The chambered nautilus, last living relic of an ancient era that saw the seas aswarm with strange creatures made of spiral shells and tentacles, has long been considered a case of arrested evolution.

To biologists who examined its symmetrically spiraling, compartmentalized shell, it appeared unchanged from the time, beginning 525 million years ago, when 17,000 species of nautiloids dominated the seas. Land animals had not evolved, nor had animals with bones.

Then, according to the fossil record, the nautiloids and their close relatives the ammonites went into decline, the last of them, except for the genus Nautilus itself, dying out 65 million years ago in the mass extinction that also finished the dinosaurs. Nautilus persisted into modern times as a "living fossil."

Now, however, David Woodruff, a biologist at the University of California at San Diego, has found evidence to challenge this old idea. Far from being a genetically monotonous group without potential to evolve, nautilus contains more genetic diversity from one individual to the next than exists among many rapidly evolving species, including human beings.

It is this reserve of genetic variability, the opposite of being purebred, that makes new species possible through the combination of traits from different parents.

The biologist's studies indicate that all of the five or six known species of nautilus came into existence within the last 5 million years, two evolving less than a million years ago, having arisen about as recently as Homo sapiens.

Within each nautilus species, Woodruff said, "They are richly endowed with a genetic heritage that suggests they can evolve quite quickly. This appears to be an actively speciating group and we may even be in the early stage of a new nautilus radiation."

A radiation, in the jargon of evolution theory, is a proliferation of many species from one or a few ancestral forms, usually in just a few million years. Several times in the past, the nautiloids, like many other groups, have alternated between periods of radiation and decline.

"The genes say this is not a living fossil but an actively evolving group," Woodruff said. "The reason people call them living fossils is that the change doesn't show up very much in the shell, which is all we know of the extinct species. The changes are in the soft parts and sometimes just within the chemistry of the cells."

The known nautilus species differ considerably in size but only slightly in the soft parts or in the shapes of the shells.

Woodruff spoke at a symposium on the nautilus that was part of last month's annual meeting of the American Association for the Advancement of Science.

Although the beautifully shaped and colored shells of the nautilus have captured scientific and popular attention at least since Aristotle, who gave them their name, it was not until 1832 that scientists had an example of the soft parts to examine. The publication of that study, based on one specimen shipped by sailors from the New Hebrides to England, was hailed as a major scientific event at the time.

The more detailed nature of the organism remained elusive until just the last 10 years when aquarists learned how to keep live specimens in tanks long enough to study. Of the five definitely known species (some say there is a sixth), one was first seen alive only last year and another only the year before.

Just last year John Arnold of the University of Hawaii became the first person to see a nautilus embryo, culminating a search by many scientists that had gone on for decades. He cut open an egg case laid by a captive specimen in the Waikiki Aquarium and found something white and moving inside. Previous efforts had yielded empty egg cases.

Arnold reported that the egg case is relatively large. A female nautilus six or seven inches in diameter may lay an irregularly shaped egg case measuring 2 inches by 1 1/4 inches. He estimates that it takes the embryo about a year to reach hatching size of perhaps an inch in diameter. The nautilus then grows slowly, reaching maturity in about 15 years and then reproducing periodically for several years.

Such developments have helped launch the nautilus as one of the most popular species to be newly taken up for intense scientific study. Bryn Mawr College, near Philadelphia, hosted a major international scientific conference on the animals last month.

"Having access to nautilus is somewhat like having a single living dinosaur," said Bruce Saunders, a Bryn Mawr College paleontologist who has trapped and photographed the animals for many years. Saunders said those who study evolution are just beginning to realize that the nautilus may be able to tell them many things about the nature of life in the distant past.

Biologists find the nautilus interesting because it is the only living representative of a group of creatures that once dominated the Earth, the externally shelled cephalopods.

Except for the nautilus, all of today's cephalopods, such as squids and octopi, have internal shells. Cephalopods are marine mollusks distinguished most obviously by having tentacles. The nautilus has about 90 tentacles.

All known nautilus species live in deep water, typically between 500 and 1,000 feet down, around reefs in the Indo-Pacific, ranging from the Philippines to Australia and Indonesia.

Although the nautilus has two primitive, lensless eyes, it sees little because almost no light penetrates to these depths. The nautilus uses its tentacles as a kind of nose to detect scents of food wafting through the ocean currents. The nautilus is chiefly a scavenger -- a marine vulture, as one scientist put it -- but also captures the occasional shrimp.

The nautilus swims slowly by jet propulsion, sucking water in from the sides and squirting it out a flexible tube that can be aimed forward or backward to propel the animal in the opposite direction.

Nestled among the bases of the tentacles is a powerful, parrotlike beak that can tear flesh from fish carcasses or crunch into dead crab claws. The nautilus is believed to have been the first animal to evolve jaws.

As the nautilus grows, it adds a new chamber every few months. Once the new wall is in place, the soft parts of the animal slide toward the shell's mouth, the vacated space fills with seawater and the animal begins secreting a new wall. When complete, that wall seals off a new water-filled chamber. For the next few months or years, the flooded chamber is slowly pumped dry through the thin tube penetrating the wall.

The growth pattern is timed so that by the time a chamber occupies the uppermost portion of the shell, it contains only gas, mainly nitrogen. The buoyancy of the empty chambers keeps the whole animal oriented properly in the water and exactly balances the animal's weight. As a result it can hover in mid-water without using energy.