Fossils, Rocks, and Time

1. Fossils, Rocks, and Time

2. What is a fossil? • Remains or traces (footprints and molds) of once living organisms preserved in the rocks • Most are remains of extinct organisms

3. How is a fossil preserved? • To become a fossil an organism has to be quickly covered by sediments before being damaged or eaten • Minerals can replace bone and shell • Later the fossil must be exposed to be found by scientists • Tar and amber (tree sap) can also preserve organisms • It is more likely to be covered by sediment if in water so land fossils are less common than marine • http://www.pbs.org/wgbh/evolution/library/04/3/quicktime/l_043_01.html

4. Geologic Time • Earth is about 4.5 billion years old = 100% • Draw a pie graph of the following: • Precambrian 4055 million years = 88% • Paleozoic 300 million years = 7% • Mesozoic 179 million years = 4% • Cenozoic 66 million years = 1%

5. Geologic Time in 4 Eras • Precambrian – birth of planet until complex life forms • Paleozoic – Age of ancient life – trilobites, corals, fish, plants and reptiles • Mesozoic – Age of midieval life – dinosaurs dominate – warm and wet climate • Cenozoic – Age of recent life – mammals and modern organisms – cooler and drier climate

6. Relative Dating • Relative Dating – ages based on position of layers compared to other layers • 1785-1800 Hutton states layers represent a period of geologic time and that the older layers are on the bottom • Index fossils are used to compare rock layers from different areas of the earth

7. Fossils as Evidence • Darwin saw fossils of extinct organisms • Organisms that did not look like any living thing • Scientists could see that fossil organisms appeared and disappeared over time • Used to believe repeated catastrophes wiped out these extinct organisms • Darwin had a different explanation: older species give rise to younger species

8. Fossils show series of changes over time Ankle bones similar to hippos Limbs become more reduced and adapted for swimming

9. Fossils show series of changes over time

10. Reptiles to Bird-- Archeopteryx Unlike all living birds, Archaeopteryx had a full set of teeth, a rather flat sternum ("breastbone"), a long, bony tail, gastralia ("belly ribs"), and three claws on the wing which could have still been used to grasp prey (or maybe trees). However, its feathers, wings, furcula ("wishbone") and reduced fingers are all characteristics of modern birds.

11. Absolute/Radiometric Dating • Ties an actual number of years to a rock layer • Based on known radioactive decay rates of various elements – discovered in 1896 • Precise dating methods since 1950 • Radioactive decay an atom of one element (parent) loses a proton or neutron to become another element (daughter)

12. Half - Life • Decay rate of an element • Half-life = time it takes for ½ of a particular radioactive element in a sample to decay into the daughter element • Uranium-238 to Lead 206 4.5 billion years • Rubidium-87 to Strontium 87 48.8 billion years • Potassium-40 to Argon-40 1.25 billion years

13. Carbon 14 • Half life of 5,730 years • Useful for more recent events • Ex. Sample living plant = 100% c-14 • Fossil plant now 50% C-14 = 5,730 years old = 1 half-life • Fossil plant now 25% C-14 = 11,460 years old = 2 half–life • Fossil plant now 12.5% C-14 = 17,190 years old = 3 half-life

14. Radiometric Dating • Allows scientists to determine how many years ago certain fossil organisms were on the Earth • Examples: Dinosaurs 230-65 million years ago Jawless fish first appeared 500 million years ago