Geologic Time Scale

How are Fossils Formed?

The right conditions for creating a fossil depend on both the type of organism and the environmental conditions in which it dies.

These conditions combine to determine an organism’s preservation potential.

Preservation potential is a term for an organism’s likelihood of becoming a fossil.

Preservation potential also includes how well the organism will be preserved.

Fossils and Dating Geologic Events

According to geological evidence, Earth is about 4.6 billion years old.

The oldest fossils that have been found are those of bacteria that lived about 3.8 billion years ago.

For hundreds of millions of years, the only living things were single-celled organisms.

Fossils and Dating Geologic Events

Scientists use the fossil record to create systems called geologic time scales.

The fossil record is a sum of all of the fossils, or evidence of living things on Earth.

Geologists study the fossil record to make inferences and draw conclusions about environments on Earth at different times in Earth history.

Fossils and Dating Geologic Events

The most widely used geologic time scale is split into a series of time periods within longer time periods.

The longest time period is usually an eon.

Eons are split into eras.

Eras are split into periods, which in turn are split into epochs, and then ages.

Fossils and Dating Geologic Events

Each boundary between divisions is marked by the appearance, disappearance, or change in dominance of particular fossil groups.

For example, the division between the Devonian and Carboniferous periods is marked by the appearance of primitive trees and the first land animals with backbones.

Some divisions are also accompanied by changes in the or a major natural disaster that led to the extinction of millions of species.

Large mass extinctions have occurred at least five times in Earth’s history.

Geologists have given names to periods of time in geological history.

What geological epoch are we living in?

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Geologic Time Scale

Fossils

In some areas of the country, you can even find fossils in plain sight, lodged in the stone of a bluff or a quarry.

When fossils are found like this, scientists can use the information in the stone around them to estimate the time period during which the animal or plant lived.