(RM) Fossils Lesson

Fossils

Fossils, the remains of organisms preserved in sedimentary rocks, are part of the evidence scientists use to infer changing conditions at the earth's surface through time. Fossils are the preserved remains of past life on Earth. They are found in rocks deposited in the environment in which they lived. Fossils are generally most abundant in marine sedimentary rocks and generally, they are not found in igneous and metamorphic rocks. Fossils and present day bones are not the same. Fossils help scientists infer how Earth's surface has changed. Fossils are clues to what past environments were like.

There are many different types of fossils. 

Body fossils, though rare, include bones, teeth, or entire organisms preserved by freezing or being trapped in wax asphalt (tar), or amber. This is the only method for preservation of soft tissue.

Impression fossils, also known as carbon films show outlines of plants, feathers, or fish that die in sediment. As they decay, they leave a carbon deposit that shows as a dark print of the organism.

Trace fossils, Tracks, tail marks, burrows, teeth marks, and body outlines are considered impression fossils. These impressions form in soft sediment and are covered before they can be washed away or destroyed.

Mold and cast fossils are also impression fossils. This type of fossil basically makes a sort of copy of an ancient organism. A mold is formed when an organism is buried in sediment and decays, leaving a hole (the mold) in its place. If this mold is later filled with sediment, it produces a three-dimensional model (the cast) that resembles the organism.

Mineral replacement fossils are formed when an organism is buried in sediment. Water seeping into the bone dissolves the bone, which is replaced by minerals. Petrified wood is also an example of a mineral replacement (or permineralization) fossil. Petrified means "turning into stone".

Most Common Fossils

The most common fossils are found in sedimentary rock. Sedimentary layers act as evidence of the changing climate or movement of the continents during the passage of time. The Law of Superposition or Steno's law states that in a pile of undisturbed sedimentary rock, the oldest bed will lie at the bottom and the youngest on top. Layers of strata in different locations may have the same composition but carry fossils of a different time period, therefore a technique of zoning or an index fossil is used.

See description of diagram Links to an external site.

Index fossils are specific animals or plants that had a broad geographical distribution but existed for relatively short periods of time. These fossils allow geologists to establish a parallel between layers of sediment. Some excellent guide fossils are ammonites; each species lived for relatively short periods of time but had such a broad geographical distribution. They can be found today in stratigraphic rock layers often separated by great distances. The appearance of the same ammonite in different layers in different localities gives evidence that those layers were deposited at the same time. Each time period is marked by an abundant radiation of many new life forms or the mass extinction of past life forms.

Geologic Time

See description of diagram Links to an external site.

The geologic time scale provides geologists the world over with a shared time reference. The geologic time scale is divided into eons, eras, periods, epochs and ages with eons being the longest time divisions and ages the shortest. Many of the period names have historical precedents dating back to the 18th and 19th centuries when outcrops of rock across Europe were being systematically mapped using the rules of relative timekeeping (original horizontality, original continuity, superposition, cross-cutting relationships and faunal succession). In accordance with these rules, the geologic time scale is arranged so the oldest time divisions are at the bottom and the youngest are at the top. It was not until the twentieth century that absolute ages could be assigned to the geologic time scale using radiometric dating techniques. Even now these absolute dates occasionally undergo minor revision.

[CC BY 4.0] UNLESS OTHERWISE NOTED | IMAGES: LICENSED AND USED ACCORDING TO TERMS OF SUBSCRIPTION