MARINE FOSSILS paint an idyllic scene of animal life in its infancy off the shores of the naked continents some 670 million years ago: Soft coral fronds arch from the ocean floor, jellyfish undulate in the currents and marine worms plow through the ooze.

But a geologically brief 100 million years later, at the dawn of the Cambrian period, the seascape abruptly changes. Animals suddenly appear cloaked in scales and spines, tubes and shells. Seemingly out of nowhere, and in bewildering abundance and variety, the animal skeleton emerges.

For more than a century, paleontologists have wrinkled their brows trying to explain why -- after at least 100 million years of soft, serene multicellular existence -- life so hurriedly turned hard. Sophisticated hypotheses abound, some linking the skeletal genesis to changing chemistries of the seas and skies. Yet a recent analysis of these ideas suggests that the oldest -- and perhaps most basic -- of these explanations deserves the spotlight. From old fossil quarries in Canada and from new ones in Greenland comes fresh evidence supporting the notion that the skeletal revolution was more than a chemical reaction: It was an arms race. Postcards From the Ledge High in the Canadian Rockies of British Columbia, in an extraordinary 540-million-year-old fossil deposit called the Burgess Shale, a mid-Cambrian marine community comes to life. Like many less exceptional deposits, the Burgess harbors mild-mannered mollusks, trilobites (the ubiquitous, armored "cockroaches" of the Cambrian seas) and clam-like brachiopods (creatures with hinged shells and armlike feeding parts).

But other imprints in the smooth black shale dispel any image of a peaceful prehistoric aquarium. In these waters lurked a lethal cast of predators, eyeing little shells with bad intent: Sidneyia, a flattened, ramheaded arthropod (the phylum of animals with jointed legs and segmented exterior skeletons), with a penchant for munching on trilobites, brachiopods and small cone-shelled creatures called hyolithids; Ottoia, a chunky burrowing worm that preferred its hyolithids whole, reaching out and swallowing them with a muscular, toothed proboscis; and even some trilobites with predatory tastes.

Thanks to a rapid burial under fine sediment, which sealed out scavengers and agents of decay, the Burgess Shale preserves a unique snapshot of life in the heyday of the skeletal revolution. Though Burgess excavations began early in this century, only in the past 20 years have paleontologists begun detailed reconstruction of the shale's hunters and hunted. Their findings have helped resurrect the arms-race hypothesis: the 80-year-old idea that skeletons evolved primarily as fortresses against an incoming wave of predators.

Witness Wiwaxia, a small, slug-like beast sheathed in a chain-mail-like armor. With two rows of spikes running along its back, Wiwaxia was the mid-Cambrian analogue to a marine porcupine. Paleontologist Simon Conway Morris of the University of Cambridge in England, who reconstructed the creature from a mashed mass of fossil scales and spines, says Wiwaxia was likely dressed for defense.

Even more telling are the chinks in its armor. Some of Wiwaxia's spines appear to have broken and healed, says Conway Morris, who suspects that predators snapped them off. In recent years, he and others have also noticed bite-sized chunks missing from fossil trilobites.

The healed wounds of trilobite and Wiwaxia specimens suggest that predators strongly influenced the elaborate new skeletal designs of the mid-Cambrian, asserts evolutionary biologist Geerat Vermeij of the University of California at Davis. In a paper published in December 1989, he reexamines several of the major hypotheses explaining the skeletal revolution, and concludes that predation was the primary factor. Nonlethal injury to skeletons, Vermeij writes, "would demonstrate that the organism sustaining the injury was able to survive despite the onslaught, and therefore that some of its attributes (including those of the skeleton) served a protective function."

What sort of creature could gouge such wounds in a husky trilobite? British paleontologists Derek Briggs of Bristol University and Harry Whittington of the University of Cambridge believe they have found a likely culprit embedded in the Burgess Shale. In 1985, they unveiled their reconstruction of Anomalocaris, the largest of Cambrian predators. Fitting no other major animal design known, this half-meter-long "terror of the trilobites," as Briggs and Whittington have called it, glided through the seas with ray-like fins and chomped with a ring of spiked plates that dispatched trilobite shells like a nutcracker, the two speculate. Its bite probably formed a W shape, nicely matching some of the trilobite wounds they have examined.

In all fairness, Anomalocaris has been caught holding the weapon but never the victim. Other Burgess predators have not fled the scene so quickly, however. For instance, paleontologists have found fossils of the arthropod Sidneyia and the carnivorous worm Ottoia with shelly animals still in their guts.

