Types of Fossil Evidence
Why are there so few fossils relative to the number of species that have existed? It is most likely because organisms decompose before they have a chance to become fossilized. Only those organisms that are quickly buried in sediment after their death become preserved.
There are numerous types of fossil evidence that have been left behind by organisms. Some of the types of fossil evidence commonly used by scientists include trace fossils, molds and casts, replacement fossils, petrified fossils, amber and original material fossils.
A trace fossil is any indirect evidence left by an organisms. Examples of trace fossils include footprints, burrows and fossilized feces that have been left behind by organisms. They simply leave material evidence that the organisms existed rather than evidence of the actual structure of the organism.
This image shows trace fossils of shells that once existed.
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Molds and casts are also important fossil evidence. Molds are impressions of entire organisms that have been left behind. Casts are molds that have been filled with sediment over time.
This image depicts a mold fossil of a Trilobite.
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Replacement fossils are fossils where the original material of an organisms is replaced with mineral crystals that can leave detailed replicas of hard or soft parts. These types of fossils can appear amazingly realistic and give scientists a better visual image of the organism.
Petrified fossils are formed when empty pore spaces of the organism are filled in with minerals that crystallize. A classic example of this is petrified wood.
This image shows a piece of wood that has been petrified.
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Amber fossils were made known to the popular imagination by movies depicting dinosaurs. They are composed of organisms that have been preserved in tree sap that hardened over time into amber.
This insect has been fossilized in amber.
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Original material fossils are fossils of organisms that have been preserved as a result of mummification or freezing. Recent headlines have been about entire animals, such as woolly mammoths, that have been found frozen in arctic ice. This leaves the entire organism preserved to be studied.
Lesson Summary
Let's review. Fossils are evidence of organisms that have existed. The fossils may consist of clues about the organism, such as its tracks or feces, or they may be a part of the organism, such as a bone or other body part. Fossils give scientists clues about the structure and lifestyle of the organisms.
