Image above:
Topics covered on this page: What are fossils?; Types of Fossil Preservation; Lagerstätten; Ancient Biodiversity; Biostratigraphy; Discovering ancient environments.
Credits: Most of the text of this page is derived from "Fossils" by Warren D. Allmon, chapter 3 in The Teacher-Friendly Guide to the Geology of the Southeastern U.S., 2nd. ed., edited by Andrielle N. Swaby, Mark D. Lucas, and Robert M. Ross (published in 2016 by the Paleontological Research Institution; currently out of print). The book was adapted for the web by Elizabeth J. Hermsen and Jonathan R. Hendricks in 2021. Changes include formatting and revisions to the text and images. Credits for individual images are given in figure captions.
What are Fossils?
Fossils (from the Latin word fossilis, meaning "dug up") are the remains or traces of organisms that lived in the geologic past (older than the last 10,000 years), now preserved in the Earth's crust. Paleontologists use fossils as a record of the history of life. They tell us that an incredible multitude of organisms lived prior to the species that we see on Earth today; that most species that ever lived have become extinct; and that living things have changed through evolution over time, from one species into another, and adapted to changing environments. Fossilized organisms are also extremely useful in understanding the ancient environment that existed when they were alive.
Most organisms never become fossils, but instead decompose after death, and any hard parts are broken into tiny fragments. In order to fossilize, an organism must be buried quickly before it is destroyed by weathering, decomposed, or eaten by other organisms. This is why fossils are found almost exclusively in sediment and sedimentary rocks. Igneous rocks, which form from cooling magma or lava, and metamorphic rocks, which have been altered by heat and pressure, are unlikely to contain fossils (but may, under special circumstances).
Types of Fossil Preservation
Body fossils
Body fossils are fossils that consist of an actual part of an organism, such as a bone, shell, or leaf, are known as body fossils.Body fossils may be preserved in a number of ways, common examples of which are given below:
Not transformed
The fossil is preserved in its original form. For example, the original mineral skeleton of an organism is preserved. This is most likely to occur in young fossils.
Mineral replacement
Mineral replacement: Chemical replacement of the material making up a structure (like a shell or a bone) by a more stable mineral.
Recrystallization
Replacement by a different crystal form of the same chemical compound. For example, the replacement of aragonite in a shell by calcite. Both minerals have the same chemical formula (calcium carbonate, or CaCO3) but a different crystal structure.
Permineralization
Filling of empty spaces in bone, a shell, wood, or other structures by minerals. Coal balls, a strange type of plant preservation found in coal-producing deposits in the eastern United States, are a type of permineralization.
Petrifaction or petrification
Complete replacement of the original organic material by minerals, as in petrified wood.
Molds and casts
Impressions of the exterior or interior of a structure, such as a shell.
Impressions and compressions
Imprints of structures that may or may not have a carbon film. These terms are often used for leaves.
Inclusions
Inclusions are organisms or parts of organisms that have been trapped in resin, particularly amber. Amber itself is fossil resin, which was produced by a number of different ancient plants depending on the age and location
Ichnofossils
Ichnofossils or trace fossils are the fossil records of the actions or behaviors of organisms, such as footprints, burrows, and feeding traces.
Chemical fossils
Chemical fossils are chemicals produced by an organism that leave behind an identifiable trace in the geologic record, and it is these fossils that provide some of the oldest evidence for life on Earth.
Lagerstätten
The US contains numerous examples of exceptional preservation, also called Lagerstätten. In a Lagerstätte, the "soft" tissues of an organism, such as skin, muscles, and internal organs, are typically not preserved as fossils. 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. Examples of Lagerstätten by region include:
Midwestern US:
- Carboniferous (Pennsylvanian) Mazon Creek Fossil Beds southwest of Chicago, Illinois.
Northeastern US:
- Late Cretaceous Raritan Formation (mesofossils and amber), New Jersey.
