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Petrified fossil a fossil formed when minerals slowly replace some or all of an organism, turning it to stone Carbonaceous film a type of fossil created when an organism is buried under many layers of sediment and pressure and heat builds up leaving a thin film of carbon residue Original remains a fossil that is the actual body or body parts of an organism Trace fossils fossils that aren't exactly part of the organism. These include footprints, burrows and eggshells.They give insight into an organism's behavior. Mould A type of fossil in which the hard parts of the organism have dissolved, leaving a cavity in the rock Cast A type of fossil in which sediments or minerals have filled a mould and hardened into rock Principle of superposition states that in undisturbed rock layers, the oldest rocks are on the bottom and the rocks become progressively younger toward the top Strata layers of sedimentary rock Relative dating method to determine the order of events and relative age of rocks by examining the position of rocks in a sequence Index fossil a fossil that is used to establish the age of rock layers because it is distinct, abundant, and widespread and existed for only a short span of geologic time Half-life the period of time in which half of a radioactive substance decays Radiometric dating the process of determining the age of a geological specimen by measuring the relative amounts of radioactive particles that are present in the specimen Radiocarbon dating A method used to determine the age of organic remains by measuring the relative amount of radioactive carbon found in the remains Eons the largest division of time on the geological scale Eras one of the four longest subdivisions in the history of Earth Periods On the geologic time scale, a subdivision of an era Precambrian Era the first of the four eras on the geologic time scale Rodinia the name of the earliest supercontinent thought to have broken apart approximately 750 million years ago Paleozoic Era the second era on the geologic time scale; the era in which the first plants and animals appeared Mesozoic Era the third era on the geologic time scale; the era in which dinosaurs were the dominant life form on Earth Cenozoic Era the fourth and current era on the geologic time scale; the era in which humans evolved Pangaea the name of the second supercontinent thought to have existed approximately 350 million years ago Laurasia the northern part of the supercontinent Pangaea, which split off approximately 200 million years ago Gondwanaland the southern part of the supercontinent Pangaea, which split off approximately 200 million years ago Petroleum a type of oil found in rock formations in Earth's crust; refined into products such as gasoline or jet fuel Fossil fuels fuels made of decomposed plants and other organisms that have been hardened or fossilized; take millions of years to develop Bitumen a heavy, almost solid form of petroleum; some deposits are found near the surface of Earth and can be mined or heated and pumped to the surface