These are marine mollusks with or without chambered or solid shells
which may be internal or external. The living animal possesses a
well-developed head with eyes, horny jaws, and many tentacles fused with
the foot. Cephalopods are the most advanced of all mollusks and include
the squid, octopus, pearly nautilus, and the extinct ammonoids. Members
of this class range from Cambrian to Recent in age but were much more
abundant in ancient seas than they are today. Their remains constitute a
very useful group of fossils, particularly in Paleozoic and Mesozoic
rocks.
Most paleontologists have divided the Cephalopoda into three subclasses,
the Nautiloidea, Ammonoidea, and the Coleoidea (known also as subclass
Dibranchiata and subclass Decapoda); each of these is discussed below.