The age of rock given as the number of years since the rock formed.

The geologic principle that states rocks horizontal layers of sedimentary rock ,each layer is older than the layer above it and younger than the layer below it.

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

The process in which the nuclei of radioactive elements breakdown,releasing fast moving particles and energy.

The process of determining the age of an object using the half-life of one or more radioactive isotopes.

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

One of the three long units of geologic time between the precambrian and the present.

One of the units of geologic time into which geologist divides eras.

An animal without a back bone.

An animal with a backbone.

A vertebrate whose body temperature is determined by the temperature of its environment,and that lives its early life in water and its adult life on land.

A vertebrate whose temperature of its environment ,that has lungs and scaly skin, and that lays eggs on land.

When many species of living things die off off at the same time.

A vertebrate whose body temperature is regulated by its internal heat,and that has skin covered with hair or fur and glands that produce milk to feed its young