1. Chapter 17 Section 1 paleontologists- scientists who study fossils -they infer what past life forms were like -arrange fossils according to time in which they lived -all this info together is the fossil record fossil record- provides evidence of history of past life on Earth, shows how different organisms change and evolve over time -about 99% of all species that have ever lived on Earth have become extinct

2. How Fossils Form -fossils can be bones, eggs, footprints, animal droppings, shells, pollen -many fossils form in sedimentary rock **List the three steps of fossil formation (page 418) Interpreting Fossil Evidence-Two Techniques: 1)Relative Dating -age of fossil is determined by comparing its placement with that of fossils in other layers of rock

3. -layers of sedimentary rock form in order by age -also use index fossils- distinctive fossil used to compare the relative age of fossils -relative dating allows paleontologists to estimate a fossil’s age compared to other fossils, but no info on absolute age or age in years 2) Radioactive Dating -used to assign absolute age to fossils -age is calculated based on amount of radioactive isotopes a fossil contains

4. -uses half-life to determine age half-life- length of time required for half of the radioactive atoms in a sample to decay Geologic Time Scale -used to measure evolutionary time -major changes in fossil animal and plants are used to mark where one segment ends and another begins -after Pre-Cambrian Time, the basic divisions are eras and periods

5. Eras Pre-Cambrian Time (oldest), Paleozoic, Mesozoic and Cenozoic (most recent) Periods Vendian, Cambrian, Ordovician, Silurian, Devonian, Carboniferous, Permian, Triassic, Jurassic, Cretaceous, Tertiary, Quaternary