Fossil of the Week

10/6/10 – Chambered Nautilus

The living Chambered Nautilus is often considered a classic “living fossil.” It is a cephalopod – in the same group as squid and octopuses. Its subclass, the Nautiloidea*, once claimed extremely diverse and abundant marine predators, with approximately 2,500 fossil species currently known. Today, there are only two living genera and four to ten species. The genus Nautilus is the most common of the two modern genera and contains perhaps six species through the Indo-West Pacific. Pictured here is a modern specimen of Nautilus pompilius, the Chambered Nautilus (PRI acc. 1414), which is the largest and most common of the living species. Like our last Fossil of the Week, it was named by Linnaeus in Systema Naturae in 1758.

The first nautiloids appeared in the Upper Cambrian Period, approximately 490 million years ago, when cephalopods first developed shells. The shell allowed them to become neutrally buoyant (that is, no energy expenditure is required to maintain a constant depth) allowing them to expand their habitat off the sea floor. The group then underwent an evolutionary radiation in the early Ordovician Period, approximately 480 million years ago, becoming an extremely important group of predators in the Early Paleozoic. Fossil nautiloids were diverse and abundant, with many different morphologies known, from straight cone-shaped shells to coiled shells. Today, the only shape remaining is the discus-like, planispiral shell of N. pompilius. The living nautiloids appear to have undergone extremely little change during the nearly 500 million years since their first appearance.

Nautilus has often been used as an analog for understanding its better-known but extinct cousins, the ammonoids. Although there are many similarities in shape and appearance, the ammonoids are a much younger group that first appeared in the Silurian Period (420 million years ago). The ammonoids might in fact have been descendents of the older nautiloids. Nevertheless, reconstructions of the soft parts of extinct coiled ammonites owe much to studies of Nautilus specimens. We know that Nautilus today uses jet propulsion to swim shell-first through the water, and it is likely that some ammonoids moved in a similar way.

Text by Ursula Smith (reprinted from “Fossil Focus” in American Paleontologist, Winter 2008)

*For more about fossil nautiloids, see Fossils of the Week 6/16/10 – The Fourth Variant and 5/5/10 - Bradfordoceras.