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Small Discovery Has Large Implications

August 8, 2005
University Of Southern California
Microscopic fossils found in China emerge as the oldest examples of animals that display bilateral symmetry -- two halves that are mirror images of each other. The find by a USC paleontologist and his peers focused on critters that date back millions of years.

Bottjer was among the group that discovered the fossils -- period-sized blobs believed to have skimmed the ocean floor with suction-cup mouths some 580 to 600 million years ago.
Credit: Photo Philip Channing

They were the width of a few hairs pressed together, but themicroscopic fossils discovered in China were enormous in theirimplications.

The fossils turned out to be the oldest examples of a bilaterian --animals that display bilateral symmetry, meaning their right and lefthalves are mirror images. The remarkable 2004 discovery pushed back thegenesis of complex animal life by as many as 50 million years.

USC College paleontologist David J. Bottjer was among the group thatdiscovered the fossils -- period-sized blobs believed to have skimmedthe ocean floor with suction-cup mouths some 580 to 600 million yearsago.

In the August edition of Scientific American magazine, Bottjer wroteabout his experience and these minute, yet developed, creatures.Looking like teensy gumdrops or squashed helmets, they contain tissuelayers, a gut, mouth and anus.

In Bottjer's article, which includes color graphics, he describedcollecting a truckload of black rocks in Guizhou Province in 2002 withother researchers, including then-USC graduate student Stephen Q.Dornbos. The group joined forces in their quest for the earliestbilaterians at the urging of Eric Davidson, a molecular biologist atCaltech.

Bottjer, a professor of earth and biological sciences, recalled thecertainty of another participant, Jun-Yuan Chen, a paleontologist atthe Chinese Academy of Sciences in Nanjing. Chen, a pioneer in thestudy of early animal life, was certain that specimens of bilateriananimals would be found in the ancient rock heap. He was right.

But it took incredible patience and work to uncover the fossils,which measure about 200 micrometers across. The team sliced the samplesinto thousands of see-through-thin layers and examined them under amicroscope. Finally, among the 10,000 slides, the collaboratorsdiscovered 10 examples of the fossil type they had been seeking. Aftermore months of painstaking analysis, the group confirmed the exampleswere fossils of miniscule bilaterian animals.

"We were pretty excited when we saw what we had," Bottjer recalled. "It was sort of a 'holy cow!'-like experience."

They named the find Vernanimalcula, meaning small, spring animal.The name refers to the time they lived after glaciers covered theplanet.

The discovery is crucial. It suggests that the earliest ancestors tomodern-day animals developed before the Cambrian explosion. Thatso-called explosion period, 488 to 542 million years ago, envelops thetime on Earth when most animal groups first appeared.

In his article, Bottjer suggests that the famous Cambrian explosionwas more accurately "the exploitation of newly present conditions byanimals that had already evolved the genetic tools to take advantage ofthese novel habitats."

Rather than solely genetics, it may have been the critters' abilityto grow large that led to the explosion. The growth spurt, Bottjersaid, may have been caused by a drastic rise in dissolved oxygen inseawater. More oxygen for breathing reduces size constraints.

Despite the findings, the quest for fossils of early bilaterians has not ended.

"There's got to be older stuff out there," Bottjer said. "We have tohope that we can find even older rocks that contain these tiny things."

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