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Exquisite fossils could be early missing link
13 May 2006
From New Scientist Print Edition.
Jeff Hecht

"Creationists, please note," says Simon Conway Morris of the University of Cambridge. Beautifully preserved fossils of soft-bodied animals found in China show that the origin of phyla, the broadest category in the classification of animal life, is less mysterious than was thought.

Gaps in the fossil record mean that animal groups sometimes seem to pop up without having obvious ancestors. One example is the comb jellies or ctenophores, soft-bodied animals that live throughout the world's oceans. Now, with the discovery of eight fossils, palaeontologists think its ancestors came from the Ediacaran biota, the planet's first large complex creatures.

The animal has been named Stromatoveris (see above left), for "mattress spring" - it lived attached to the seabed by a stalk. The fossils were found in deposits in Chengjiang in Yunnan province, and are about 515 million years old. This means the animal lived in the midst of the Cambrian explosion of evolutionary diversity, yet frond-like structures show it actually evolved from creatures thought to have died out some 542 million years ago, at the end of the Ediacaran period.

The discovery means that there was not an abrupt transition between the Ediacaran and Cambrian periods, and that the mass extinction postulated by some may not have taken place, says Conway Morris, a member of the team that worked on the find, led by Degan Shu of Northwest University in Xi'an, China.

Branches preserved in the fossils suggest that the animals are related to comb jellies. In other words, an evolutionary overhaul of the body plan of something like Stromatoveris gave rise to an entire phylum of animals (Science, vol 312, p 731).

"We can make a powerful case that the remarkable ctenophores, with a very distinctive body plan of eight comb-rows, gelatinous composition and odd body axes, are actually derived from a component of the Ediacaran biota," says Conway Morris. Without the find, he says we would never have guessed that this type of Ediacaran animal was an ancestral ctenophore.

The Ediacaran biota appeared about 580 million years ago, then diversified and spread over shallow sea floors worldwide. Most had bizarre structures and bore little resemblance to later organisms.

From issue 2551 of New Scientist magazine, 13 May 2006, page 13

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