Almost all fossils are preserved in sedimentary rock. Organisms that live in topographically low places such as lakes or ocean basins have the best chance of being preserved. This is because they are already in locations where sediment is likely to bury them and shelter them from scavengers and decay.
Mudstone, shale, and limestone are examples of sedimentary rock likely to contain fossils. As the layers of sediment build up on top of one another, they create a physical timeline. The oldest layers, along with the organisms that were fossilized as they formed, are deepest. The youngest layers are found at the top.
Reading the layers is complicated by the fact that as continents move and mountains rise, the layers are often tipped sideways and altered in other ways. Igneous rock is formed by the cooling of molten rock, either at the surface lava , or underground as granite or other similar rocks.
The heat of molten rock usually incinerates organisms rather than preserving them. There are some exceptions, such as tree trunks surrounded by lava and preserved as casts, but these are rare. Since rapid burial in sediment is important for the formation of fossils, most fossils form in marine environments, where sediments are more likely to accumulate.
Fossils come in many types. Those that consist of an actual part of an organism, such as a bone, shell, or leaf, are known as body fossils ; those that record the actions of organisms, such as footprints and burrows, are called trace fossils.
Body fossils may be preserved in a number of ways. These include preservation of the original mineral skeleton of an organism, mineral replacement chemical replacement of the material making up a shell by a more stable mineral , recrystallization replacement by a different crystal form of the same chemical compound , permineralization filling of empty spaces in a bone or shell by minerals , and molds and casts, which show impressions of the exterior or interior of a shell.
Chemical fossils are chemicals produced by an organism that leave behind an identifiable trace in the geologic record. Chemical fossils provide some of the oldest evidence for life on Earth.
Index fossils are used to determine the age of many deposits that cannot be dated radiometrically. An ideal index fossil lived during a short period of time, was geographically and environ- mentally widespread, and is easy to identify.
Some of the most useful index fossils are hard-shelled organisms that were once part of the marine plankton. Paleontologists use fossils as a record of the history of life. Fossils are also extremely useful for understanding the ancient environment that existed in an area when they were alive. The study of the relationships of fossil organisms to one another and their environment is called paleoecology. Fossils are also the most important tool for dating the rocks in which they are preserved.
Because species only exist for a certain amount of time before going extinct , their fossils only occur in rocks of a certain age. The relative age of such fossils is determined by their order in the stacks of layered rocks that make up the stratigraphic record older rocks are on the bottom and younger rocks on the top—a principle called the Law of Superposition.
Such fossils are known as index fossils. The most useful index fossils are abundant, widely distributed, easy to recognize, and occur only during a narrow time span. Exceptions to this rule occur when conditions favor rapid burial and mineralization or very slow decay. The absence of oxygen and limited disruption of the sediment by burrowing are both important for limiting decay in those deposits where soft tissues are preserved.
This fossil assemblage includes the remains of many animals with hard mineralized skeletons, such as echinoderms and brachiopods, but many of these fossils also preserve non-mineralized parts, such as tentacles, gut tracts, and soft appendages.
See Chapter 1: Geologic History to learn more about accreted terranes. Of the sedimentary rocks, most fossils occur in shale, limestone and sandstone. Earth contains three types of rocks: metamorphic, igneous and sedimentary. With rare exceptions, metamorphic and igneous rocks undergo too much heat and pressure to preserve fossils.
So most fossils are found in sedimentary rocks, where gentler pressure and lower temperature allows preservation 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.
Mud forms when larger rocks erode into tiny, usually microscopic, particles. These particles settle in the calm waters of lakes, swamps and the ocean, covering creatures that live there.
Mud and clay combine with minerals and other particles over time to harden into shale. The hard parts of the creatures covered with mud undergo preservation as fossils when consolidated with other materials inside the shale.
Shale splits easily into layers to reveal any fossils inside. Fossils inside shale often include brachiopods, fossilized plants, algae, crustaceans and arthropods trapped in the hardened mud. The very small mud and clay particles allow small details of organisms to be preserved, like the rare fossils of soft-bodied organisms found in the Burgess Shale.
Limestone forms when calcite from the water crystallizes or when fragments from coral and shells cement together. Limestone often contains fossils of shelled sea creatures. Entire reef formations and communities of organisms are found preserved in limestone. The types of fossils found in limestone include:. Most limestone forms in shallow tropical or subtropical seas. In some cases, fossils make up the entire structure of limestone.
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