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Earliest sign of evolutionary arms race.

When the curtain opened on Earth's Cambrian period, many of the newly evolved animals sported an unusual dress not seen in earlier times. This was when animals first began wearing hard shells, fashioned from calcium carbonate and other minerals in the oceans. Paleontologists have proposed many theories to explain why mineralized exoskeletons came into style so quickly at that time (SN: 8/25/90, p.120). Now, two researchers say they have found hard evidence that addresses the question.

Stefan Bengtson of Uppsala University in Sweden and Yue Zhao of the Chinese Academy of Geological Sciences in Beijing studied a tube-like animal called Cloudina that lived right before the start of the Cambrian period and was the earliest known animal with a mineralized exoskeleton. From the exterior, the shell of Cloudina looks a bit like a stack of icecream cones. Scientists have yet to determine what kind of animal lived inside these tubes.

Of the more than 500 specimens recently found in China's Shaanxi province, 17 are tubes with holes in their sides, Bengtson and Yue report in the July 17 SCIENCE. The two researchers propose that predators bored these holes. Bengtson and Yue also described their work at the Fifth North American Paleontological Convention, held in Chicago in late June.

Some researchers have proposed that exoskeletons may have served as a detoxification mechanism, allowing animals to rid their bodies of potentially lethal calcium that may have accumulated in the seas at that time. Other researchers have suggested that exoskeletons helped animals develop body shapes efficient for moving and feeding. But the borings in Cloudina support the theory that exoskeletons appeared as a shield against the world's first predators, say Bengtson and Yue.

If so, the tactic apparently worked in some instances. Bengtson and Yue found several Cloudina specimens with holes going only partially through their shells, perhaps indicating that predators had abondoned their efforts to find a tasty meal inside.
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Title Annotation:animal appearance changes in Cambrian period
Publication:Science News
Article Type:Brief Article
Date:Aug 15, 1992
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