A trace of an ancient organism that has been preserved in rock.

An animal that has a backbone.

A long and distinct period of history with a particular feature or characteristic.

Event in which many types of living things become extinct at the same time.

A technique used to determine the actual age of a fossil on the basis of the amount of a radioactive element it contains.

States that younger layers of rock are deposited on top of older layers.

A gap in the geologic record that shows where rock layers have been lost due to erosion.

The atoms in one element break down to form atoms of another element.

A vertebrate that lays eggs and has lungs and scaly skin.

An animal that does not have a backbone.

A record of the geologic events and life forms in Earth's history.

The number of years since the rock formed.

The age of a rock compared to the ages of other rocks.

Able to live both on land and in water.