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A Trip Through Geologic Time The Fossil Record • fossils tell the history of life on earth • not all organisms form fossils • conditions necessary for fossil formation: – protected from destruction and decay – sediments bury it and become rock – hard parts Types of Fossils • petrified fossils – mineralized copies of original bones – minerals in water replace original matter – parts turned to stone • molds and casts – mold - cavity in the shape of an organism – cast - minerals fill in the cavity • trace fossils – footprints, tracks, trails, and burrows • coprolites – tell about what an organism ate • unchanged fossils – amber, tar pits Interpreting Fossils Relative Age Dating principle of superposition -younger rock on top of older rock -basis for most relative age dating of fossils Disruption of Layers • folding & faulting disrupt order of layers • erosion removes layers Rules for interpreting layers 1. Rock layers are horizontal before they deform 2. A fault or igneous dike did not exist when the layers formed, and is younger than the layers it cuts across Absolute Dating • absolute dates expressed in years • Radiometric dating – compares amount of radioactive isotope present to the amount of decay present • half-life – time it takes for half of isotope to decay Isotope Half-Life (Years) Used to Date Thorium-232 14 billion Very old rocks Potassium-40 1,300 million Old rocks & fossils in them 5,730 Fossils less than 50,000 years old Carbon-14 The Geologic Time Scale • Geologic Eras: – Precambrian – Paleozoic Era – Mesozoic Era – Cenozoic Era Precambrian Era The Dawn of Life • 4600 mya - 540 mya • Earth’s history begins • Seas form from volcanic emissions & comet impacts. • Continents form & grow. • Oxygen builds up in atmosphere from photosynthesizing organisms (stromatolites) Paleozoic Era “Old Life” - Life Comes Ashore • 540 mya - 248 mya • Age of Invertebrates • Began with “Cambrian Explosion” of new life forms, due to preceeding mass extinction. • Abundant sea life. First fishes appear, first amphibians evolved from fishes, first reptiles evolved from amphibians. • First terrestrial life (arthropods & other invertebrates) invades from sea. • Carboniferous swamps create coal deposits. • Pangaea formed near end of era. Mesozoic Era “Middle Life” - Age of Dinosaurs • 248 mya - 65 mya • Era of many changes • Dinosaurs, birds, & mammals all evolved from reptiles. Mammals avoid competition with/predeation by dinosaurs by being nocturnal & eating insects. • Pangea broke up 200 mya. • Flowering plants became dominant plant life. Cenozoic Era “New Life” - Age of Mammals • 65 mya – present • Mammals dominated. • Primates, human ancestors, & modern humans evolved. • Land bridge formed between N. & S. America, disrupted ocean currents & triggered Ice Ages sheets of ice advance and recede several times. • Alps and Himalaya Mountains form from collisions of continents. Check These Out! Geologic Time Machine Fossils, Rocks, and Time Ice Age Paleoecology USGS FAQ about Dinosaurs Carbon-14 Dating Paleontology Without Walls Dinosaur Images Paleomapping Project The End