Fossilized shells are common fossil finds.
Fossils are created when organic material becomes trapped in a soft substance that hardens over time. These substances include tree resin, naturally occurring tar, ice, loose sand and soil, and natural concrete. Fossils are most often found in forms of sedimentary rock. Sedimentary rock is made up of the hardened buildup of sand, clay and other materials that are deposited by wind and water.
Shale
Shale is a smooth rock that is made from layers of clay. The layers make the rock flaky and the clay gives it a gray, brown, black or red color. A variety of fossils have been discovered since 1909 in the Burgess Shale, a very large shale deposit in the Canadian Rocky Mountains. These include what are widely regarded as the most well-preserved fossils of specimens from the Cambrian period, including trilobites, worms, and crinoids, or "sea lilies."
Limestone
Limestone is white or off-white calcium-based rock that is created from the buildup of shells. Limestone is usually found in oceans or places that were once oceans, so most of the fossils found in limestone are from aquatic plants and animals. The Redwall and Temple Butte limestone deposits in the Grand Canyon have produced marine fossils including clams, trilobites and corals as well as the fossils of freshwater fish.
Dolomite
Dolomite is similar to limestone in that it is formed from calcium-rich animal shells, but it contains more magnesium. In 1999, fossilized organisms derived from cyanobacteria were discovered in a 3.45 billion-year-old dolomite deposit in the Pilbara Range in northwest Australia, reports BBC News. Cyanobacteria is one of the oldest living organisms, and the fossils are considered to be among the oldest ever found.
Sandstone
As its name indicates, sandstone is a coarse-textured rock composed of fused sand particles. Though you can often slough off grains of sand from sandstone with your fingers, the rock can contain fossils from organisms as delicate as jellyfish. The Navajo Sandstone in the Colorado Plateau houses fossilized stromatolites, which are originally formed from cyanobacteria. Like the cyanobacteria fossils found in the Pilbara range, these are among the oldest fossils ever discovered, according to the National Park Service.
Siltstone
Siltstone is a gritty rock made from mica, clay, silica and other minerals. It is similar to shale in that it has layers and shale often has large amounts of silt in it, but siltstone is harder and its layers are thinner. Fossilized corals, bryozoa (jellyfish-like animals), and gastropods (a class of animals including snails and slugs) have been found in the siltstone deposits of the Owens Valley formation in Death Valley National Park.
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