The first known cephalopod was only a millimetre long and lived about 520 million years ago

The locomotion of modern cephalopods, such as squid and cuttlefish, relies on buoyancy control achieved mainly by a specialized structure called a siphuncle. The discovery of fossils of Eoceras — a millimetre-sized cephalopod with a siphuncle — suggests that the group had already emerged in the early Cambrian, around 520 million years ago.