Marine fossils paint an idyllic scene of aquatic animal life in its infancy some 670 million years ago (mya): soft coral fronds arch from the ocean floor, jellyfishes 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 picture suddenly changes. Animals abruptly 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 tried to explain why life turned hard. Hypotheses abound, some linking the skeletal genesis to changing chemistries of the seas and skies. Yet a recent analysis of old fossil quarries in Canada and new ones in Greenland is providing evidence supporting the notion that the skeletal revolution was more than a chemical reaction—it was an arms race.
High in 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 mollusks, trilobites (the ubiquitous, armored "cockroaches" of the Cambrian seas), and clam-like brachiopods. 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, ram-headed arthropod with a penchant for munching on trilobites, brachiopods, and cone-shelled 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. These 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.
Take 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. The chinks in its armor are telling. Some of Wi-waxia,s spines appear to have broken and healed. The healed wounds of trilobite and Wiwaxia specimens suggest that predators strongly influenced the elaborate new skeletal designs of the mid-Cambrian.
What sort of creature could gouge such wounds in a tough trilobite? One likely culprit is Anomalocaris, the largest of Cambrian predators. This half-meter-long creature glided through the seas with ray-like fins and chomped with a ring of spiked plates that dispatched trilobite shells like a nutcracker.
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.
New finds strengthen the case for an early Cambrian arms race. From an extraordinary fossil bed discovered in 1984 in north Greenland, predating the Burgess shale by perhaps as much as 15 million years, comes a jigsaw puzzle already assembled: a suspiciously familiar, slug-like beast sheathed in chain-mail armor, proposed to be the long-sought ancestor of the armored slug Wiwaxia.
The creature sports a disproportionately large, saucer-like shell at each end of its elongated body. From another fossil discovery at a quarry in south China, which appears even older than the Greenland site, emerges the bizarre Microdictyon. Unveiled in 1989 by Chinese paleontologists, Microdictyon is a wormish creature with a row of pointed appendages and a body studded with oval phosphate plates. About 30 quarries
worldwide are beginning to yield Burgess-quality fossils, with perhaps many more waiting to be discovered.
Since that explosion of new forms some 530 mya, however, few new marine animals have evolved. Analysis of the evolution of marine animals suggests that a sufficient variety of life forms in an environment suppresses further innovation. About 530 mya, during the Cambrian period, after a long period in which animals were essentially jellyfishes or worms, marine animal life exploded into a variety of fundamentally new body types. Arthropods turned up inside external skeletons, mol-lusks put on their calcareous shells, and seven other new and different body plans appeared; an additional one showed up shortly thereafter. But since then, there's been nothing new in terms of basic body types, which form the basis of the toplevel classification of the animal kingdom called phyla.
Research presented at a 1994 meeting of the Geological Society of America lends support to the idea that once evolution fills the world with sufficient variety, further innovation may be for naught. There are only so many ways marine animals can feed themselves—preying on others or scavenging debris, for example. And there are only so many places to
do it: on the sea floor, beneath it, or some distance above it. When all the nooks and crannies of this "ecospace" are filled, latecomers never get a foot in the door.
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