Shale is a form of sedimentary rock created from silt, sand and mud. Sedimentary rocks such as shale are fossil-rich areas because plants and animals trapped in mud get fossilised as the mud and sand dries out and hardens. In the United States, rich fossil deposits can be found in shale deposits in Utah, Nebraska, California and Kentucky. Fossilised plants, algae, crustaceans and arthropods can be found in shale. According to the Kentucky Geological Survey, brachiopod fossils are extremely abundant.
Algae and Cyanobacteria – Shale formations contain the fossilized remains of many types of plants. Among the most common of these are algae such as Yuknessia, a form of green algae. According to the University of California at Berkeley, the House Range shale deposits in Western Utah contain many non-mineralized algae and cyanobacteria fossils. These sometimes appear as dark irregular patches on the stone.
Trilobites – Trilobites are a common fossil find in places such as the Burgess Shale Formation. First appearing over 500 million years ago, the trilobites adapted to survive in many habitats. Some lived and hunted on the sea floor while others floated or swam. Their hard sectioned shells were made of calcite like the shells of clams and other brachiopods. According to the University of California at Berkeley, trilobite fossils have been collected by humans for a long time. One of the artifacts found at a 15,000-year-old settlement in France was a trilobite fossil made into an amulet.
Plant Fossils – Many plant fossils have been found in shale formations. In Kentucky, the black shale of the Knobs Region holds the fossil logs of seed ferns and large-scale trees. Many fossil-bearing shale formations are associated with coal mines such as the mines near Danville, Illinois. The shale formation associated with this mine holds a large section of a fossilized forest including many examples of seed fern giant lycopsids. According to the University of Illinois, the lycopsid was a type of giant club moss with hard circular scales covering its trunk.
Brachiopods – One of the most common fossils found in shale is the brachiopod. Many species of brachiopods exist today on the rocks and mud of the sea floor. All brachiopods are bivalve animals featuring bilateral symmetry. The top shell is a mirror image of the bottom shell, just as the left side of your body is a mirror image of your right side. There are many examples of brachiopods in the fossil record. According to the Kansas Geological Survey, brachiopod fossils first appeared in rock from 525 million years ago. The species was nearly wiped out during the great Permian mass extinction event that killed 90 percent of all living species approximately 250 million years ago.
Crinoids – Crinoids are among the oldest animal species on the planet. Related to starfish and sea urchins, crinoids are echinoderms. Echinoderms are animals with tough spiny outer coverings that display a radial symmetry. Crinoids are often referred to as sea lilies because of their flower-like shape. According to the Kansas Geological Survey, the crinoid had three body parts. The long stem was fashioned of ring-shaped sections and attached the animal to the sea floor. The calyx sat atop the stem and contained the mouth, stomach and anus. Atop the calyx, long food gathering arms waved in the water, capturing particles of food with tiny cilia and pushing it down to the mouth.