An allosaurus skull.  Click here to go back website start. National Park Service Museum Collections presents 'Dinosaur! National Monument'
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Park Ranger with Diplodocus sp. femur
Drawing of fossil Drawing of fossil fish
Fossils are remains or traces of ancient life. They provide the clues that paleontologists use to learn about prehistoric animals and plants. When a plant or animal dies, it usually decays or is eaten by other animals. However, if it is buried before that happens, it may become a fossil.

Hard materials, such as wood, bones, shells, and teeth, have a better chance of being buried and fossilized because they decay more slowly than soft tissues like leaves or skin. Sometimes, hard animal or plant parts may be preserved unchanged. More often, they are partly or totally filled in by minerals such as silica from surrounding rocks. The minerals are probably carried into the fossil by water seeping through the rock. However, scientists are still not sure how this happens.

Fossils changed by minerals are said to be mineralized. They can also be called petrified, or turned into stone. The minerals may have completely replaced the original tissues or filled in the tiny spaces in and between the cells. Either way, mineralization can preserve many details, such as growth rings in petrified wood. Minerals may contain iron or other elements that make the fossil very colorful.

The bones found in Dinosaur National Monument's Douglass Quarry have been mineralized with silica. In a living animal, bone consists of a hard framework laced with cells, blood vessels, marrow, and other soft materials. The Douglass Quarry bones still have their original hard framework. The soft parts have been replaced by minerals.

Mineralization makes fossils much harder and heavier than living bone. Nevertheless, the bone structure is still visible when thin slices are viewed under a microscope.

Drawing of fossil leaf

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Last modified: 1/21/2004