Fossils and Geologic Time

Early History of Earth Hot interior and surface Volcanoes released gases

Water vapor Carbon dioxide Nitrogen Mot much oxygen

First organisms 3.9 billion years ago

Fossils – Clues to the Past A fossil is evidence of an organism that lived

long ago.

trace cast petrified

imprint mold amber

Fossils – Clues to the Past How do fossils form?

Animal or plant covered in mud or sand Minerals seep into the body Pressure turns mud or sand into rock

Fossils – Clues to the Past How do we know how old fossils are?

1. Relative dating – Rock forms in layers Younger layers at top Tells us which fossils are

older or younger

Fossils – Clues to the Past How do we know how old fossils

are? 2. Radiometric Dating –

Rocks have radioactive isotopes – unstable atoms that breakdown over time

We know how long it takes to break down

Absolute age

Geologic Time Scale A calendar of Earth’s history

based on fossils66 mya---

245 mya---

544 mya---

Geologic Time Scale

Precambrian Until 544 million years ago Unicellular prokaryotes Multicellular eukaryotes

66 mya---

245 mya---

544 mya---

Geologic Time Scale

Paleozoic 544 to 245 million years ago Fish, land plants, amphibians,

reptiles Ends with mass extinction

66 mya---

245 mya---

544 mya---

Geologic Time Scale

Mesozoic 245 to 66 million years ago Small mammals,

dinosaurs, birds Mass extinction event

66 mya---

245 mya---

544 mya---

Geologic Time Scale Cenozoic

66 million years ago to today Mammals 66 mya---

245 mya---

544 mya---

Continental Drift

Pangaea 245 million years ago

Breaks up135 million years ago

Moving apart66 million years ago