Chapter 5, Section 2 The Fossil Record Monday, January 11, 2010 Pages 157 -- 164

Objectives Describe how most fossils form Explain how a scientist determines a fossil’s age. Explain what fossils reveal. Describe the main events of the Geologic Time Scale. Distinguish between gradualism and punctuated equilibria.

Vocabulary Words Fossil Sedimentary rock Petrified fossil Mold Cast Relative dating Absolute dating Radioactive element Half-life Fossil record Extinct Gradualism Punctuated equilibria

What is a fossil? A fossil is the preserved remains or traces of an organism that lived in the past. Other fossils include things like footprints or burrows left in mud that later turned to stone. Fossil of a spider.

How Do Fossils Form? Fossils can form from bones, tooth, shell, or other part of an organism. Most fossils form when organisms that die become buried in sediments. Over millions of years, the sediments build up and layers harden to become sedimentary rock.

Petrified wood from Arizona. Petrified Fossils Petrified fossils come from remains that are buried in sediments that eventually turns to rock. Gradually, the minerals replace the remains, changing them into rock. Petrified wood from Arizona.

Amonite mold - left and mold - right. Molds and Casts Sometimes shells or other parts that are buried by sediments are gradually dissolved. The empty space that remains in its place in the shape of the organism is a mold. A cast is a copy of the shape of the organism that made the mold. Amonite mold - left and mold - right.

Preserved remains of an elephant that fell in ice. Entire organisms have been found preserved in substances other than sediments. Low temperatures preserved the animal’s soft parts. Other ancient animals have been preservd in tar pits (tar is a sticky form of oil). Preserved remains of an elephant that fell in ice.

Determining a Fossil’s Age The age of a fossil can be determined in two ways: Relative Dating Absolute Dating

Relative Dating Relative dating is used to scientists to determine which of two fossils is older. Mainly used when looking a various layers. The layers closest to the outside are younger. Relative dating will not determine the actual age of the fossil.

Absolute Dating Absolute dating allows scientists to determine the actual age of a fossil. Fossils are found near rocks that contain radioactive elements. Radioactive elements are unstable and break down to form other elements. The half-life of a radioactive element is the time it takes for half of the atoms in a sample to decay. The amount of radioactive element in a sample is compared to the amount of the element into which it breaks down.

Fossil Record Fossil record is the millions of fossils that scientists have collected. The fossil record is incomplete. Fossil records provide clues about how and when new groups of organisms evolved. Most of the species preserved as fossils are now extinct. Members of this species are no longer alive.

Geologic Time Scale The geologic time scale is also known as the calendar of Earth’s history. Largest span is the Precambrian Time.

Gradualism Gradualism is the process that evolution occurs slowly but steadily. According to this theory, tiny changes in a species gradually add up to major changes over very long periods of time. This is what Charles Darwin believed.

Punctuated Equilibria Species evolve during short periods of rapid change. Periods of rapid change are separated by long periods of little or no change. According to this theory, species evolve quickly when groups are isolated and adapt to new environments.

Homework Workbook 5.2 (1/13) Worksheet 5.2 (1/13) Vocabulary quiz 5.2 (1/14)