Rapid evolution in early trilobites fueled by high variation
Trilobites were even more ubiquitous on the Cambrian seafloor than they are now in museum gift shops. Their abundance in the fossil record — partly a function of their hard, easily preserved exoskeletons — bore witness to the Cambrian Explosion, Earth’s most dramatic boom of multi-celled animal life, says pale biologist Mark Webster of the University of Chicago. Now a new study is teasing another great story out of the ancient creatures. Webster compiled morphological data for nearly 1,000 of the 17,000 different species of trilobites, a class of marine arthropods that died out by 250 million years ago, from 49 previously published sources. By tracking different morphological features — the number of body segments, for example —Webster found that trilobite species exhibited more variation during the Cambrian than in later periods, he reported in Science July 27. “Once you go beyond the Cambrian, the diversity of forms within any one species drops off,” he says. This high within-species variation provided more raw material upon which natural selection could operate, Webster says, potentially accounting for the high rates of evolution in Cambrian trilobites. Such findings may have implications for our understanding of the nature of evolutionary processes, he says.