Fossils of the Northwest Central US

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. Most organisms never become fossils, but instead decompose after death, and any hard parts are broken into tiny fragments. In order to become fossilized, an organism must be buried quickly before it is destroyed by weathering, is decomposed, or is 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).

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, and it is these fossils that provide some of the oldest evidence for life on Earth.


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. The Northwest Central States contain numerous examples of such exceptional preservation, also called lagerstätten, including the Miocene Clarkia fossil beds in northern Idaho, the Bear Gulch Beds of central Montana, the Florissant fossil beds of Colorado, the Agate bone beds of Nebraska, and the Eocene Green River Formation of Utah, Colorado, and Wyoming.

Discovering Ancient Environments

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, or whether it 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 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.

Paleontologists use fossils as a record of the history of life. Fossilized organisms are also extremely useful for understanding the ancient environment that existed 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 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. This use of fossils to determine relative age in geology is called biostratigraphy.

Index fossils are used to determine the age of many deposits that cannot be dated radiometrically. This practice is called biostratigraphy. An ideal index fossil lived during a short period of time, was geographically and environmentally 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.

Ancient Biodiversity

Since life began on Earth more than 3.7 billion years ago, it has continuously become 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. The diversity of life has, in general, 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 significant 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 (Figure 3.1), and these specific mass extinction 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.

Different fossils are found in different regions because of the presence of rocks deposited at different times and in a variety of environments. The availability of fossils from a given time period depends both on the deposition of sedimentary rocks and the preservation of these rocks through time.

Figure 3.1: The history of life in relation to global and regional geological events and the fossil record of the Northwest Central US (Time scale is not to scale).

Figure 3.1: The history of life in relation to global and regional geological events and the fossil record of the Northwest Central US (Time scale is not to scale). NOTE: Clicking on this image will bring up a high-resolution version that may take a few minutes to load. This version will expand to the size of your browser window, so enlarge the window for maximum visibility.

Fossils of the Northwest Central US

The rocks of the Northwest Central United States contain abundant and spectacular fossils, and preserve an excellent fossil record of many aspects and intervals in the history of life (see Figure 3.1). Indeed, this area contains some of the most spectacular fossil deposits in the world, and in many places, fossils are almost everywhere. In the remainder of this chapter, we will highlight the major types of fossils present in most of the geologic periods represented by rocks in each state. The references at the end of the chapter should be consulted for details, especially for identifying particular fossils you might find.