David C. Kopaska-Merkel, Geological Survey of Alabama
The Steven C. Minkin Paleozoic Footprint Site in Walker County is the most prolific source of vertebrate trackways of its age in the world. It is an important resource for scientists for a number of reasons: the fossils found within it are well-preserved, abundant, and diverse, and thus scientists can study multiple examples of a given species or behavior; the deposit records the footprints of some of the earliest reptiles; and it contains the oldest known examples of fish schooling and group behavior of tetrapods. The site is named for the man who led the effort to preserve the site.
The fossil trackways are preserved in shale that formed from mud on a freshwater tidal flat about 315 million years ago. Scientists know that this was the likely environment from a combination of clues. For example, the trackway layer lies on top of a coal deposit that formed on land, and associated rocks contain known marine (sea-dwelling) fossils, indicating that the trackways formed near the shoreline. The shale contains traces of land-dwelling organisms (amphibians and reptiles) and aquatic organisms (fish), but no exclusively marine organisms. Marine deposits nearby contain brachiopod shells and the traces of resting trilobites, both absent from the trackway deposits, but no remains of land dwellers.
The coal deposit, once a swamp forest, is one of the reasons that the trackways were discovered. The Minkin site is located at the former Union Chapel coal mine. In 1999, a high school teacher learned that the grandmother of one of his students owned the coal mine. The teacher visited the mine and soon found something he knew was important: tracks of an ancient amphibian. He reported the find to the Birmingham Paleontological Society, and some of its members visited the site. Later, some former members of the society formed a new group, the Alabama Paleontological Society, which continued to visit the site. They informed professional paleontologists, and the two groups together began a race to collect the specimens and struggled to preserve the site. By law, surface mines in Alabama must be reclaimed shortly after mining operations end, and the Minkin site was scheduled for reclamation. The preservation effort was spearheaded by geologist and amateur paleontologist Steve Minkin, and was named for him following his untimely death. It represented a true collaboration between dozens of amateurs and professionals. Their efforts were successful, and the Minkin site is now preserved in perpetuity thanks to the New Acton Coal Mining Company, the Alabama Department of Conservation and Natural Resources, and dedicated paleontologists. In 2005, this same group published the first major scholarly work about the trace fossils and the site: Pennsylvanian Footprints In the BlackWarriorBasinof Alabama. The contents range from historical essays to technical discussions of the trackways.
The Minkin site features a cliff, the high wall of the former mine, partially surrounded by a large mass of broken rock that was moved during the coal-mining process. This rock consists primarily of slabs of shale that range from a fraction of an inch to several feet in thickness. The fossils are found on or in some of these slabs. Nearly 3,000 specimens have been collected so far and many more await discovery. In addition to the hundreds of footprints and trackways left by reptiles and amphibians, the Minkin site also features undulating traces made by fish swimming in very shallow water; horizontal and vertical burrows of fly larvae; walking, resting, and jumping traces made by horseshoe crabs and other arthropods; and worm burrows. The trace fossils occur with the carbonized remains of ferns, treelike cycads, giant horsetails, and other plants. These plants lived in swamps adjacent to tidal flats on deltas and rimming bays and lagoons. These tidal flats were home to fish, insects, worms, amphibians, horseshoe crabs, and other creatures that left traces of their activities in soft mud (now shale). Fragments of plant material were periodically washed out onto the tidal flats or into the adjacent shallow bays.
The Minkin site is not generally open to the public, but visits are easily arranged. The Alabama Paleontological Society conducts monthly collecting expeditions, and interested persons are welcome to participate. Specimens of scientific value must be donated to a museum so that they are available for future study.
Buta, Ronald J., and David C. Kopaska-Merkel. Footprints In Stone, Fossil Traces of Coal-Age Tetrapods. Tuscaloosa: University of Alabama Press, 2016.
Buta, R. J., Rindsberg, A. K., and Kopaska-Merkel, D. C., eds. Pennsylvanian Footprints in the Black Warrior Basin of Alabama. Birmingham: Alabama Paleontological Society Monograph 1 (2005).
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