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What's a fossil?

These tree sections are made of stone. A volcanic eruption covered the trees with ash 3 million years ago turning them into fossils. (Copyright Lee Krystek 2000)

A fossil is the prehistoric remains of a plant or animal. Fossils are typically preserved when they are buried under many layers of sand and mud. Under great pressure the sand and mud become sedimentary rock. Minerals seep into the fossil replacing the organic matter and creating a replica in stone.

A fossil can be a trace fossil or a body fossil. A body fossil is what was actual organic material from a creature or plant (like a bone). Trace fossils are signs of plant and animal activity that have been preserved in rock. For example, dinosaur tracks, trails and dung are all trace fossils.

A body fossil is usually composed of the bones of an animal. Preservation of the "soft" tissue of a body is rare especially over long periods of time. Most body fossils are made when an animal dies and falls, or is washed into a stream, and is covered with mud.

In a few cases, though, fossils have been preserved by freezing ( as with mammoths in Siberia and Alaska) or when the animals have become mired in a tar pit (such as the famous La Brea tar pit near Los Angeles, California).

Sometime a fossil is the result of a volcanic eruption. Ash and mud from the volcano can cover animals and plants killing and preserving them at the same time.

Fossils come in all sizes. The largest are dinosaur bones (which may be ten or more feet in length for a single bone) and the smallest are plant spores only a few hundredths of an inch across. The study of fossils is called Paleontology. The study of fossil plants is Paleobotany.

Fossils exist all over the world. As hills erode, fossils become visible and a Paleontologist can find and remove them for study. Sometimes a small area only a few hundred feet across will be jammed with skeletal remains. This is called a "bone bed." A bone bed maybe the site of a sandbar on an ancient river. During floods, the bodies of many animals floated downsteam, grounded on the bar, and quickly got covered with silt and were preserved.

Removal of fossils from the rock is done very carefully to avoid damage to the specimen. Photographs are taken to record the position of the bones as they were found. These aid scientists as they attempt to reconstruct fossil skeletons.

Fossils are laid down in the rock layers in order of age. The oldest fossils are deeper and the new fossils farther up in another "strata" of rock. Geologic action, though, jumbles this over a wide area so that fossils from the Cretaceous Era (65 million years ago) might be on the surface only a few miles from fossils of the Ice Age (10,000 years ago). A Paleontologist knows where to go to find the right age rock for the type of fossils he is seeking.

The petrified lump of dinosaur dung is a "trace" fossil. (Copyright Lee Krystek 1996)

Next Stop on Dinosaur Safari

Copyright Lee Krystek 1996. All Rights Reserved.