Fossil Evidence http://glencoe.mcgraw-hill.com/sites/007874184x/student_view0/brainpop_movies.html# EQ: How do fossil records provide the history and changes of life on Earth?

Have you ever looked at a map of the world? Did you notice how the coastlines of continents on opposite sides of the oceans appear to fit together like the pieces of a puzzle?

Is this a coincidence? How is it that the coastlines fit together so well? Is it possible that the continents were actually together sometime in the past?

Wegener’s Continental Drift Hypothesis The hypothesis that the continents once formed a single landmass, broke up, and drifted to their present locations.

Pangea Wegener made many observations before proposing his hypothesis of continental drift. He thought that all the present continents were once joined in a single, huge continent called Pangea

Pangea “Pangaea” means “all earth” It existed about 245 million years ago It split into two huge continents about 180 million years ago Laurasia Gondwana These two continents split again about 65 million years ago to form the continents we know today

Pangea Continental Drift explained a few puzzling questions Why do the continents seem to fit together like a giant puzzle? Why are fossils of the same plant and animal species found on continents that are on different sides of the Atlantic Ocean? Why are rocks with different magnetic alignments found in the same place (paleomagnetism)?

Fossil Evidence Fossils are the remains or physical evidence of an organism preserved by geological processes

Fossil Evidence How can we use fossils to learn about the past? They give a rough sketch of the history of life on Earth They show how the Earth’s environment has changed They show how organisms have changed. They help scientists date rocks.

Fossil Evidence Scientists also noticed that fossils of the same species of extinct plants and animals are found in rocks of the same age, but on continents that are now widely separated. Wegener believed that the organisms would not have been able to travel across the oceans.

Fossils can be found in… Sedimentary rocks Amber Petrified wood/bone Asphalt Or they may be found preserved by ice!

Fossil Evidence Usually when an organism dies, it is quickly consumed by other organisms. But sometimes, the organism is quickly buried by sediment after they die. These sediments slow down decay. The hard parts of organisms, like shells and bones are more resistant to decay than soft tissues. When the sediments lithify, the hard parts can become preserved Fossils are usually found in sedimentary rocks

Relative Dating One big puzzle for geologists to solve is the order in which events occurred during the past. They rely on rocks and fossils to help them find out what life existed and the conditions of the Earth in the past. Determining whether and object or event is older or younger than another object or event is called relative dating

Relative Dating Sedimentary rock tends to be stacked in “layers”, like paper in a pile, with the oldest layers on the bottom and the youngest on the top These layers are called strata

Relative Dating This principle, that the youngest rocks lie above older rocks is called The Law of SUPERPOSITION

But not all rocks are arranged with the youngest on top and the oldest on the bottom Forces can disturb rocks Rocks can be pushed into another sequence Rocks can be tilted…. Folded…. and broken Sometimes geologists find rock sequences upside down!

These disturbances make a scientist’s job very tricky! To make their job easier, geologists combine information from known undisturbed rock sequences. From this information, they create a geologic column, which is an ideal sequence of rock layers made of rocks and fossils arranged from oldest to youngest.

The columns are used to…. Interpret rock sequences Identify the layers in puzzling rock sequences

Index Fossils Scientists have found that particular types of fossils appear only in certain layers of rocks. By dating the rock layers above and below these fossils, scientists can figure out the time span in which the fossilized organisms lived.

Index Fossils If a type of organism lived for only a short period of time, its fossils would only show up in a few layers. These types of fossils are called index fossils.

Index Fossils Index fossils are fossils of organisms that lived during a relatively short, well-defined geologic time span Examples include: Ammonites 230-208 million years ago Trilobites 400 million years ago