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How do scientists know that different kinds of plants & animals lived in the past? Fossils • • • • Preserved remains/traces of living things Clues to the past Usually found in sedimentary rocks Bones, shells, teeth, seeds, woody stems become fossilized • It is rare for the soft part of an organism to become a fossil • Fossils provide information for: – What the environment looked like – Climate – Shape of Earth’s surface How do fossils form? • Living things are buried by sediments • Slowly harden into rock • The shape of the organism is preserved Who studies fossils? • Paleontologists! Kinds of Fossils: • • • • • • Petrified fossils Molds Casts Carbon Films Trace fossils Other Petrified Fossils • Petrified means “turning into stone” • Minerals replace all/part of an organism Molds & Casts • Mold is a hollow area in sediment in the shape of an organism/part of an organism • Cast is a copy of the shape of an organism when a mold is filled with mud, ash, lava • Most common types of fossils Carbon Films • Extremely thin coating of carbon on rock • Carbon comes from living things buried underground • Carbon can preserve delicate parts of plants & insects Trace Fossils • Provide evidence of the activities of ancient organisms • Ex: footprint, burrows, trails Other • Some processes preserve the remains of organisms with little/no change • Ex: tar, amber resin, freezing How are fossils classified? • Paleontologists classify fossils in order in which they lived from earliest to latest into a fossil record • Fossil records show how organisms have changed over time How do geologists use fossils to decide how old the rocks are? • • • • Relative age Absolute age Law of Superposition Index fossils Relative Age • The rock’s age compared to the ages of other rocks Absolute Age • The number of years since the rock formed • Geologists use radioactive dating to determine this – Carbon-14 dating – Potassium-Argon dating Law of Superposition • In horizontal sedimentary rock layers, the oldest layer is at the bottom and each layer above it is younger Index fossils • A fossil found in many different areas • Represents a type of organism that existed only briefly • Geologists look for index fossils in rock layers • Index fossils are useful because they tell the relative age of the rock layers they are in Index Fossil Example • Trilobite: hard-shelled animal with 3 distinct body sections • Many types of trilobites existed over 500 million years ago • Geologists must find one trilobite that is different in some way than the others to date the rock layer in which it was found – Ex: Large eyes What if a small piece of time is missing? • Unconformity is a gap in the geologic record • Some rock records are lost due to erosion What is the geologic time scale? • A record of the life forms and geologic events in Earth’s history • First developed by studying rock layers & index fossils • Later radioactive dating helped to determine the absolute age of each divisions of the time scale Different units of the time scale: • Eras • Periods • Epochs Eras • Long units of time – Precambrian – Paleozoic – Mesozoic – Cenozoic Periods • Sub-divided units of an era • Range in length from 2-10million years • Names came from where geologists first discovered the rocks/fossils of that period – Ex: Jurassic Epochs • Sub-divided units of a period Precambrian Era • 4.6billion-544million years • Earliest life forms similar to bacteria of today called cyanobacteria • Made food through the use of photosynthesis Paleozoic Era • • • • 544million-245million years Explosion of life forms Animals began coming onto land Evolving: – – – – – Hard-shelled organisms Invertebrates (jellyfish) Vertebrates (jawless fish) Amphibians Small reptiles • Mass extinction ended this era Mesozoic Era • 245million-65million years ago • Called the “Age of Reptiles” • Evolving: – – – – Dinosaurs Mammals Birds Flowering plants • Jurassic Period-dinosaurs ruled Earth for about 150million years • Mass extinction occurred when an object from space hit Earth’s surface causing a dust cloud which blocked the Sun – No dinosaurs survived! Cenozoic Era • 65million-Present • Evolving: – Marine mammals (whales) – Grass – Mammals, birds, & insects continue to flourish – Fossil records show that humans appeared about 3.5million years ago