The nautilus is often called a living fossil because its distinctive lineage has ancient roots and has changed relatively little in appearance.
Nautiluses are cephalopods with external, chambered shells. As the animal grows, it adds a new chamber and seals off the older one. A tube called the siphuncle helps regulate gas and liquid in the chambers, allowing the animal to control buoyancy.
Unlike octopuses and most squids, nautiluses retain a visible shell and have many simple tentacles without suckers. They live on tropical Indo-Pacific slopes, often at depths of several hundred metres, and are active mostly at night.
The phrase “living fossil” does not mean the animal stopped evolving. It means that a surviving organism resembles ancient fossil relatives more closely than most living lineages do. Ammonites are extinct relatives, not living nautiluses.