The cephalopods are members of the molluscan class called Cephalopoda. These exclusively marine animals are characterized by bilateral body symmetry, a prominent head, and a set of arms or tentacles modified from the primitive molluscan foot. Cephalopods occupy most of the depth of the ocean, from the abyssal plain to the sea surface. Their diversity is greatest near the equator and decreases towards the poles.

Early cephalopods are thought to have been predators near the top of the food chain. They underwent pulses of diversification during the Ordovician period to become diverse and dominant in the Paleozoic and Mesozoic seas. In the Early Palaeozoic, their range was far more restricted than today; they were mainly constrained to sub-littoral regions of shallow shelves located in the low latitudes. A more pelagic habit was gradually adopted as the Ordovician progressed.

Deep-water cephalopods, whilst rare, have been found in the Lower Ordovician—but only in high-latitude waters. The mid Ordovician saw the first cephalopods with septa strong enough to cope with the pressures associated with deeper water, and could inhabit depths greater than 100-200 metres.

The class now contains two, only distantly related, extant subclasses: Coleoidea, which includes octopuses, squid, and cuttlefish; and Nautiloidea, represented by Nautilus and Allonautilus. In the Coleoidea, the molluscan shell has been internalized or is absent, whereas in the Nautiloidea, the external shell remains. About 800 living species of cephalopods have been identified. Two important extinct taxa are the Ammonoidea (ammonites) and Belemnoidea (belemnites).