The Fossil Record (Ch 19.1) Unit 5: Evolution

Fossils and Ancient Life Fossils tell us the story of extinct species (species that have died out) May organisms have died without leaving a trace which leaves the fossil record incomplete Fossil range in terms of size, type, and type of preservation Fossils in Sedimentary Rock Sediments (sand, silt, or clay) build up over a dead organism that has sunk to the bottom of a body of water Over time, the layers of sediment are pressed together by the water pressure preserving the organism What Fossils Reveal Paleontologists (researchers who study fossils) learn about the structures of organisms, their environment, and the ways in which they lived They can determine evolutionary relationships among organisms based off the fossil record

Dating Earth’s History Relative Dating Places rock layers and their fossils in temporal sequence – allows paleontologists to determine whether a fossil is older or younger than other fossils Index fossils are distinctive fossils that are used to establish and compare the relative ages of rock layers and the fossils they contain Easily recognized and will occur only in a few rock layers Example: Trilobites Radiometric Dating Relies on radioactive isotopes which decay or break down into stable isotopes to tell the absolute age of fossils Half-life: time required for half of the radioactive atoms in a sample to decay Different radioactive isotopes have different half-lives – elements with short half- lives are used to date younger fossils and elements with longer half-lives are used to date older fossils Carbon-14  used for younger fossils Potassium-40 / Uranium-238  used for older fossils

Geological Time Scale EONS ERAS The geological time scale is based on both relative and absolute dating. The major divisions of the geological time scale are eons, eras, and periods. Major changes in the rock layers signified the divisions in the time scale Radiometric dating was used to verify specific time frames (radioactive isotopes) Divisions of the Geological Time Scale Each division has different lengths Eons are divided into eras which are then divided into periods g EONS ERAS Hadean Eon: the first rocks were formed Archean Eon: first life appeared Proterozoic Eon: 2.5 billion years ago – 542 million years ago Phanerozoic Eon: 542 million years ago – present Paleozoic Mesozoic -- Dinosaurs Cenozoic

Geological Time Scale

Life on a Changing Planet Physical Forces climate  shifts in global temperature will affect living things Example: wind currents / ocean currents geological forces  building mountain ranges and moving continents (plate tectonics) Building mountains, opening coastlines, changing climates, and geological forces have altered habitats of living organisms repeatedly throughout Earth’s history. Biological Forces The actions of living organisms over time have changed conditions in the land, water, and atmosphere of planet Earth. Example: iron deposits and photosynthetic bacteria causes the increase in oxygen in the Earth’s atmosphere