The nautilus is the marine animal known as a living fossil with a coiled shell. Nautiluses are cephalopods, related to octopuses, squid and cuttlefish, but they are the only living cephalopods with an external chambered shell.
Their shells are made of aragonite and form a tightly coiled, logarithmic spiral. Internal chambers, called camerae, help control buoyancy: the animal adjusts the fluid and gas within them to rise or sink. The shell also provides protection, and the nautilus can withdraw into it and close the opening with a leathery hood.
“Living fossil” is a useful popular description, but it does not mean the animal has remained completely unchanged. Modern nautiluses have continued evolving, and genetic studies show substantial diversity. They are not living ammonites, another group of extinct shelled cephalopods.
Nautiluses are slow-growing scavengers and predators that live mainly on reefs and seafloors in the Indo-Pacific. Their shells have also been collected for ornaments, contributing to conservation concerns.