Facts on Fossils
Fossils are the evidence that creatures and plants existed long ago. Fossils can be thousands or even millions of years old. Though most of these plants and animals disappeared without a trace, given the right circumstances a record was made in stone. Fossils come in all sizes, from giant dinosaur skeletons to almost invisible marine creatures.
Types
There are five ways that fossils can form. The most common is petrification, also called permineralization. In this process the original organic tissue of the animal or plant is replaced with minerals including calcite, pyrite or silica from the surrounding rocks.
Carbonization occurs when all elements such as hydrogen, oxygen and nitrogen are removed, leaving only a carbon trace of the original creature.
Insects or plant matter trapped in tree sap, which hardens into amber, are an example of unaltered preservation, a very special type of fossil.
Some fossils are casts, or molds, of the original where the rock formed around the plant or animal, which then decayed, leaving a hollow space that is refilled with mineral deposits. This is called authigenic preservation.
Recrystalization occurs when crystals form within the original structure, eventually replacing it and resulting in a mineral copy.
Oldest Fossils
The oldest fossils on the earth come from two places: The Burgess Shale Formation in Canada and The Chengjiang Deposits in the Yunnan Province of China. The oldest marine fossils exist in Isua greenstone in Greenland.
Biggest Fossils
The largest ammonites, a marine fossil, are the Pachydiscus seppenradensis, which can reach a diameter of 6 ½ feet (2 meters). They are from the Cretaceous Period and were found in Germany.
The largest recorded nautiloid fossil is from the Ordovician Period and measured 13 feet (3 ½ meters) long.
The Megaladon is a prehistoric shark fossil, almost 50 feet long, while the Sauroposeidon measures about 60 feet (18 meters).
Geology
Most fossils are found in sedimentary rock. Sedimentary rock is made of bits of sediment: sand, silt and mud along with minerals that accumulate and harden over time. Geologic forces that reveal fossils over time are erosion (the wearing away of the surface of the rock by the action of water and wind) and uplifting, which occurs when two of the plates of the earth's crust collide. Mountains, deserts, cliffs and eroded hillsides are all good places to search for fossils.
Extreme Temperatures
Sometimes fossils are preserved by extremes in climate, such as the conditions found in the desert or in areas of permafrost. These fossil records often show that the climate and vegetation of an area changed dramatically and suddenly at some point in time.
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