A shark would not be a good index fossil because
A. the species has survived for too long.
B. salt water would degrade the fossil over time.
C. no fossil records exist for sharks.
D. it doesn’t have structures that can be preserved in a fossil.
The Correct Answer and Explanation is :
The correct answer is D. it doesn’t have structures that can be preserved in a fossil.
Sharks are made primarily of cartilage, a flexible and less durable material than bone. Most fossils of animals and plants are preserved through mineralization, where organic materials are replaced by minerals over time, forming a hard, rock-like structure. Cartilage, unlike bone, doesn’t mineralize as well, which makes it more prone to decay and decomposition over long periods. While shark teeth, which are made of enamel (a hard mineralized substance), can be preserved as fossils, the shark’s body structures, such as cartilage, typically do not fossilize well.
To be a useful index fossil, an organism must have certain characteristics:
- Wide distribution – It must have lived in many parts of the world so that its fossil remains can be found in many places.
- Short duration – The organism should have existed for a relatively short period of geological time, so that its fossils can help date the rock layers it is found in.
- Abundance – It should be found in large numbers in the fossil record, making it more likely to be found and identified.
Sharks do not meet the second and third criteria as well as other organisms, such as brachiopods or ammonites. While sharks have been around for hundreds of millions of years, their fossils are limited primarily to their teeth. The lack of body fossils (due to the decay of cartilage) means sharks are not ideal index fossils. Fossilized teeth can be useful for identifying the general time period of a rock layer, but the lack of more complete body fossils makes it difficult to correlate precise geological time intervals.
In conclusion, the absence of well-preserved body structures and the tendency for cartilage to decompose rather than fossilize limits the utility of sharks as index fossils.