Yet even the inside of a stomach might not spell doom for the ingested prey -- given a properly equipped skeleton. Vermeij cites the example of some modern-day clams with hermetically sealed shells, swallowed whole by one type of starfish. "They can sit there for 14 days in the digestive system," he says. "Finally the sea star excretes them, and they just go their merry way." The Calcium Question Though it depicts the skeletal drama unfolding a staggering 540 million years ago, the Burgess Shale missed the opening act. The curtains went up about 30 million years earlier, when a flourish of fauna with hard little tubes, cones, scales and needles -- collectively called the "small shelly fauna" -- burst upon the scene. Many of these structures consisted of calcium compounds, leading some paleontologists to look to oceanic chemistry for skeletal explanations. Geochemical models suggest that oceanic calcium levels were increasing as animal skeletons became more diverse and elaborate. Noting that high concentrations of calcium in animal tissue can prove lethal, proponents of the detoxification hypothesis contend that skeletons evolved as calcium receptacles for early soft-bodied creatures needing to dump the excess mineral from their tissues.

Vermeij questions that hypothesis, citing the energy expense of producing such shells. "If an organism really needs to get rid of calcium in a desperate way, it will do it any way it can," he says. On the other hand, shells made under less dire conditions, Vermeij suggests, would tend to be more tidily constructed. "But the problem is, you're seeing inefficient forms in places such as fresh water, where the levels of calcium carbonate are generally low."

Paleontologist Kenneth M. Towe at the Smithsonian Institution in Washington adds, "If {detoxification} is the reason, how did those organisms without calcium skeletons detoxify themselves? You don't need a skeleton to detoxify calcium. You can just dump it into the ocean."

Levels of atmospheric oxygen also appeared to rise in concert with the skeletal revolution. Some researchers have proposed that the new metabolic energy supplied by oxygen allowed for larger animals, which in turn required more rigid structural supports. "But most of the early shell-bearing animals were extremely small," counters Vermeij. He favors the idea that oxygen merely afforded animals the luxury of fancier skeletal architecture.

"Predation, rather than detoxification or size increase, was largely responsible for the origin and subsequent elaboration of calcareous skeletons," Vermeij asserts.

From the treacherous maw of Anomalocaris to the healed wounds of Wiwaxia, much of the support for the arms race argument hinges on the Burgess Shale collection. But what about the small shelly fauna that emerged 30 million years earlier? For an arms race hypothesis to be complete, predators must have roamed then, too.

"Unfortunately, if you {look at} a typical shelly fauna, a normal fossil fauna, you won't see any predators at all, because most predators in a paradoxical sense are soft-bodied and are not preserved," says Conway Morris. Hence, most evidence for predators of shelly fauna rests on the injuries the attackers left behind.

In 1968, Stefan Bengtson of the University of Uppsala, Sweden, described several early Cambrian specimens of the small, clam-like Mobergella, each with a neat round hole through the thin, conical apex of its shell. To this day, Bengtson remains unsure whether the shells represent an entire skeleton or merely a cap to a tubular burrowing animal. But he has strong convictions about the holes in Mobergella, believing they did not get there by daily wear and tear. "The predator that attacked" Mobergella, he explains, "appeared to have been drilling through the thinnest part of the shell to get to the soft interior."

Something might have wanted to bore into the skeletal fortress of the early Cambrian Mickwitzia as well, but it would have run up against a second line of defense. Paleontologist Mark McMenamin of Mount Holyoke College in South Hadley, Mass., notes that minute holes, or punctae, pepper the shell of the clam-like Mickwitzia. These, he believes, served as conduits for secreted antipredator chemicals. In Mexican fossil beds, McMenamin has found that predators riddled the unperforated shells of other animals but left Mickwitzia alone. Sermons in Stones With such a cast of armed combatants dominating the Cambrian arena, early paleontologists nearly overlooked an inconspicuous little spectator cowering on the sidelines. Flat as a ribbon, limp as a worm and eyeless, the two-inch creature -- eventually named Pikaia -- wisely kept out of the fray.

In the early 1900s, Pikaia's discoverer, Charles Doolittle Walcott, examined the strange fossil and tossed it, taxonomically speaking, in a heap with the rest of the Burgess Shale worms. Not until Conway Morris reopened the fossil album more than 60 years later did Pikaia glow in a new light. Unlike all other animals then known from the Cambrian, Pikaia featured a single central nerve cord running the length of its back -- placing it in the phylum Chordata (which contains vertebrates). Offshoots of the chordates would someday sprout the fins of fishes and fill the oceans, or grow Brontosaurus legs and lumber about in Jurassic swamps. Some would spread feathered wings and cruise the continents by air. Some would don pants and study ancient fossils.

