Final answer:
The least useful index fossil would be one that has a wide age range and is not limited to a specific geologic time period.
Step-by-step explanation:
The least useful index fossil would be one that has a wide age range and is not limited to a specific geologic time period. This means that the fossil could be found in rocks spanning different time periods, making it less reliable for dating purposes.
An example of a less useful index fossil would be a species that existed for a long time, such as a type of brachiopod or rugosa. These organisms can be found in rocks from multiple geologic periods, so their presence wouldn't give a precise age for the rock layer.