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-
High in Canadian Rockies of British Columbia, in an extraordinary 540-
In these waters lurked a lethal cast of predators, eyeing little shells with bad intent: Sidneyia, a flattened, ram-
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-
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, mollusks 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-