In November 1986, geologists with the West Virginia Geological and Economic Survey's Coal Resources Section had the opportunity to examine the fossilized footprints of an ancient amphibian, a rare fossil find in West Virginia. The trails of footprints, called trackways, were uncovered during the removal of overburden in a surface coal mine on the west side of Piney Swamp Run, Elk District, Mineral County (Figures 1 and 2).

Figure 1. Trackway (boxed area) left in siltstone by an amphibian about 300 million years ago, and recently unearthed at a surface mine in Mineral County. Camera lens cap, center, shows scale. (All photos by the author)

This discovery was similar to the fossilized amphibian trackway investigated by Survey geologists in Tucker County in 1982 (see the article "300 Million Year Old Footprints Found in Tucker County" in the 1982 issue of Mountain State Geology). Both of these finds are important because there are few well-documented examples of fossilized animal tracks within the State's boundaries.

How Old?

Upon arrival at the site, the geologists' primary task was to identify and date the rock units containing the trackways. Dating the rocks also dates the animal that left the footprints. Identifying the rock units gives an idea of the environment in which the animal lived.

Two trackways were found, both in a dark gray siltstone. Neither trackway was in its original place, having been disturbed by the overburden removal. However, from observations made at the site, it was concluded that the trackways must have come from the interval of rocks between the roof of the Pittsburgh coal and the base of the Sewickley coal. This puts the trackways in the Monongahela Group (Late Pennsylvanian age) and means that they, like the 1982 Tucker County find, are almost 300 million years old. At that time, this area of Mineral County was low-lying, wet, and swampy. Since the trackways were in siltstone (laid down as a layer of sediment carried in by meandering streams) and not coal, the environment would not have been as lushly vegetated as was necessary for a coal-forming peat swamp.

Figure 2. Closeup of an individual footprint in siltstone. Lens cap shows scale.

How the Fossils Formed

Fossilized footprints such as these are formed under a fairly narrow set of conditions. An animal heavy enough to form an impression must walk slowly across sediments that are fine-grained, cohesive, and moist. If the sediments are too coarse, the footprints will lack detail. If the sediments are not cohesive, only a depression and not a footprint will be left behind (like walking on the dry part of a beach). If the sediments are cohesive but strong enough to bear the weight of the animal, no impression will be made.

Moisture is usually the controlling factor. It may give some cohesion to a material that is not otherwise cohesive. For example, a detailed footprint can be made in the moist sand at a beach, whereas wet or dry sand lacks cohesion and hence the strength to retain the impression of the foot.

There is a similar moisture range in clayey soils which have the right strength to take the impression of the foot and hold it without distorting. Then, before the footprint is destroyed by rain, erosion, or other forces, it must be filled in with a sediment having a different texture so that, assuming the sediments lie undisturbed until they harden, they will separate at the footprint. In a swamp, such conditions are most likely to be found when high water has receded at the end of a rainy season.

What the Fossils Reveal—and What They Don't

Figure 3. Hypothetical example of a trackway. Measurements taken can give information on an animal's walking behavior, size, and broad clues as to its type. (Illustration by Ray Strawser)

Preservation was not very good on the Piney Swamp Run trackways. One was so poorly preserved that there were no details in the individual footprints. The better trackway had one footprint which showed five toes and may have been the animal's hind foot. (Modern salamanders have five toes on their hind feet.) Due to the size of the specimens, the nature of the rock, and the poor preservation of the footprints, the trackways were not collected. However, photographs were taken, and notes and casts were made for future study (see box).

The 1982 Tucker County footprints were associated with mollusks, fern-like plants, and trace fossils other than the footprints. Fossils of these types were not noted at the Mineral County site. Thus, a less complete picture of the environment the animal lived in can be reconstructed for the Piney Swamp Run site.

Mossman and Sarjeant (1983) have given a good overview of fossil vertebrate footprints. They point out that to be scientifically valuable, several measurements should be made on the trackway: stride, pace, step angle, and trackway breadth (Figure 3). For quadrupeds an additional useful measurement is the distance between the midpoint of the line connecting two hindfoot prints with the midpoint of the line connecting two forefoot prints. (This gives the animal's trunk or body length.) Measurements on the Piney Swamp Run trackways are shown in Figure 4. (Measurements of the 1982 Tucker County trackway are shown for comparison.)

Compared with those found in Tucker County, the individual tracks at Piney Swamp Run are larger. The best-preserved footprint was about 41h inches at its longest and about 3¼ inches at its widest. A footprint from Tucker County measured only about 21h inches at its longest and about 2¼ inches at its widest. None of the Tucker County footprints had five toes showing but at least one of the footprints from the Piney Swamp Run site had five toes.

Figure 5. Making plaster casts of the Piney Swamp Run footprints. Casts at left are from the 1982 Tucker County find, and were used for comparison.

Were the trackways made by the same species of amphibian? The footprints are of similar but not identical size. The animal could have had four toes on the forefeet and five toes on the hind feet, as do modern salamanders. The various measurements of walking behavior are similar. Could the difference in footprint size be due to a large individual or a different type of individual? Right now there is not enough information to say. These footprints are all we have as a record of the amphibian.

Still, each discovery like this adds to the evidence of what West Virginia was like in prehistory, and the nature of the ancient animals who lived here.

Further Reading

300 Million Year Old Footprints Found in Tucker County: T. R. Jake and B. M. Blake, in 1982 Mountain State Geology magazine, p. 23-25. Publication MSG-82, $1.00.

Reference

Mossman, D. J., and Sarjeant, W.A.S., January, 1983, The footprints of extinct animals: Scientific American, vol. 248, no. 1, p. 75-85.

Thanks to being alerted by Mr. David Idleman of the West Virginia Department of Energy, Survey geologists were able to examine these fossil trackways, and thus share this unique find with the people of West Virginia.

Article reprinted from the 1988 Mountain State Geology magazine.