Final answer:
The identification of index fossils is essential for dating geological formations. Shells, such as those from Belemnites, may serve as index fossils if they come from a short-lived, wide-spread species. Without species-specific information, it's challenging to definitively identify index fossils from the given options.
Step-by-step explanation:
The question poses a scenario where a student is asked to identify index fossils from a cross section of rock. Index fossils are fossils that meet certain criteria: they are distinctive, abundant, had a wide geographic distribution, and existed for a relatively short and specific geologic time, thereby allowing geologists to use them as reference points to determine the age of the rock layers in which they are found.
Given the options in the question (a. bone and shell b. bone and teeth c. shell and leaf d. leaf and teeth), we must select the ones that are most likely to be abundant, widely distributed, and from a known and short geologic era. Keep in mind that while bones and teeth can be distinctive and plentiful, they are less likely to be used as index fossils due to their broader existence in the geologic record.
Shells and leaves could potentially serve as index fossils if they are from species that existed for a short time. For instance, Belemnites are an example of marine animals whose fossilized remains (specifically their shells) can act as index fossils for rocks dating to the Mesozoic era. Similarly, certain fossilized leaves might come from plants that thrived for a brief period and had wide-reaching distribution, potentially making them viable as index fossils.
In conclusion, without specific information about the species of leaf or shell, it's difficult to definitively choose between the given options. However, if the shells represent a species like those of Belemnites, common in the Mesozoic era, then they would likely make good index fossils. The same could apply for leaves if they belong to a species known to have been short-lived and widespread.