The relative age of rocks and fossils can be determined using two basic methods: law of superposition and index fossils.
Index Fossils
Every fossil tells us something about the age of the rock it's found in, and index fossils are the ones that tell us the most. Index fossils are those that are used to define more distinct periods of geologic time.
A good index fossil is one with four characteristics:
- distinctive and unique looking enough to identify easily
- widespread and found all over Earth
- abundant where a great number of these fossils have and can be found
- lived a relatively short period of geologic time
Trilobites are considered a very good index fossil because they were mobile animals and tended to inhabit large, even global areas during the Paleozoic Era. Trilobites were also hard-shelled invertebrates, so they fossilized easily. These fossils are also easily identifiable and large enough to study without a microscope.
Some common index fossils include ammonites, crinoids, rugose corals, brachiopods, bryozoans, and mollusks.
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Topic Tips
- Index fossils are very useful to determine the relative age of many different fossils and rock layers.