Fossils are the preserved remains of animal and plant life often found embedded in rocks. Earth contains three types of rocks, metamorphic, igneous and sedimentary. Metamorphic and igneous rocks undergo too much heat and pressure to be able to preserve fossils as most fossils are found in sedimentary rocks, which exert gentle pressure and allows preservation of fine details of past life-forms. Fossils become a part of sedimentary rocks when sediments such as mud, sand, shells and pebbles cover plant and animal organisms and preserve their characteristics through time.
Shale is a sedimentary rock formed when parts of larger rocks broke off and settled in the bottom of the ocean, covering creatures that lived there. Mud and clay combined with minerals and other particles over time to harden into shale. The hard parts of the creatures covered with mud underwent preservation as fossils when consolidated with other materials inside the shale. Split shale easily into layers to uncover the fossils inside. Normally found intact, fossils inside shale often include brachiopods, fossilized plants, algae, crustaceans and arthropods trapped in the hardened mud.
Limestone forms when calcite from the water crystallizes or when fragments from coral and shells cement together. Limestone is a type of sedimentary rock that contains fossils in the form of sea creatures. Entire reef formations and communities of organisms are found preserved in limestone. The types of fossils found in limestone include coral, algae, clams, brachiopods, bryozoa and crinoids. Most limestone forms in shallow tropical or subtropical seas. In some cases, fossils make up the entire structure of limestone.
Cemented together grains of sand become sandstone. Since sandstone is a coarse material, fossils found in them are not as well preserved as those in shale and limestone. Sandstone rarely contains delicate fossils. Found in a wide range of environments, including beaches, oceans, sand bars, dunes, rivers, deltas, deserts and flood plains, sandstone contains fossils of creatures such as trilobytes, brachiopods, crustaceans, bryozoans and plants.
Conglomerate rocks made up of a combination of small rounded pebbles, usually containing quartz, are cemented together over time. They form faster than shale, limestone and sandstone and are under more pressure from fast-moving waves, so they are not as likely to incorporate fossils. Conglomerate rocks do provide fossils periodically, however, in the pebbles that make up the rocks. Sponges, brachiopods and gastropods are some fossils found in conglomerate rocks.