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Fossils push back origin of land animals.

Fossils Push Back Origin of Land Animals

Paleontologists have discovered fossilized fragments of the oldest known land-adapted creatures: centipedes and tiny, spider-like arachnids dating back about 414 million years. The finds are forcing scientists to revise their thoughts about animal colonization of the continents, one of the most important steps in evolutionary history.

The fossils come from the city of Ludlow in Shropshire, England, where they were embedded within rocks from Earth's Silurian period, report Andrew J. Jeram of the Ulster Museum in Belfast, Paul A. Selden of the University of Manchester and Dianne Edwards of the University of Wales, who announced their discovery in the Nov. 3 SCIENCE. Prior to the Ludlow finds, the oldest known land animals dated to the early Devonian period, about 398 million years ago.

"This represents a very substantial step back in time," says William A. Shear, who studies the early evolution of land animals at Hampden-Sydney (Va.) College. "What this tells us is that we can look much farther back in the fossil record and expect to find more communities like this. We'll probably have to look much, much farther back to find the actual transitional forms [from which these land creatures evolved]."

Jeram and his colleagues say the age of the fossils suggests that the earliest land animals -- ancient arthropods -- emerged from the ocean soon after plants began spreading over the continents. Until now, paleontologists conjectured that animals lagged far behind plants in their adaptation to terrestrial life.

The researchers uncovered the fossils by dissolving Silurian rocks in hydrofluoric acid, which leaves behind exoskeleton fragments. Then they examined the animal parts under a microscope, attempting to decipher how the fragments fit together.

"It's a bit like doing a jigsaw puzzle -- or maybe a dozen jigsaw puzzles that have been thrown together -- without knowing what the picture looks like," Selden told SCIENCE NEWS.

The acid treatment unveiled pieces of legs, back plates and trunks from centipedes of unknown size and the body of a spider-like animal called a trigonotarbid arachnid. The trigonotarbid fossil measured 1.3 millimeters long, suggesting an animal about the size of a common flea. The most complex land plants of that era grew only a few millimeters tall and would have looked like an outdoor carpet covering the landscape, Shear says.

Because both trigonotarbids and centipedes were predatory animals, the researchers reason that early terrestrial communities must have included other arthropods that served as prey. The Ludlow remains did not offer clear evidence of such creatures, but Selden suggests the prey animals were small arthropods that munched on tiny, easily digestible bits of decayed plant material. This contrasts with the modern world, where animals at the lower end of the food chain subsist on live vegetation.
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Author:Monastersky, Richard
Publication:Science News
Date:Nov 10, 1990
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