats of living jumping rodents are currently underway. Evidence from body fossils tell us that the families of jumping mice (Zapodidae) and kangaroo mice and rats (Heteromyidae) lived in Colorado from 9 to 11 million years ago.
Fossil tracks and trackways give us a snapshot of a moment in time when an animal walked upon the earth. They tell us the size of the foot, the direction of travel, and sometimes posture, gait, and speed. Fossil tracks are made when an animal crawls or steps on a soft substrate such as sand or mud and leaves an imprint. After the imprint has dried, the tracks are covered by more sand or mud, preserving the footprint for posterity.
Fossil tracks of vertebrates are uncommon, and fossil tracks of mammals are extremely rare. The University of Colorado Museum of Natural History holds over 150 specimens of fossil tracks and trackways of reptiles, birds and mammals for perpetuity in its paleontological collections.