Nautiloidea

Nautiloidea is a subclass of the class Cephalopoda that includes the living nautiluses and a long, largely extinct record of other shelled cephalopods. Members are defined by their external shells, which enclose a series of internal chambers separated by septa, and a siphuncle that runs along the shell's ventral side to regulate buoyancy. The animal occupies the final chamber, using the gas-filled chambers to control vertical position in the water.

Fossil nautiloids first appear in the late Cambrian. They peaked in diversity during the Paleozoic, especially the Ordovician to Devonian, and many lineages went extinct by the end of the Permian. In the modern seas, only two genera remain: Nautilus and Allonautilus, which inhabit shallow tropical waters and exhibit comparatively simple shell sutures.

Ecology and morphology: Nautiloids were marine predators with a circle of tentacles for capturing prey. They were generally slow-moving and occupied a range of habitats from near the sea floor to open water. Buoyancy was achieved by adjusting the buoyant gas in the shell's chambers via the siphuncle.

Taxonomy and significance: Nautiloidea is a key group for understanding early cephalopod evolution. It contains the living Nautilidae as well as numerous extinct genera from orders such as Nautilida and orthocerids. Their fossil shells, often well preserved, make nautiloids useful index fossils for Paleozoic rocks.