The Archaeologist is an optional crew role - only the delver, scout, burrower, and guard must be assigned; furthermore in the event a crew falls below 4 explorers an explorer must take on multiple crew roles.
What isn't clear is what is done with the archaeologist role in crews of 4 players. Is one player allowed to fill their required role and the archaeologist in this case or only in cases where they fall below the minimum requirements? Is the archaeologist only in player for crew of a 5 or more players?