WHAT IS A FOSSIL? There are three types of rocks: Igneous rocks, Metamorphic rocks and sedimentary rocks. Igneous rock form by the cooling of magma. Metamorphic rocks form by exposing already existing sediments to high temperatures or pressure (deep in the earth’s crust). Fossils only occur in sedimentary rocks which are described below. Most sedimentary rocks are formed when erosion products are deposited in layers, and over time lithifies into rock. Erosion products are the materials that form during erosion of rocks by water, ice, wind, etc. This is how conglomerates (pebble stone), sandstones and shale’s form. These rocks are ultimately erosion products of igneous rocks.
The best-preserved fossils form when the organism is quickly buried. If the organism is not buried quickly, external factors such as scavengers, decay and the weather can deteriorate the quality of the fossil to be. The right conditions for fossilization are exceptional. In most cases an organism will not fossilize. If the organism is buried in an anoxic environment, even the soft parts can be preserved. Mostly only the harder parts, such as shells or skeletons, are fossilized. The fast burial of organisms occurs mainly in sediments of watery environments (i.e. clay, sand in rivers and oceans). Fossils of land animals are more rare. On land you need for example a volcanic eruption or flashflood to bury the organism fast enough with sediment.After burial it depends on the chemical composition of the sediment whether the organism is well preserved or not. In the sediment chemical and mineralogical composition of the fossil can be altered. The internal structure remains preserved. 
Otherwise the fossil can be dissolved in water. In this instance the cavity can later be filled up by another mineral.
"Fossils A Evolution In the making" Some animals were quickly buried after their death (by sinking in mud, being buried in a sandstorm, etc). Over time more and more sediment covered the remains. The parts of the animals that didn’t rot (usually the harder parts like bones and teeth) were encased in the newly formed sediment.
This is the result of compaction and several chemical processes in the sediment. When sediments are buried deep inthe earth’s crust, or exposed to high temperatures, new types of rocks can be formed as a result. Geologists are always on the lookout for sedimentary structures to reconstruct the environment during the deposit.
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In the right circumstances (when there is no scavengers, quick burial, not much weathering) parts of the animal turned into fossils over time. After a long time the chemicals in the buried animals bodies underwent a series of changes. As the bone slowly decayed, water infused with minerals seeped into e b h a t n c h
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