First cephalopods

The first cephalopods, such as Plectronoceras from northern China, appear in the Late Cambrian, about 500 million years ago. They were small, with a cone-shaped shell divided by walls into chambers. A tube called the siphuncle ran through the chambers and let the animal pump fluid in and out to adjust its buoyancy, an invention that allowed active swimming. Cephalopods grew from small molluscs creeping along the bottom into free swimmers with tentacles and a beak.

In the Ordovician, shells of nautiloids elongated into straight cones, called orthocones. Some, such as Cameroceras, grew to several metres and were probably among the largest predators of their time, ruling the seas before large fish. Later lines coiled their shells, and the ammonoids that appeared in the Devonian became some of the most common marine fossils of the next 300 million years. Squid, octopuses and cuttlefish lost the external shell, and the modern Nautilus is a living survivor of the original design. The group is a good example of an innovation in buoyancy control that turned a bottom dweller into a hunter.