Wednesday 12/16/15 Learning 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

Fossils The preserved remains or traces of organisms that lived in the past

How Do Fossils Form? Hard parts of organisms form fossils Bones, Shells Most fossils form when organisms that die become buried in sediments (particles of soil and rock)

Types of Fossils Petrified Fossils Petrified Fossils Remains of organisms that are actually changed to rock Molds and Casts Molds and Casts Mold: 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 Preserved Remains Can be preserved in other substances other than sediments ICE

Determining a Fossil’s Age Relative 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 Organisms Extinct: no members of that species are still alive Most of what scientists know about extinct species is based on the fossil record

The Geologic Time Scale The calendar of Earth’s history The largest span of time in the Geologic Time Scale is Precambrian Time This is when living things appeared on Earth Page

Mass Extinctions Extinction of many types of organisms at the same time Cretaceous 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