The mountains were once sediment at the bottom of an ocean.

The ancient environment in the area did not favor fossilization.

The ancient environment in the area was much drier than it is today.

The mountains formed before the animals died, and their remains were fossilized.

0 years

650 million years

2.6 billion years

3.9 billion years

Ferns became less abundant during the Cenozoic era.

Ferns did not have the resources they needed during the Cenozoic era.

Species A disappeared after a mass extinction event caused the extinction of all ferns.

Species A became extinct because of increased competition from other species of ferns.

They existed for a relatively long period of geologic time, and they can be used as index fossils.

They existed for a relatively short period of geologic time, and they can be used as index fossils.

They existed for a relatively long period of geologic time, and they can be used as examples of Cenozoic era fossils.

They existed for a relatively short period of geologic time, and they can be used as examples of Paleozoic era fossils.

a plant covered by a lava flow

a small lizard covered by tree sap

a deer killed and eaten by predators

a bacterium dead on the bottom of the ocean