• Fossil of a Tyrannosaurus Rex"20070505 tyrant's gaze" is Copyrighted by Flickr user: schizoform (s shepherd) under the Creative Commons Attribution license.
  
  

Fossils

• The word "fossil" means "dug up" and is taken from the Latin word "fossilis." Fossils are the physical remains of plants and animals that lived from thousands to millions of years ago. Most people are familiar with dinosaur bones. These fossils date back to 65 million years ago, when the dinosaurs became extinct. The earliest fossils found date back to 600 million years ago and are proof of the existence of complex life forms. Fossils can take such forms as footprints hardened in sandstone, insects buried within amber, or, as in the case of the dinosaurs, skeletons that have been turned to stone. There are several ways in which a fossil can be created.
  
  

Permineralization

• Also known as petrification, this is the process in which the organic tissues of the plant or animal are slowly replaced by minerals from the rock encasing the organism. The petrified trees in the National Petrified Forest in Arizona are good examples. The organic matter in the trees was replaced with silica derived from volcanic ash. Groundwater allowed the mineral to seep into the fallen logs and over time the silica fossilized into quartz. The infusion of other minerals including iron during the petrification process caused the quartz to form in many colors including the purple of amethyst, the clear browns of smoky quartz and the yellow shades of citrine.
  
  

Unalterization

• Fossils preserved by unalterization are left intact and are preserved exactly how they appeared when they died. Amber, which is fossilized tree sap, is the best example of preservation by unalterization. Insects, bits of plants and even small animals such as frogs have been found encased in the hardened yellow substance, some of which have been preserved for 40 million years. These fossils are of particular interest to scientists because it is possible to study the entire organism.
  
  

Carbonization

• Carbonization, or coalification, is the result of organisms decomposing and leaving only the carbon elements behind. This happens when plants or animals get buried in sediment and over time the sediment hardens into rock. A fossil imprint of the animal is left by the carbon, sometimes providing excellent detail of the once living organism. The discovery of the Archaeopteryx, a type of winged dinosaur, is a perfect example of this type of fossil preservation. The detail in these carbon imprints proved the presence of feathers, giving credence to the theory that birds are descended from dinosaurs.
  
  

Authigenic Preservation

• Authigenic preservation is similar to carbonization. Organisms that have been buried in sediment gradually dissolve. The sediment, which has hardened around the organism, forms a mold or a cast of the animal that was once inside. The most common organisms fossilized in this manner are from the sea, including scallops, mussels, sponges and sea urchins.
  
  

Freezing

• Prehistoric mammals such as the woolly rhinoceros and the mammoth have been found imbedded in ice in the polar regions of both North America and Russia. These specimens date back to the Ice Age and many have been found completely intact. These are especially valuable finds because scientists can study the entire animal, including the stomach contents, to learn about the diet of these ancient creatures. Conditions must be right for such detailed preservation as the animals must be kept frozen from death until discovery.
  
  

Desiccation

• Desiccation, or drying, is another way of preserving fossil remains. This type of fossilization is not as common and only occurs in more arid parts of the world. Animal and human mummies have been found with skin and organs preserved, some quite intact. In December 2007, a dinosaur mummy with most of its bones, skin and muscle intact was found in North Dakota. The skin was detailed enough to show different-size scales, and when examined in a CAT scan, the musculature of the animal's tail and hind quarters was revealed. This three-dimensional find, named Dakota, had scientists redefining the shape and size of the hadrosaur, or duck-billed dinosaur.