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EARTH AND SPACE SCIENCE. Chapter 8 The Rock Record 8.3 The Fossil Record. 8.3 The Fossil Record Objectives. Describe four ways in which entire organisms can be preserved as fossils. List five examples of fossilized traces of organisms.
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EARTH AND SPACE SCIENCE Chapter 8 The Rock Record 8.3 The Fossil Record
8.3 The Fossil Record Objectives • Describe four ways in which entire organisms can be preserved as fossils. • List five examples of fossilized traces of organisms. • Describe how index fossils can be used to determine the age of rocks.
Introduction • Fossils are remains or indicators of a form of life from a previous geologic unit of time. • Fossils almost always occur in sedimentary rock and can be used to help determine the age of the rock. • Fossils also provide clues about the past geologic events, past climates, and the evolution of life over time. • Paleontology is the study of fossils.
Introduction • As remains of an organism are covered, they are protected from further degradation (although soft parts normally decay rapidly).
Interpreting the Fossil Record • The geologic history of the Earth is recorded in the fossil record. • Fossils provide important clues about how environments have changed throughout Earth’s history and how life has adapted to those changes. • The occurrence of a fossil reef in Kentucky, for example, tells us that the area was once under a shallow ocean in a more tropical latitude.
Fossilization • The remains of dead organisms are often consumed by other organisms or completely broken down by bacterial decomposition and weathering of hard parts. • Only remains that are buried or covered rapidly will be preserved in the fossil record. • Normally, hard parts such as teeth, wood, shells, and bones are preserved. • In rare cases, soft tissues are preserved.
Fossilization • Sometimes, only a cast of the original organism remains. • Some fossils are just evidence that a creature was there, but incorporate no body parts, such as coprolites or tracks.
Types of Fossils • Fossils may be broken down into two major categories. • Actual remains – some part of the organism’s body is preserved • Trace fossils – indicator of a past form of life that is not an actual body part • Trace fossils provide important information about habit and behavior of organisms that sometimes cannot be learned from body fossils.
Types of Fossils • Actual remains may be unaltered, such as a fossil oyster who’s shell remains unchanged through time, or they may be altered like petrified wood in which minerals have replaced the wood at the cellular level. • Soft tissues are sometimes preserved when tree sap engulfs small organisms, such as insects, and polymerized into amber. • Also, soft tissues can be preserved in the permafrost in arctic regions in which an animal has remained frozen for thousands of years, such as a mammoth.
Index Fossils • Index fossils are remains of creatures that only lived during a short time span, are easily recognized, and are widely distributed. • Index fossils help scientists determine the age of a strata by the fossils presence.
Index Fossils and Absolute Age • Since index fossils are remains of creatures that lived only during a short time span, their presence can allow scientists to assign an absolute age to a rock unit if they are present in the rock unit. • For example, if a hamulid worm tube that lived from 85 m.y.a. to 68 m.y.a. were found in the same rock unit as an echinoid that lived from 72 m.y.a. to 65 m.y.a., then the rock unit can be accurately described as being 70 million years old (± 2 million years).
References • Ash Falls State Park -http://www-museum.unl.edu/research/vertpaleo/ash-feat.html • Cincosaur Track – Ron Buta photo • Exogyra costata - http://www.ucmp.berkeley.edu/mollusca/bivalve?D=A • Hamulus squamosa - http://www.catnapin.com/public/Fossil/CoralReef/Worms.htm • Micraster leskei - http://www.educarm.es/paleontologia/galequin.htm
