Wednesday 12/16/15Learning Goal: Explain how fossils form, how scientists determine a fossil’s age, and the Geologic Time Scale.

Warm-up:How do most fossils form?

Homework:Chapter 6 test on Wednesday!

How do most fossils form?

A dead organism becomes buried in sediment

Chapter 6 Section 3

The Fossil Record

FossilsThe preserved remains or traces of

organisms that lived in the past

How Do Fossils Form?

Hard parts of organisms form fossilsBones, Shells

Most fossils form when organisms that die become buried in sediments (particles of soil and rock)

Types of FossilsPetrified Fossils

Remains of organisms that are actually changed to rock

Molds and CastsMold: a hollow space in sediment in the shape

of an organism (cake pan)Cast: a copy of the shape of the organism that

made the mold (cake)Preserved Remains

Can be preserved in other substances other than sedimentsICE

Determining a Fossil’s AgeRelative Dating Radioactive Dating

a technique used to determine which two fossils are older

• Oldest layer at bottom• Younger layer at top

a technique used to determine the actual age of fossil on the basis of the amount of a radioactive element it contains

• Radioactive elements: unstable elements that decay, or break down, into different elements

• Half-life: the time it takes for half of the atoms in a radioactive element to decay

Radioactive Decay

What Do Fossils Reveal?

Fossil record: the millions of fossils that scientists have collected

Extinct OrganismsExtinct: no members of that species are

still aliveMost of what scientists know about extinct

species is based on the fossil record

The Geologic Time ScaleThe calendar of Earth’s historyThe largest span of time in the Geologic

Time Scale is Precambrian TimeThis is when living things appeared on Earth

Page 194-195

Mass ExtinctionsExtinction of many types of organisms at

the same timeCretaceous Period, about 65 million years

ago (plants, animals, dinosaurs)

Gradualism

A theory that proposes evolution occurs steadily in tiny changes over long periods of time

This is how Darwin thought evolution occurred

Punctuated Equilibria

the theory that species evolve during short periods of rapid change