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FOSSILS Types and how they form Image source: http://www.nature.nps.gov/geology/photodb/photos/flfo_4879.JPG 1. What are fossils? Fossils are evidence of past life; they are preserved remains and traces of animals and plants. 2. Where are they found? Fossils are usually found in sedimentary rocks. 3. How do fossils form? The living thing must usually Be buried quickly (stops rapid decay) Have hard parts (skeleton or shell) Other things that promote fossilization are: Low oxygen water (stops decay) Extreme temperatures Salty environment Rapid mineralisation (hardening of the living tissue) 4. How are fossils useful? Fossils called index fossils allow rock layers to be correlated. This means that rock layers in different areas can be matched up and be said to be of the same age. Index fossils must Be spread over a wide area Be of a species that only existed for a short geological time Fossils also show that life has evolved over time. Fossil evidence can be used to show that certain countries were once joined (provides evidence for continental drift/plate tectonics). Fossil type How is it formed? Petrified Minerals slowly replace the hard parts of a living thing. Eventually, the parts of the living thing break up and leave only the hardened minerals behind. Description (add sketch if possible) Looks a lot like the original organism except that it is hard rock e.g. might look like a tree trunk. Carbonised A dead organism trapped in a A black imprint of the rock can be ‘cooked’ by heat and living thing. pressure. The organism breaks down leaving only carbon behind. Mould A decomposed organism trapped A hollow shaped like a in rock leaves a space behind living thing. that is like the imprint of the organism. Cast When minerals seep into a mould, they harden and create a fossil that looks like a rock version of the original creature. Organism made of rock. Unaltered Very rarely, complete organism can be found preserved, for example in tar or sap. Looks exactly like the organism. trace Markings left by an animal (footprint, trail, or burrow)