What do index fossils found in rock layers in different areas of the world indicate about the rock layers formed?

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Index fossils can be used to determine when layers of rock were formed. They may also help us to identify the kind of environment the rock layers result from. Geologists and paleontologists use index fossils as a means of measuring geologic time.

Using index fossils relies on pre-established dates (or periods) for the existence of certain species of plants and animals. For example, we know that ammonites existed between 245 and 65 million years ago. If a paleontologist finds an ammonite fossil in an layer of rock (or a piece of rock) which has yet to be dated, they could infer that the rock formed between 245 and 65 million years ago.

Index fossils may also help us to date other species found in the same layer of rock. This is especially helpful for establishing timelines and tracking change (evolution) of species.