Northwest Central US (northern Rockies and Great Plains):
- Carboniferous (Mississippian) Bear Gulch Limestone, Montana.
- Paleogene (Eocene) Green River Formation, Colorado, Utah, and Wyoming.
- Neogene (Miocene) Clarkia fossil beds, northern Idaho.
- Neogene (Miocene) Agate Bone Beds, Nebraska.
South-central US:
- Carboniferous (Pennsylvanian) Hamilton Quarry, Kansas.
- Late Cretaceous Smoky Hill Chalk, Kansas.
Southeastern US:
- Cambrian Conasauga Formation of northwestern Georgia.
- Triassic-Jurassic insect fossil beds in Virginia's Culpepper Rift Basin.
Southwestern US:
- Cambrian Marjum Formation and Wheeler Shale, Utah.
- Triassic Chinle Formation (Petrified Forest), Arizona.
- Paleogene (Eocene) Green River Formation, Colorado, Utah, and Wyoming.
- Paleogene (Eocene) Florissant fossil beds, Colorado.
Western US (West Coast, Nevada, Hawaii, and Alaska):
- Cambrian Indian Springs Lagerstätte, Nevada.
- Pleistocene La Brea Tar Pits, California.
Ancient Biodiversity
Since life began on Earth more than 3.7 billion years ago, it has continuously grown more abundant and diverse. It wasn’t until the beginning of the Cambrian period, around 541 million years ago, that complex life—living things with cells that are differentiated for different tasks—became predominant. This event at the beginning of the Cambrian, called the Cambrian Explosion, resulted in the emergence of most major animal phyla. The diversity of life has generally increased through time since then. Measurements of the number of different kinds of organisms—for example, estimating the number of species alive at a given time—attempt to describe Earth's biodiversity. With a few exceptions, the rate at which new species evolve is significantly greater than the rate of extinction.
Most species have a lifespan of several million years; rarely do species exist longer than 10 million years. The extinction of a species is a normal event in the history of life. There are, however, intervals of time during which extinction rates are unusually high, in some cases at a rate of 10 or 100 times the normal pace. These intervals are known as mass extinctions. There were five particularly devastating mass extinctions in geologic history, and these specific events have helped to shape life through time. Unfortunately, this is not just a phenomenon of the past—it is estimated that the extinction rate on Earth right now may be as much as 1000 times higher than normal, due mostly to human activity, and that we are currently experiencing a sixth mass extinction event.
Biostratigraphy
Fossils are 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 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. Some of the most useful index fossils are hard-shelled organisms that were once part of the marine plankton. This use of fossils to determine relative age in geology is called biostratigraphy. The geologic time scale is in part based on sequences of fossils correlated from around the world.
Discovering Ancient Environments
The study of the relationships of fossil organisms to one another and their environment is called paleoecology. The kinds of animals and plants living in a particular place depend on the local environment. The fossil record preserves not only fossil organisms, but also evidence of what their environments were like. By studying the geological and biological information recorded in a rock that contains a fossil, scientists can determine some aspects of the paleoenvironment.
Grain size and composition of the rock can tell us what type of sediment surface the animal lived on, what the water flow was like, and whether the sediment was transported in a current. Grain size also tells us about the clarity of the water. Fine-grained rocks such as shales are made of tiny particles of silt or clay that easily remain suspended in water. Thus, a fossil found in shale might have lived in muddy or very quiet water. Filter-feeding organisms, such as clams or corals, are not usually found in muddy water because the suspended sediment can clog their filters.
Sedimentary structures, such as asymmetrical ripples and cross-beds, can indicate that the organism lived in moving water. Mud cracks or symmetrical ripples are characteristic of shoreline or intertidal environments. Broken shells or concentrated layers of shells may indicate transportation and accumulation by waves or currents. Color of the rock may indicate the amount of oxygen in the water. If there is not enough oxygen in the water, organic material (carbon) in sediments will not decompose, and the rock formed will be dark gray or black in color.