Worksheet - Index Fossils *EDITABLE* in 2024 | Earth science ... - Free Printable

Let’s go through each question one by one.

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Question 1:

We’re asked: What characteristic makes a fossil useful as an “index fossil” for figuring out the relative age of rock layers?

An index fossil is like a time marker in rocks. To be useful, it needs to:

- Be found in many places (so you can compare rocks from different areas)
- Exist for only a short time (so if you find it, you know exactly when that layer was formed)

Looking at the options:

a. wide time range and narrow geographic range → ✘ Too long-lived, not precise
b. narrow time range and wide geographic range → ✔ Perfect! Short time = precise dating; wide area = easy to find and compare
c. narrow time range and narrow geographic range → ✘ Hard to find elsewhere
d. wide time range and wide geographic range → ✘ Not precise enough

✔ So the correct answer is b

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Question 2:

Index fossils existed for a relatively short time. What happened to the species that are now used as index fossils?

If they only lived for a short time and we find their fossils, that means they don’t exist anymore — they died out.

Options:

a. They became extinct → ✔ Yes! That’s why they’re only in certain old layers
b. They changed into other species → ✘ Not necessarily — extinction is simpler and more direct
c. They migrated to new environments → ✘ Doesn’t explain why they’re gone from the record
d. They stopped reproducing → ✘ Same as extinction, but “extinct” is the scientific term

✔ So the correct answer is a

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Question 3:

We have a diagram with rock layers A–H (A on top, H on bottom). Fossils (1) to (4) are shown in different layers.

We need to pick which fossil could be an index fossil.

Remember: An index fossil must be in a layer that represents a short time period — so ideally, it should appear in only ONE layer, not many.

Look at where each fossil appears:

- Fossil (1): Found in layers G and F → appears in two layers → probably lived too long → ✘
- Fossil (2): Found in layer E only → only one layer → good candidate → ✔
- Fossil (3): Found in layers D and C → two layers → ✘
- Fossil (4): Found in layers B and A → two layers → ✘

Only fossil (2) is found in just one layer — meaning it likely lived for a very short time, making it a good index fossil.

✔ So the correct answer is 2

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Final Answer:
1. b
2. a
3. 2