Sod busters along the old Cambrian trail

October 25, 2002

If you've ever had to scrape a barnacle, you can blame a trilobite for your trouble. The advent of trilobite-tilled, worm-worked muck a half-billion years ago may have helped feed an explosive evolution of early animal life in the oceans - including the sort of critters that today attach themselves to boats and piers.

In the placid days before what's called the Cambrian Explosion of animal life, seafloor sediments were laid down in neat, firm layers, coated by microbial mats and remained undisturbed. Animals had no need to attach themselves to anything because the seafloor was firm and reliable. Then along came animals that had figured out how to mine the seafloor for food, changing the course of evolution forever. David Bottjer of the University of Southern California will explain the pivotal evolutionary role of what he calls the Cambrian "sod busters" at the annual meeting of the Geological Society of America on Monday, October 28, in Denver, CO.

To learn how mud mining animals might have spurred evolution, Bottjer looked at ancient seafloor sediments in China and Canada from the period of time when the habit of churning, or "bioturbation," was spreading across the globe. He found that messier sediments held the remains of more kinds of advanced animals than the contemporary areas where microbial mats managed to hang on. The discovery suggested to Bottjer that churned up conditions created environmental pressure that led to the evolution of animals with noticeably different, even modern, lifestyles.

"It's kind of a feedback thing," said Bottjer. "The change is creating change in other organisms." The new churning animals were themselves part of the Cambrian explosion of new kinds of beasts, but they also probably accelerated the burgeoning creation of new life forms by changing the environment and forcing earlier animals to adapt to harsher, backwater environments where the sodbusters couldn't survive, asserts Bottjer. The churning up of the sea floor also forced filter-feeding animals to evolve ways to attach themselves to rocks and shells so they could stay above the mess.

By later in the Cambrian the seafloor had became very unstable, says Bottjer. Earlier animal forms had to move to rocks or shells of other organisms to stay alive. "They had adaptations that we see in the sea every day today." It was, in effect, the invention of the barnacle lifestyle, although barnacles as we know them today evolved much later.

In contrast, during the earlier Precambrian animals had no need to adapt to muck. In those days the microbial mats ruled the world. So some of the earliest multicellular animals were probably "mat scratchers" that searched the pudding-like microbial surface for food. Others were "mat stickers" that stuck into mats and underlying sediments, like a fencepost, Bottjer explains. All that ended when worms, mollusks and trilobites came along and started tearing things up.

During the GSA Annual Meeting, Oct. 27-30, contact Christa Stratton at the GSA Newsroom in the Colorado Convention Center, Denver, Colorado, for assistance and to arrange for interviews: 303-228-8565.

The abstract for this presentation is available at:

Post-meeting contact information:

David J. Bottjer
Dept. of Earth Sciences
University of Southern California
Los Angeles, CA 90089-0740

Ann Cairns
Director of Communications
Geological Society of America

Geological Society of America

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