The premises tell us that plesiosauromorphs and certain birds share a physical similarity, but we don’t care why they share that similarity. It could be from a common ancestor, random luck, or magic, with no difference to the argument.

The paleontologist is only concerned with plesiosauromorphs, so information about other species is not necessary. As such, the existence of other marine reptiles with similar fins wouldn’t affect our argument.

(C) would strengthen our argument, but it is too strong and therefore not necessary. Negating it yields, “gigantic marine mammals didn't need to chase prey over long distances to survive,” which still leaves open the possibility that plesiosauromorphs hunted in that manner.

This answer choice is far too strong to be necessary. We only need some marine mammals that chase prey over long distances to be specialized for long-distance swimming, not most.

If we negate (E) to say that fin shape doesn’t affect swimming in the same way that a bird’s wing affects flying, then long, thin fins would no longer be an indication of long-distance swimming. The argument would fail to make sense, so (E) is necessary.