Final answer:
The least useful fossil as an index fossil would be one that lived throughout the fossil record, like bryozoans and echinoids, because it would not indicate a specific geologic time period.
Step-by-step explanation:
According to the fossil record, the least useful fossil as an index fossil would be one that lived throughout the fossil record and thus does not indicate a specific geologic time period. While none of the exact examples provided in the student's question (Sutcliffia, Medullosa, Archaeopteryx, Bjuvia) are discussed in the context of the provided reference information, we know that index fossils are typically those that were abundant, existed for a short period of time, and had a wide geographic distribution. Bryozoans and echinoids, which lived throughout the fossil record, are not good index fossils because they do not meet the criteria of existing for only a short period. Based on the context, it would be reasonable to infer that among the options given, the fossil that would fit the description of being least useful as an index fossil would be one similar to bryozoans and echinoids insofar as it existed for a long duration throughout the fossil record.