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. Small shelly fossils show that cephalopods with siphuncle structures lived in the early Cambrian period.