Hm?

Medullosa would be the least useful index fossil.

Explanation:

When it comes to the past of Earth, what has happened, what was present, why or why not something was there, dating, geology uses numerous different methods. One of those methods, which is relatively accurate and is a good indicator of the layer and other things around it, are the index fossils.

The index fossils are fossils that can be found in numerous different places around the world. They also appear only in a specific and not very long timeline. From the fossils we have presented, while we don't have information about their dispersal, we do have timelines of when the organisms existed. Medullosa is the obvious candidate that would be the least useful if it was to be used as an index fossil. This is due to the enormous timeline of the existence of this organism, stretching somewhere around 90 million years and through two periods, the Carboniferous and Permian, which will make it very hard to find any use of it in determining anything significant.