The image presents a geological cross-section of four different locations (Location 1 to Location 4), each showing layers of sedimentary rock with various fossils labeled A, B, C, and D. The question asks:
"Index Fossil?" — meaning we are to determine which fossil is the
index fossil.
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🔍 What is an Index Fossil?
An
index fossil is a fossil of an organism that:
- Lived during a
specific, relatively short period in Earth’s history.
- Was
widespread geographically (found in many places).
- Is
easily identifiable and abundant.
These fossils are used by geologists to
date rock layers because their presence indicates a particular time period.
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🧩 Analyzing the Diagram
Let's examine the distribution of fossils A, B, C, and D across the four locations:
#### Fossil A:
- Found in
all four locations (1, 2, 3, 4)
- Present in the
lowest layer in all locations
- Appears consistently at the
same stratigraphic level
➡️ This suggests it lived during a
specific time and was
widely distributed.
#### Fossil B:
- Found in Locations 1, 2, and 4
- Missing from Location 3
- Found in
different layers depending on location
➡️ Not widespread (missing from Location 3) → not ideal as an index fossil.
#### Fossil C:
- Found in Locations 1, 2, and 4
- Also missing from Location 3
- In some locations, it appears above or below others
➡️ Not present everywhere; inconsistent stratigraphy → not reliable.
#### Fossil D:
- Found only in Locations 1, 2, and 4
- Missing from Location 3
- Appears in uppermost layers
➡️ Not found in all locations → limited geographic range.
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✔ Conclusion:
Only
Fossil A appears in
all four locations, in the
same relative position (bottom layer), suggesting it existed during a specific time and was widely distributed.
Even though the exact age isn't given, the fact that it appears
consistently across multiple locations and layers implies it is a strong candidate for an
index fossil.
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🟩 Final Answer:
👉
Fossil A is the index fossil.
Because it is:
- Widespread (present in all four locations),
- Found in the same relative stratigraphic layer,
- And likely represents a short-lived, abundant species.
This makes it useful for correlating the ages of rock layers across different regions.
✔ Answer: Fossil A
Parent Tip: Review the logic above to help your child master the concept of index fossils worksheet.
