Fossil Record and Ancient Life: Discovering Extinct Species

1. Chapter 19 Section 19-1: The Fossil Record

2. Fossils and Ancient Life • Tell us about extinct species • Vary in size, type, degree of preservation • Only form under certain conditions • Incomplete

3. Types of Fossils • As large as entirely preserved animal, with skin, hair, scales, or feathers • As small as bacteria, developing embryos, pollen grains • Fragments – teeth, bone pieces, bits of leaves • Traces – footprint, burrows, tracks, droppings • Mostly form in sedimentary rock, some in amber

4. Fossils in Sedimentary Rock • Form when small particles of sand, silt, clay, or lime mud settle to the bottom of a body of water • Sediments bury dead organisms that have sunk to the bottom

5. Fossils in Sedimentary Rock • Sediments continue to build up and over many years water pressure compresses lower segments, turning them to rock • Soft body structures decay quickly, usually only hard parts like wood, shells, teeth, bone remain • If the organism is buried quickly, it is protected from aerobic decay and leaves imprints

6. What Fossils Can Reveal • Paleontologists (study fossils to learn about ancient life) compare body structures to living organisms, infer evolutionary relationships • Bone structure/trace fossils show how animals moved • Plant leaves/pollen suggest environment • Info compiled can recreate ecosystems

7. Relative Dating • Places rock layers and their fossils into temporal sequence – lower layers are older • Index fossils are distinctive fossils used to establish and compare the relative ages of rock layers and the fossils they contain • Same index fossil found in 2 widely separated rock layers, the rock layers similar in age

8. Relative Dating • Good index fossil species must be easily recognized, occur in only a few rock layers • Layers will be found in many places • Trilobites

9. Radiometric Dating • Relies on radioactive isotopes which decay into nonradioactive isotopes at a steady rate • Compares amounts of radioactive isotopes in samples to determine precise age • Half-life : The time required for ½ the radioactive atoms in a sample to decay • Different half-lives for different elements

10. Radiometric Dating • Carbon-14 has a short half-life (5730 years) and is commonly used to date young fossils • Fossils as old as 60,000 years • Naturally found in atmosphere, all living things • Decays to nitrogen-14 • Compare C-14 in fossil with C-14 in air • Older fossils dated by dating rock layers • Potassium-40: half-life 1.26 billion years • Uranium-238: half-life 4.5 billion years

11. Geologic Time Scale • Timeline of Earth’s history • Basic divisions are eons, eras, periods

12. Establishing the Time Scale • Studied rock layers, index fossils, to organize things by relative age • Noticed patterns/changes at boundaries between certain layers, used these to determine divisions • Later, radiometric dating used to assign specific ages • No regular pattern

14. Naming the Divisions • Naming started before anything older than the Cambrian Period identified • Called everything before Precambrian time

15. Precambrian Time • Actually 90% of Earth’s history

16. Life on a Changing Planet • Earth/climate always changing, organisms evolved to new conditions • Global “heat wave” in Mesozoic Era temps 6-12 ° C higher than today • Ice Ages temps were 5° C cooler • Geological forces produced mountain ranges, moved continents • Volcanoes, local climates

17. Plate Tectonics • Solid continental plates move slowly over Earth’s molten core • Continental drift • Continents collided to form supercontinents, then split apart and reformed

18. Plate Tectonics • Mountain ranges rise where landmasses collide • Continents change position causing major ocean currents change course • Changes affect local and global climate

19. Geological Cycles and Events • Affects distribution of fossils and living organisms • Fossils of Mesosaurus, a semiaquatic reptile, have been found in both South America and Africa

20. Physical Forces • Evidence indicates that over millions of years, giant asteroids have crashed into Earth • Toss up so much dust, possibly blocking out sunlight and causing global cooling • Could have contributed to or caused worldwide extinctions

21. Biological Forces • Earth’s early oceans contained large amounts of soluble iron and little oxygen. • During the Proterozoic Eon, photosynthetic organisms produced oxygen gas, removed carbon dioxide from the atmosphere • Reduced the greenhouse effect, cooled the globe • Organisms today shape the landscape by building soil from rock, and sand and cycle nutrients through the biosphere