Stephen C. Minkin Paleozoic Footprint Site: an Alabama Treasure
The Steven C. Minkin Paleozoic Footprint Site in Walker County is the most prolific source of vertebrate trackways of its age in the world. To date, more than 4,000 fossil specimens have been collected from the site. This is important because
fossils are well-preserved, abundant, and diverse,
scientists can study multiple examples of a given species or behavior,
this deposit records the footprints of some of the earliest reptiles,
the oldest known examples of schooling and herding were found here.
The fossil trackways are preserved in dark gray shale above coal. The shale formed as mud on a freshwater tidal flat about 313 million years ago.
Reconstruction of the ancient environmental setting of the Minkin site (star). The site was located in the freshwater part of a major tide-dominated delta. Illustration made by Jim Lacefield; used by permission.
Stratigraphic chart giving the names of the geologic units of the Mississippian and Pennsylvanian Periods. The fossils come from a thin unit called the Cincosaurus beds, which is Westphalian A in age (about 313 million years old). The age of the unit is determined primarily from associated plant fossils.
A combination of clues led to the environmental interpretation. Associated coal (formed on land) and rocks containing known marine (sea-dwelling) fossils indicate 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 brachiopods and trilobite resting traces, both absent from the trackway deposits, but no remains of land dwellers.
The coal, once a swamp forest, is one reason the trackways were discovered. The Union Chapel mine, now the Minkin site, was located where valuable coal deposits were known to be shallowly buried. The trackways were found in 1999 after a student told his high school science teacher that his grandmother owned a coal mine.
The teacher, Ashley Allen, visited the mine and soon found something he knew was important: tracks of an ancient amphibian. He reported the find to what was then the Birmingham Paleontological Society (now Alabama Paleontological Society), and other members visited the site. Soon they informed professional paleontologists. Thus began the race to collect the specimens and also a struggle to preserve the site. This was because, by law, surface mines in Alabama must be reclaimed shortly after mining operations end, and this was about to happen to the Minkin site. This preservation effort, a true collaboration between amateurs and professionals, was spearheaded by Steve Minkin, which is why the site was named for him following his untimely death. The group included dozens of amateurs and professionals. The Minkin site is now preserved in perpetuity thanks to the New Acton Coal Mining Company, the Alabama Department of Conservation and Natural Resources, the Geological Survey of Alabama, and dedicated paleontologists.
The same group of amateur and professional paleontologists that ensured site preservation published the first major scholarly work about the trace fossils and the site, titled Pennsylvanian Footprints In the Black Warrior Basin of Alabama. The contents range from historical essays to technical discussions of the trackways.
Besides trackways made by reptiles and amphibians, with some slabs containing dozens of footprints, the Minkin site has produced
the undulating traces made by fish swimming in very shallow water;
horizontal and vertical burrows made by fly larvae;
walking, resting, and jumping traces made by horseshoe crabs, millipedes, and other arthropods;
insect wings, a spider, and other arthropod body fossils; and
ferns, treelike cycads, giant horsetails, and other plant fossils.
The following photographs illustrate a small part of the amazing diversity of trace fossils found at the Minkin site. Scales on all photographs in centimeters.
Cincosaurus cobbi, trackway of small reptile moving from left to right. The tracks of small reptiles like this are the most common vertebrate trace fossils found at the site.
Nanopus reideae, trackways of small amphibians that crossed paths on the tidal flat. Amphibian tracks differ from those made by reptiles in several ways. For instance, (1) the forefeet of amphibians have only four toes and (2) they are usually significantly smaller than the hindfeet.
Kouphichnium, trackway of horseshoe crab, which moved from left to right. Invertebrate trackways like this are among the most common trace fossils found at the site.
Diplichnites, trackways of two millipedes that prowled the mud flats in search of food.
Undichna, paired wavy lines inscribed by the fins of two fish swimming in very shallow water. One fish moved vertically across the slab; the other moved from top to lower left (direction of movement indicated by "points" of the curves pointing backward).
Impressions of a pair of wings from a large insect. Only six or seven body fossils have been found at the site, but each represents a different kind of arthropod.
Impression of a small spider.