EXPLOITING THE CAMBRIAN EXPLOSION

Everyone's talking about the Cambrian Explosion, it seems. While evolutionary biologists discuss the actual rate of adaptive radiation, a handful of creationists have hailed it as proof of...something...I forget exactly what. The Cambrian Explosion was, of course, that event from half a billion years ago, when suddenly—well, relatively suddenly—over the course of about 8 million years, a great diversity of metazoan life appeared on Earth, with representatives of most of the phyla that have ever turned up on the planet. For most of the preceding 3 ½ billion years, life on Earth had been unicellular. The cause of this explosion is poorly understood. Is it an illusion suggested by gaps in our knowledge of the fossil record? Was it caused by climatic or geological changes, or the evolution of Hox genes or sexual reproduction? Or was it simply a response to opportunities that could only be exploited by multicellular life? How about a combination of factors? Whatever the case, it gives me a chance to dust off this old illustration I did for a geology calendar that my friend John Christensen and I never finished designing back in the '80s: each week was to represent 10 million years—the whole of metazoan life on Earth compressed into one year. This was to be the illustration for January, and it depicts some of those cool Cambrian denizens cavorting in a shallow sea before a stromatolite reef. So without further ado, let's introduce them:

From left to right, the arthropod Marella scuttles across the sea floor, as the priapulid Ottoia emerges from its burrow to attack the mollusc Hyolithes. A pair of onychophorans (Aysheaia) graze on sponges as three Odaraia swim by in the company of a little Sarotrocercus. Behind the sponge is the phyllocarid Pseudoarctolepis. Nearby, the peculiar mollusc-like Wiwaxia and the trilobite Tricrepicephalus creep along. Closer in is the arthropod Habelia, and closer still, another Marella. A Canadaspis feeds upon a dead trilobite. The peculiar, worm-like Amiskwia swims near the surface, while in the background, the Great White Shark of the Cambrian, the two-foot-long Anomalocaris carries a fresh-caught trilobite in its arms. In the center sits the arthropod Branchiocaris, and the flatworm-like chordate Pikaia crawls alongside another Hyolithes. To the right of Branchiocaris is the little Burgessia. In the foreground, another Wiwaxia creeps along as a Marella gleans the carapace of a Naraoia. Behind Naraoia is the trilobite Asaphiscus. Further back are three forms of sessile animal life: a pair of articulate brachiopods, several Dinomischus, and the early echinoderm Gogia. In the foreground, an acrobatic Opabinia struggles with its prey, the polychaete Canadia. Last of all, a pair of tiny Lejopyge trilobites ride air bubbles on the surface, while the little arthropod Plenocaris perches in the foreground.
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illustration: BURGESS SHALE FAUNA (1989) acrylic 15" x 20"