Thus while many of the Cambrian contestants were parrying with lethal weapons and fancy defenses, mankind's possible precursor was playing the part of a worm. Fossil censuses reveal that Pikaia was a relatively minor part of Cambrian ecology. It had almost no known relatives, and its numbers accounted for but a tiny fraction of the period's living masses. Yet it got by where others did not. An unparalleled host of major animal designs flourished within the Cambrian seas, many of which would not outlast the following 500 million years of evolutionary winnowing.

Not that Pikaia was ill-endowed. Its coordinated muscles likely powered an agile swimmer, Conway Morris speculates, quick enough to slip through the grasps of clumsy attackers.

Was Pikaia's success inevitable, mandated by survival-of-the-fittest dogma? Harvard paleontologist Stephen Jay Gould suspects a large measure of luck. "Wind the tape of life back to Burgess times, and let it play again," he writes in "Wonderful Life" {see box}: "If Pikaia does not survive in the replay, we are wiped out of future history -- all of us, from shark to robin to orangutan. And I don't think that any handicapper, given Burgess evidence as known today, would have granted very favorable odds for the persistence of Pikaia."

Human ancestors may have figured little in the Cambrian frenzy; but their descendants are making serious steps toward explaining it all. Conway Morris says about 30 quarries worldwide are beginning to yield Burgess-quality fossils, with many more sites yet to be discovered. And he believes it's only a matter of time before paleontologists track down the original cast of predators that might have helped incite the skeletal stampede. Fossils now show, for instance, that the trilobite-chomping Anomalocaris identified at Burgess also roamed the early Cambrian seas.

"When we find the equivalent of the Burgess Shale right at the base of the Cambrian period, as surely we will, we will find that there are all sorts of interesting predators," he predicts. "Ultimately, I think we're going to be able to integrate the whole thing into quite a nice story."

Survival of the Luckiest?;Stephen Jay Gould on Why Evolution May Be a Sort of Lottery William Stolzenburg

HARVARD paleontologist Stephen Jay Gould, in "Wonderful Life: The Burgess Shale and the Nature of History" (W.W. Norton, 1989) argues that the record embedded in the Burgess Shale -- a fossil snapshot of the period just after the "Cambrian explosion" of 570 million years ago which "marks the advent of virtually all major groups of modern animals" -- challenges our traditional ideas of evolution.

The conventional notion depicts evolution as an inexorable march of progress through time -- "reinforcing a comfortable view of human inevitability and superiority," Gould writes. Another familiar metaphor is what he calls "the cone of increasing diversity -- an upside-down Christmas tree. Life begins with the restricted and simple, and progresses ever upward to more and more and, by implication, better and better" forms. This viewpoint, Gould argues, biased research for the past hundred years in favor of viewing more recently evolved animals as "improved" and their ancestors as primitive. Thus standard evolutionary "tree" diagrams show that all coelomate animals (those with a body cavity) arose from a single type of flatworm that flourished around 700 milion years ago. Conversely, any living organism must have evolved from a less sophisticated ancestor, and any species that died out must have been inferior or ill-suited to "progress."

But the astonishing diversity of creatures recently revealed by research on the Burgess Shale fossils, Gould asserts, shatters that notion by demonstrating that "most Burgess organisms do not belong to familiar groups, and that the creatures from this single quarry in British Columbia probably exceed, in anatomical range, the entire spectrum of invertebrate life in today's oceans," including "some 20 to 30 kinds of arthropods {creatures with jointed limbs} that cannot be placed in any modern group." The shale record shows that many intriguing and complicated species died out altogether despite apparent adaptive advantages. Thus extinction may not imply inferiority; it may simply be bad luck. "The history of life," Gould writes, "is a story of massive removal followed by differentiation within a few surviving stocks, not the conventional tale of steadily increasing excellence, complexity and diversity."

If evolution is less like an upward spiraling cone and more like a bush -- some of whose branches simply stop while others grow and split -- then can we still think of man as having a special place in nature? We prefer to think so, especially since mankind developed so extremely recently in geological history. But "if humanity arose just yesterday as a small twig on one branch of a flourishing tree," Gould argues, "then life may not, in any genuine sense, exist for us or because of us. Perhaps we are only an afterthought, a kind of cosmic accident, just one bauble on the Christmas tree of evolution."

William Stolzenburg is a writer for the weekly magazine Science News, from which this article was adapted.