Bellwork: In what ways has the Earth changed through time Bellwork: In what ways has the Earth changed through time? How do we know and gather information about these changes ?

Section 19.1: The fossil record The history of Life Section 19.1: The fossil record

Fossils and Ancient Life Fossils provide information about extinct species Fossils can vary greatly Different sizes, types and degrees of preservation Not every organism will become a fossil

Types of Fossils

Trace Fossils

Fossils in Sedimentary rocks Why don’t all organisms become fossils?

What information can fossils tell you? The structure of an organism Lead to ideas about evolutionary relationships The Environment an Organisms lived in Fossilized plants and leaves give an idea of the ecosystem The way in which an organism lived Footprints and bone structure give information about how an animal moved Groups of fossils give information about how organisms interacted

How do researchers know the age of fossil specimens? Two types of dating Relative dating Radiometric dating Relative dating used the presence of an index fossil or other feature to determine whether a rock is older or younger than another rock Can allow paleontologists to tell the age of a rock

What is an index fossil? Index fossils are fossils used to establish the relative age of a rock layer Often distinctive Found in many places Only existed on Earth for a small amount of time Trilobites are a great example 15,000 different species Can be used to date nearly 300 million years worth of rocks

What is radiometric dating? Radiometric dating uses the proportion of radioactive to stable isotopes to calculate the age of a sample Different elements decay at different rates The rate of decay is referred to as the half life Carbon 14 decays to Carbon 12 naturally in the atmosphere When a plant photosynthesizes it will take in a small amount of Carbon 14 Upon death, this amount of carbon is fixed – and will decay over time to carbon 12 Carbon 14 will not be replenished, so by measuring the amount of carbon 14 remaining, the age of the plant can be calculated Animals also get carbon from eating plants so can be dated this way Half life of carbon is 5730 years

How do you date samples older than 60,000 years? Carbon-14 is not the only isotope used for radiometric dating Elements such as Rubidium (half life 48.8 billion years), Uranium (half life 4.5 billion years). By choosing the correct element you can essentially date anything on Earth These are often used alongside index fossils to determine the ages of geological units

Why don’t we just radiometrically date everything? Because it is very expensive and takes a very long time!

How old is the Earth? 4.6 billion years old Divided into Eons, Eras, Periods and Epochs Precambrian is the first 3.5 billion years Our Geological period only started 1.8 million years ago

Geological time as a clock It is often hard to comprehend simply how old the Earth is, and for how little humans have occupied it This graphic puts it into perspective

How has the Earth changed through geological time?

What has affected conditions on Earth through time? Physical Forces Climate is constantly changing Great ice age 10,000 years ago, average temperature was only 5 Celsius lower, During the Mesozoic, average temperatures were only 6 – 12 Celsius higher Earth is constantly changing – Mountains, Volcanoes, New Coastlines, which has knock on effects to habitats Plate Tectonics and Super- continental Cycles are a powerful driving force

Biological effects on Earth through time Life can have a large affect in shaping Earth Early Oceans were full of dissolved Iron Atmosphere did not have much Oxygen Some of the first organisms to Evolve were photosynthetic bacteria These took in CO2 and gave out O2 CO2 is a greenhouse gas – the reduction in CO2 levels lead to drops in global temperatures As temperatures dropped, Iron reacted with atmospheric oxygen, was no longer soluble in the oceans and precipitated out This settled to the ocean floor – producing banded Iron formations

Key points Fossils are a powerful tool used to aid our understanding of the evolution of the Earth They can be dated through either relative or radiometric dating Earth is 4.6 billion years old The Earth’s surface has been shaped a number of different physical and biological factors over the course of it’s history