Is the Nautilus a Living Fossil? Ancient Shelled Cephalopods

Why is the nautilus called a living fossil?

The nautilus looks like an animal from a textbook about ancient seas. It has a coiled outer shell divided into gas-filled chambers, a feature shared with thousands of extinct cephalopods known only from fossils. Every other living cephalopod, including octopuses, squid and cuttlefish, has either lost its shell or carries a reduced one inside the body.

The chambered nautilus also keeps several features thought to be primitive. Its eyes have no lens and work like a pinhole camera, it has no ink sac, and its brain lacks the specialized learning centers of octopuses. Together, these traits made it the classic example of a “living fossil”, a term Charles Darwin used in On the Origin of Species in 1859 for living species that seem to have changed little from distant ancestors.

500 million years of nautiloids

Nautiloids first appeared in the Late Cambrian period, about 500 million years ago. During the Ordovician, roughly 485–444 million years ago, they became some of the top predators in the oceans. Many had straight, cone-shaped shells, and some of these reached more than 2.5 m (8 ft) in length.

More than 2,500 fossil nautiloid species have been described, with straight, curved, loosely coiled and tightly coiled shells. Over time most of these lines died out. The family that includes the modern nautilus, the Nautilidae, appears in the fossil record in the Late Triassic, more than 200 million years ago. Its members lived through the age of the dinosaurs and survived the mass extinction at the end of the Cretaceous.

How old are living nautiluses really?

The age of the group is not the same as the age of the animals alive today. The oldest fossils of the genus Nautilus come from the Late Eocene, roughly 35–40 million years ago, found in Washington State in the United States and in Kazakhstan. The oldest known fossils of the chambered nautilus itself, Nautilus pompilius, are from Pleistocene sediments off Luzon in the Philippines, less than about 2.6 million years old.

In other words, what has lasted for hundreds of millions of years is a general design, handed down through a long chain of different species, not the chambered nautilus itself. Fossil nautilids from the Jurassic and Cretaceous look broadly similar to living ones, but they are usually placed in other, extinct genera such as Cenoceras and Eutrephoceras.

Nautilus vs ammonites: not as close as they look

People often picture the nautilus as a surviving ammonite, but the two are different groups. Ammonoids evolved from straight-shelled cephalopods called bactritids in the Early Devonian, about 410 million years ago. The same branch probably also gave rise to the ancestors of octopuses and squid, so ammonites were likely more closely related to an octopus than to a nautilus.

Their shells differ in two easy-to-see ways:

• Sutures: where the internal walls meet the outer shell, nautiluses have simple, gently curved lines, while ammonites had wavy, often lace-like patterns.
• Siphuncle: the tube that links the chambers runs through the middle of each wall in nautiluses, but along the outer edge of the shell in most ammonites.

Ammonites were far more diverse than nautiluses for much of the Mesozoic, yet they vanished at the end of the Cretaceous, about 66 million years ago, while the nautilids carried on.

How did nautiluses survive the extinction of the ammonites?

No one knows for sure, but the leading idea concerns babies. Ammonites seem to have produced huge numbers of tiny hatchlings, about 1 mm across, that drifted in the plankton near the surface, where the effects of the asteroid impact were most severe. Nautilids lay a few large eggs on the deep sea floor, the young take about a year to develop, and they hatch as miniature adults. A slow metabolism and the ability to survive long periods without food may also have helped.

Why “living fossil” is a misleading label

Many biologists dislike the term because it suggests that evolution simply stopped. In nautiluses it clearly has not.

• Hidden species: a 2017 genome-wide study of nautiluses from across their range found that the traditional species did not match the genetic groups, with the exception of Nautilus macromphalus, and detected signs of natural selection between populations.
• New species: three nautilus species from Fiji, Samoa and Vanuatu, once lumped with the chambered nautilus, were formally described in 2023.
• Ongoing change: populations on different reefs are isolated by deep water and slowly diverging, the normal raw material of evolution.

Even the researchers behind that 2017 study used “living fossil” in its title, which shows how the phrase survives as shorthand for an old lineage with a conservative body plan.

Other animals called living fossils

The nautilus shares this nickname with the coelacanth, a lobe-finned fish known only from fossils until 1938, and with horseshoe crabs, whose body shape resembles relatives from hundreds of millions of years ago. You can compare their stories in the profiles of the coelacanth and the Atlantic horseshoe crab. The nautilus shell itself carries another popular myth, tested in is the nautilus shell a golden spiral?