| Cephalopods were once one of the dominant marine animals but there are only about 700 living species. In fact, more than 10,000 extinct species have been described from the fossil record. They appeared after other major molluscan groups, in the Middle Cambrian. But after that, their diversity took off. The most common fossil cephalopods are nautiloids and ammonoids, the soft-bodied octopuses and squids don't fossilize very well. Nautiloids have survived until today, but the ammonoids, one of the most common groups of fossils, went extinct with the dinosaurs at the end of the Cretaceous. | Cephalopods are much more variable in their diversity through time than other molluscan groups. They are hit by numerous extinctions (e.g., terminal Permian, Triassic, Cretaceous events), but typically showed rapid replacement (and subsequent radiation) by the survivors. |
