Question

An index fossil should have

• A. large geographic range and small temporal range
• B. small geographic range and large temporal range
• C. small geographic range and small temporal range
• D. large geographic range and large temporal range

Answer 1

The Correct Option is A

An index fossil is crucial for identifying and dating geological strata. These fossils are used by geologists to ascertain the relative ages of rock layers. Let's analyze the criteria that make a fossil an ideal index fossil and justify why the correct answer is "large geographic range and small temporal range".

1. Definition and Criteria of Index Fossils:
   • An index fossil is one that is geographically widespread and existed for a relatively short period. This combination allows geologists to compare the ages of different rock layers across various regions.
2. Geographic Range:
   • The fossil should have a large geographic range so that it can be used to correlate rock layers over wide areas. This widespread distribution makes it more likely that the fossil can be found in multiple locations, facilitating cross-referencing of rock strata.
3. Temporal Range:
   • The fossil should have a small temporal range because this indicates that it existed for a relatively short span of geological time. A short existence helps in precise dating, as it ensures that the fossil is indicative of a specific time period.
4. Explanation of Other Options:
   • A small geographic range and large temporal range would mean the fossil is not useful for widespread correlation as it can't be found in multiple regions or provide precise dating.
   • A small geographic range and small temporal range is also undesirable as the fossil might not be found widely enough to be useful in different locations.
   • A large geographic range and large temporal range would limit its use in precise dating since it could have appeared in multiple periods.
5. Conclusion:
   • The option that best describes an ideal index fossil is "large geographic range and small temporal range," making it efficient for dating and correlating geological strata across different regions.