They had a huge amount of diversity between the Cambrian and the Carboniferous periods, then all but a single order (Nautilida) died out when land animals became a thing, and only a single genus survived the Triassic extinction. They went on to flurish globally until only 20 million years ago, when specifically seals appeared. Nautiloids went extinct everywhere seals existed, leaving the only living species in the Indo-Pacific.

Their shells have a tendency to float huge distances, with some floating for over a decade. So not only are they a "living fossil", their shells are found even when they are not.