To determine which fossil is a good example of an index fossil, we need to consider the characteristics of index fossils. A good index fossil typically:
- Has a widespread geographic distribution (found in multiple areas).
- Existed for a relatively short, well-defined period of geological time (limited time).
Based on the options you provided:
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Fern: Appears in multiple layers (indicating a long time) and in 3 columns (multiple areas).
- This would not qualify as a good index fossil because it suggests a long time range instead of a short and specific one.
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Ammonite: Appears in only 1 layer (limited time) but in 3 columns (multiple areas).
- This aligns well with the characteristics of a good index fossil, as it existed for a limited time but was found in multiple locations.
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Trilobite: Only appears in one layer (limited time) and in one column (single area).
- This does not qualify as a good index fossil because it lacks widespread distribution.
Given these analyses, the best example of an index fossil is the Ammonite.