A shark would not be a good index fossil because A. salt water would degrade the fossil over time. B. no fossil records exist for sharks. C. it doesn’t have structures that can be preserved in a fossil. D. the species has survived for too long
The Correct Answer and Explanation is:
The correct answer is: D. the species has survived for too long.
Explanation
Index fossils are the preserved remains of organisms that lived during a relatively short, specific time period and were widespread geographically. These fossils help geologists and paleontologists determine the relative ages of rock layers. For a fossil to be a good index fossil, it must meet three main criteria:
- It must be easily recognizable.
- It must be geographically widespread.
- It must have existed for a relatively short geologic time span.
Sharks, while fascinating and ancient creatures, do not make good index fossils for a key reason: they have existed for hundreds of millions of years, with relatively little evolutionary change. The earliest sharks appeared over 400 million years ago during the Devonian Period, and many modern shark species are very similar to their ancient relatives. This long evolutionary survival makes it difficult to assign a specific rock layer to a short time period based solely on the presence of shark remains.
Because the shark lineage spans such a broad time range, their fossils do not pinpoint a narrow window of geologic time, which is essential for index fossils. Therefore, they lack the time-specificity that index fossils require.
While it is true that sharks have mostly cartilaginous skeletons, which do not fossilize as easily as bone (making option C seem appealing), many shark fossils do exist, especially teeth, which are highly durable and common in the fossil record. So, options A, B, and C are not correct.
In summary, although shark teeth are abundant and recognizable, the key disqualifying factor is their long evolutionary duration. Because sharks have existed for too long without major changes, they fail to meet the short time-span criterion required for a reliable index fossil.
