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Modern examples of ancient life.

Modern examples of ancient life

At the Smithsonian Institution's National Museum of Natural History in Washington, D.C., there is a beautiful mural of the early earth (SN: 7/12/86, p.23). The painting shows calm waves lapping a beach dotted with stromatolite -- columnar structures built layer by layer by microorganisms as they catch sediments with their sticky surfaces. This portrayal of the environment in which stromatolitic communities flourished billions of years ago is a classic one. It is based on one of the few known modern stromatolites, which reside in the relatively calm, highly saline waters of Shark Bay in Australia.

But according to Robert Dill, a geological consultant in San Diego, Eugene Shinn at the U.S. Geological Survey in Miami Beach and their co-workers, scientists should consider at least one other kind of modern environment as an analog for where ancient stromatolites lived. Dill's group recently discovered 2-meter-high stromatolites in channels between the Exuma Islands in the Bahamas. Unlike those at Shark Bay, these stromatolites are completely submerged and are subjected to very strong tidal currents of up to 3 knots. The currents are so powerful that they cause sand dunes to completely cover the stromatolites every two to three months. Dill thinks the high currents around the Exuma Islands, like the high salinity at Shark Bay, keep algae-eating creatures away from the stromatolites.

Because Shark Bay is an intertidal environment, with the stromatolites being washed by the tides, most scientists had assumed that ancient stromatolites were also intertidal, even though at many sites there is no geologic evidence for intertidal conditions in the fossil record, says Dill. The Exuma stromatolites "open up a completely new interpretation for the environments of ancient stromatolites," he says. Smaller stromatolites had been found before in the Bahamas, says Dill, but no one had recognized their significance. A paper describing the Exuma find appears in the NOV. 6 NATURE.
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Title Annotation:stromatolites in Shark Bay, Bahamas
Author:Weisburd, Stefi
Publication:Science News
Date:Nov 29, 1986
Words:319
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