The Fossil Record Ch. 6, section 3 HOW DO FOSSILS FORM • MOST FOSSILS FORM WHEN ORGANISMS THAT DIE BECOME BURIED IN SEDIMENTS. • USUALLY BONES OR SHELLS OF ANIMALS FORM FOSSILS Petrified Fossils • Fossils that are buried in sediment and turned to rock. Molds and Casts • Shells or other hard parts buried gradually dissolve and leave a blank space called a mold. (footprint) • When minerals fill the space that was left a casts is formed. Preserved remains • Animals that have been found in ice and have not changed since the day they died. Relative Dating • Scientists look at the layers of sediment and rock to determine how old a fossil is. • If a fossil is found closer to the surface it is usually younger. • If it is found in an older layer (deeper) it is usually older. Radioactive Dating • Scientists can compare the amount of a radioactive element in a sample to the amount of the element into which it breaks down. • Which can be used to determine the actual age of the fossil. Precambrian Time • • • • 4.6 billion - 544 million years ago. Formation of Earth Bacteria forms (1st living thing) Animals first appeared 600 million years ago. Paleozoic Era • 544 million - 245 million years ago • Sponges, jawless fish, jawed fish, sharks, and amphibians lived during this time. Mesozoic Era • 245 - 66 million years ago. • Dinosaurs, mammals, birds appeared. • Mass extinction happened during this time. Cenozoic Era • 66 millions years ago - present • Large mammals (mammoths, sabertooth tigers, etc) • Humans appeared Unanswered Qestions • How did animals become extinct? • Climate change- asteroid or volcanoes. We are exactly sure.