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Fossils for Beginners

The excitement of discovering a fossil inspires the imagination of amateur paleontologists everywhere. Fossils represent some of the earliest records of life on this planet. All fossils fall into one of two main categories: fossilized body parts and fossilized traces. Some of the ways fossils form include insects or plants trapped in amber, petrification and carbonization. The most common fossils have an impression of bones, claws or teeth. Fossils of soft body parts or organisms remain a rare find. All fossils develop in one of four basic ways.

Mold Fossils
- When an organism becomes trapped or covered in sedimentary sand, it leaves an imprint. If that organism had a shell, the imprint of that shell will appear in the sand. Over time, as the sand begins to harden and turn to stone, that organism and the shell will dissolve, leaving a space inside the rock containing the imprint. That imprinted space inside the rock becomes a mold fossil. When sediment or minerals fill that fossil mold, it becomes a cast in the shape of the fossilized organism.
Cast Fossils
- These fossils appear as exact stone replicas of the organism imprint from the mold fossil. Cast fossilized bones make up a good number of the dinosaur bones displayed in museums today. Many different organisms appear in museum displays as cast fossils including Turitella snails, starburst algae, sea lilies, trilobites and ammonites. Entire dinosaur nests along with their eggs make up some of the more interesting cast fossils. Coprolite, or fossilized dinosaur dung makes an interesting find for any collector of cast fossils.
Trace Fossils
- This classification of fossil serves as a record of the movements and travel behavior from the earliest dinosaurs. Trackways, or sets of footprints, show up in more than 1,500 sites around the world including coal mines, riverbeds and mountains. Because dinosaurs made lots of tracks and those footprints fossilize easily, these fossilized dinosaur tracks exist in great numbers. Fossilized footprints indicate how fast certain dinosaurs could walk or run, whether they ran in herds or remained alone and whether they walked on two or four legs.
True Form Fossils
- These fossils form when an animal, plant or other organism becomes trapped in a substance like mud, ice, tar or amber. For various reasons an organism becomes immobilized in a substance like water, sand, sediment or mud. Minerals can collect around the organism, covering it completely. As the elements and minerals dissolve, the organism slowly over time hardens into stone, and an impression remains within. An insect trapped within amber represents another good example of an unaltered preservation forming a true form fossil.

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