Fossils and Fossils and Geologic TimeGeologic Time

Early History of EarthEarly History of Earth Hot interior and surfaceHot interior and surface Volcanoes released gasesVolcanoes released gases

Water vaporWater vapor Carbon dioxideCarbon dioxide NitrogenNitrogen Mot much oxygenMot much oxygen

First organisms First organisms 3.9 billion years ago3.9 billion years ago

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

that lived long ago.that lived long ago.

trace cast petrified

imprint mold amber

Fossils – Clues to the PastFossils – Clues to the Past How do fossils form?How do fossils form?

Animal or plant covered in mud or Animal or plant covered in mud or sandsand

Minerals seep into the bodyMinerals seep into the body Pressure turns mud or sand into rockPressure turns mud or sand into rock

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

are?are? 1. Relative dating –1. Relative dating –

Rock forms in layersRock forms in layers Younger layers at topYounger layers at top Tells us which fossils are Tells us which fossils are

older or youngerolder or younger

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

fossils are?fossils are? 2. Radiometric Dating –2. Radiometric Dating –

Rocks have radioactive Rocks have radioactive isotopes – unstable atoms isotopes – unstable atoms that break down over timethat break down over time

We know how long it takes We know how long it takes to to break downbreak down

Absolute ageAbsolute age

Geologic Time ScaleGeologic Time Scale A calendar of Earth’s history A calendar of Earth’s history

based on fossilsbased on fossils 66 mya---

245 mya---

544 mya---

Geologic Time ScaleGeologic Time Scale

PrecambrianPrecambrian Until 544 million years agoUntil 544 million years ago Unicellular prokaryotesUnicellular prokaryotes Multicellular eukaryotesMulticellular eukaryotes

66 mya---

245 mya---

544 mya---

Geologic Time ScaleGeologic Time Scale

PaleozoicPaleozoic 544 to 245 million years ago544 to 245 million years ago Fish, land plants, Fish, land plants,

amphibians, reptilesamphibians, reptiles Ends with mass extinctionEnds with mass extinction

66 mya---

245 mya---

544 mya---

Geologic Time ScaleGeologic Time Scale

MesozoicMesozoic 245 to 66 million years ago245 to 66 million years ago Small mammals,Small mammals,

dinosaurs, birdsdinosaurs, birds Mass extinctionMass extinction

66 mya---

245 mya---

544 mya---

Geologic Time ScaleGeologic Time Scale CenozoicCenozoic

66 million years ago to today66 million years ago to today Mammals Mammals

66 mya---

245 mya---

544 mya---

Continental DriftContinental Drift

Pangaea 245 million years ago

Breaks up135 million years ago

Moving apart66 million years ago