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Planet Earth

Soft Silurians

By Rachel PreiserNovember 1, 1996 6:00 AM


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Animals with bones and shells dominate the fossil record. Their hard body parts endure long after traces of softer creatures have vanished. So paleontologists always greet the rare discovery of the fossils of soft- bodied, boneless creatures with excitement, as did Derek Briggs when he got his first look at some fossils that a mineralogist had collected in Herefordshire, England, about three years ago. The fossils had been shelved in the University of Leicester since their discovery, ignored until Briggs, a geologist at the University of Bristol, and his colleagues finally got a chance to study them carefully. The remains turned out to be those of soft- bodied marine animals that lived some 424 to 430 million years ago, during the Silurian Period, and include a host of worms and bizarre arthropods that have never been seen before.

Most known fossils of ancient boneless animals date from the Cambrian Period, which ended about 500 million years ago. Late in the Cambrian, however, burrowing shelled marine creatures evolved, and they often disrupted the bodies of soft creatures buried in sediments before the fossilization process could start. How then did the English fossils escape destruction by sediment-churning Silurians? The fossils were buried in volcanic sediments, and Briggs believes that the volcanic ash that covered, and probably killed, the soft-bodied organisms may have contained chemicals toxic to seafloor scavengers.

The fossils owe their remarkable preservation to a limestone encasement that probably formed around each animal as carbon dioxide produced by natural decay mixed with calcium from the newly deposited ash. The resulting fine-grained calcium carbonate, or limestone, perfectly preserved the animals’ three-dimensional structure. Soft-bodied creatures have never been found preserved in this way before; Briggs and his colleagues believe their find suggests that volcanic sediments elsewhere may hold similar fossils.

Among the oddball animals found are a half-inch-long bristly worm and a tenth-of-an-inch-long shrimplike creature with a tentacled head, segmented body, and triangular tail (two views of that creature are shown here at the upper left and lower right). More enigmatic is the half-inch- long creature in the middle, which may be an unknown relative of a group of stubby-legged worms called lobopods.

Briggs and his colleagues even found a trace of the gut of one of the fossilized worms, which suggests that exquisite internal details of previously unknown animals remain to be found in the fossil trove. That’s an exciting prospect. The Silurian is a bit of a Dark Age in the sense that we don’t know a great deal, particularly about the soft-bodied animals, says Briggs. If we can increase the number of different types of animals that we find, it has the potential of filling in a number of gaps in what we know about the evolutionary history of marine life.

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