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Trilobites: not forced off the block.

A scuba diver exploring Earth's oceans during the late Cambrian period, 520 million years ago, would have found the waters filled with trilobites -- small, oval arthropods that are a favorite of fossil collectors. Eventually though, the trilobites faded in importance, while mollusks and other creatures spread throughout the oceans. On first look, it appears those new arrivals outcompeted the trilobites, but preliminary evidence from four sites in North America suggests trilobites were not pushed out by their new neighbors.

Stephen R. Westrop of Brock University in St. Catharines, Ontario, and his colleagues investigated fossil sites in New-foundland, Wisconsin, New York state, and Canada's Northwest Territories. The rocks at these sites formed in the shallow waters near the shores of ancient seas, providing a look at trilobite history in the nearshore environment from the late Cambrian through the mid-Ordovician periods (about 460 million years ago). As expected, the researchers found trilobites declining in relative importance. Although trilobite species accounted for about two-thirds of the total species at the beginning of this time period, that percentage dropped to one-third by mid-Ordovician times. But the number of trilobite species in the nearshore did not decline over that period. Rather, the number remained constant, whereas the number of other types of organisms grew. Instead of being driven out by competition, trilobites were passive bystanders, says Westrop. Much later, the trilobites left the nearshore for the offshore, apparently because of environmental changes, he suggests.
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Title Annotation:passive pre-Cambrian fossils disappeared because of environmental changes
Author:Monastersky, Richard
Publication:Science News
Article Type:Brief Article
Date:Jul 11, 1992
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