What is a fossil?• A fossil is the preserved remains of a once- living organism.What do fossils tell us?• Fossils give clues about organisms that lived long ago. They help to show that evolution has occurred.• They also provide evidence about how Earth’s surface has changed over time.• Fossils help scientists understand what past environments may have been like.
HOW IS A FOSSIL FORMED?1. Sediment 2. Layers 3. Movement 4. ErosionAn animal is buried by More sediment layers Movement of tectonic Erosion from rain,sediment, such as accumulate above the plates, or giant rock rivers, and wind wearsvolcanic ash or silt, animal’s remains, and slabs that make up away the remainingshortly after it dies. Its minerals, such as silica Earth’s surface, lifts rock layers. Eventually,bones are protected (a compound of silicon up the sediments and erosion or peoplefrom rotting by the and oxygen), slowly pushes the fossil digging for fossils willlayer of sediment. replace the calcium closer to the surface. expose the preserved phosphate in remains. the bones.
FIVE MAIN TYPES OF FOSSILS Petrified Molds and Carbon Fossils Casts Films Trace Preserved Fossils Remains
PETRIFIED FOSSILS • The word “petrified” means “turning into stone.” • Petrified fossils form when minerals replace all or part of an organism. • Water is full of dissolved minerals. It seeps through the layers of sediment to reach the dead organism. PETRIFIED FOSSILThe Field Museum in Chicago When the water evaporates, displays a fossil of a only the hardened minerals Tyrannosaurus rex. are left behind.
MOLDS AND CASTS • A mold forms when hard parts of an organism are buried in sediment, such as sand, silt, or clay. MOLD FOSSIL • The hard parts completely dissolveThis mold, or imprint, is of over time, leaving behind a hollow an extinct mollusk called area with the organism’s shape. an ammonite. • A cast forms as the result of a mold. • Water with dissolved minerals and sediment fills the mold’s empty spaces. CAST FOSSIL • Minerals and sediment that are left This ammonite cast was in the mold make a cast. discovered in the United Kingdom. • A cast is the opposite of its mold.
CARBON FILMS • All living things contain an element called carbon. • When an organism dies and is buried in sediment, the materials that make up the organism break down. • Eventually, only carbon FERN FOSSIL remains.This carbon-film fossil of a • The thin layer of carbon fern is more than left behind can show an 300 million years old. organism’s delicate parts, like leaves on a plant.
TRACE FOSSILS • Trace fossils show the activities of organisms. • An animal makes a footprint when it steps in sand or mud. • Over time the footprint is buried in layers of sediment. Then, the sediment becomes FANCY FOOTWORKThis dinosaur footprint was solid rock. found in Namibia, Africa.
PRESERVED REMAINS Some organisms get preserved in or close to theiroriginal states. Here are some ways that can happen.Amber Tar IceAn organism, An organism, An organism,such as an insect, such as a such as a woollyis trapped in a mammoth, is mammoth, dies intree’s sticky resin trapped in a tar pit a very cold region.and dies. More and dies. The tar Its body is frozenresin covers it, soaks into its in ice, whichsealing the insect bones and stops preserves theinside. It hardens the bones from organism—eveninto amber. decaying. its hair!
For more on fossils visit: Video: Becoming a Fossil http://www.pbs.org/wgbh/evolution/library/04/3/l_043_01.html PBS Article: Major Fossil Find http://www2.scholastic.com/browse/article.jsp?id=3751945 Scholastic News Online Online Exhibit: Fossil Halls http://www.amnh.org/exhibitions/permanent/fossilhalls American Museum of Natural History Interactive Game: Fossil Hunt http://channel.nationalgeographic.com/episode/dino-death-trap-2998#tab-fossil-hunt National GeographicScholastic Classroom Magazines. www.scholastic.com Photo Credits: PAGE 1: UTAH MUSEUM OF NATURAL HISTORY (DINOSAUR); PHOTOTAKE INC./ALAMY (ANT). PAGE3: 5W INFORGRAPHIC (GRAPHIC). PAGE 4: Jason Lindsey/Alamy (DINOSAUR); Peter Bowater/Alamy (MOLD); David Lyons/Alamy (FERN); Hoberman Collection UK/Alamy (FOOTPRINT); JohnCancalosi/Alamy (MANTIS). PAGE 5: Gary Crabbe/Alamy (DINOSAUR). PAGE 6: Peter Bowater/Alamy (MOLD); Detail Heritage/Alamy (CAST). PAGE 7: David Lyons/Alamy (FERN). PAGE 8: HobermanCollection UK/Alamy (FOOTPRINT). PAGE 9: John Cancalosi/Alamy (MANTIS); R1/Alamy (TAR PITS); Gianni Dagli Orti/Corbis (MAMMOTH).