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Alpine Natural Sciences Academy Mr. Bordelon, M.S. Clues to Earth’s Past Fossils A fossil is evidence of life. Evidence such as the remains, imprints, or traces of onceliving organisms preserved in rocks are fossils. Photos taken at The Roynon Museum of Paleontology Escondido, CA What Fossils Tell Us.. By studying fossils, geologist help solve mysteries of Earth’s past. Fossils tell us when, where, and how organisms once lived. Fossil Formation Necessary conditions… First of all, the body of a dead organism must be protected from scavengers and microorganisms. One way this can happen is for the body to be quickly buried in sediment. However, quick burial alone is not enough to make a fossil. Organisms have a better chance of being preserved if they have hard parts such as bones, shells, or teeth. Fossils are most often found in sedimentary rocks. The heat and pressure involved in forming igneous and metamorphic rocks most often destroys fossil material. The hard parts in plants, such as the cellulose in cell walls, made preservation of this fossil possible. Petrified Remains Petrified (PEH truh fide) remains are hard and rocklike. Some or all of the original materials have been replaced by minerals. Much of the original matter in this petrified tree slice has been replaced by quartz and other minerals. Carbonaceous Films The tissues of most organisms are made of compounds that contain carbon. Sometimes, the only fossil remains of a dead plant or animal is this carbon. This fossil graptolite has been preserved as a carbonaceous film. Graptolites are extinct colonial animals that lived in the oceans from about 530 million to 320 million years ago. Molds and Casts In nature, imprints are made when sea shells or other hard parts of an organism fall into soft sediments such as mud. The object and sediments are then buried by more sediments. The sediments are squeezed and cemented together into rock. Holes in the rock let water and air reach the shell or hard part, causing it to dissolve, and leaving behind a hollow place in the rock called a mold. A cast (internal mold) resembling the original organism forms when a mold fills with sediments or mineral crystals from solution. Original Remains Sometimes the actual organism or parts of the organism are found—such as an insect trapped in amber. Other organisms, like woolly mammoths, have been found preserved in frozen ground. Trace Fossils Fossilized tracks and other evidence of animal activity are called trace fossils. Tracks made in soft mud, and now preserved in solid rock, can provide information about animal size, speed, and other behavioral patterns. Index Fossils Index fossils are species that lived on Earth for short periods of time, were abundant, and were widespread geographically. Because few fossils meet all the conditions to be an index fossil, groups of fossils are usually used to date rocks. Index fossils indicate age by the layering of their deposits. Fossils and Ancient Environments Fossils also can be used to tell what the environment of an area was like long ago. For example, rocks in Antarctica contain fossils of tropical plants! Brachiopods are found throughout the Midwestern United States, indicating the presence of a prehistoric shallow sea. Summary Fossils tell us about: • Past life on Earth • History of rock layers containing fossils • Past environmental conditions • Past climate conditions • Extinct animal behavior • Age (dating) of rocks Source Science Voyages: Earth & Physical Sciences. Glencoe McGraw-Hill, California Standards Edition. 2001.