UWM Paleobiologist Part of Team That Finds Fossils of Earliest Complex Animals

Stephen Dornbos (front) inspects fossils from both the Cambrian and pre-Cambrian with geosciences senior Tristan Kloss. (Photo by Alan Magayne-Roshak)

MILWAUKEE — For paleobiologist Stephen Dornbos and his colleagues, finding fossils of an ancient aquatic animal that was smaller than the size of a pinhead turned out to be something huge.

So big, in fact, that the discovery of Vernanimalcula has been featured in a stream of magazines such as Science (where it was initially described), Scientific American and Discover in the last year, the latter naming the tiny fossil "one of the Top 100 science stories of 2004" in its January 2005 issue.

What's all the excitement about?

Symmetry, says Dornbos, an assistant professor of geosciences, who served on the scientific team that uncovered the fossil in a 600- to 580-million-year-old rock quarry in southern China.

Bilateral symmetry refers to the left-right mirror-image of arms, legs, and organs that is characteristic of all higher animals. It marks the first time in evolutionary history that animals had a three-layered body, well-developed organs, and a form that included a mouth, gut and anus.

Scientists have disagreed on how old bilaterians are, and before this discovery, the oldest possible example of such symmetry dated to only about 555 million years ago, right on the cusp of the Cambrian Period. "To prove that they existed well before the Cambrian, we had to actually find evidence," says Dornbos, who was a Ph.D. student at the University of Southern California during the team’s 2002 excavation. By the time the fossils were found, he had become a post-doctoral researcher there.

Many scientists have long believed that an "explosion" of complex animals occurred at the beginning of the Cambrian Period. The age of Vernanimalcula indicates that complex animals evolved before that – up to 50 million years before the Cambrian.

Dornbos's colleagues included David J. Bottjer of the University of Southern California; Jun-Yuan Chen of the Nanjing Institute of Geology and Palaeontology, China; Eric H. Davidson of the California Institute of Technology; and Chia-Wei Li of the National Tsing Hua University, Taiwan.

One reason that so few fossils of early bilaterians exist is that there aren't many known places on Earth that will enable fossilization of soft-bodied animals, says Dornbos. Not just any rock will do.

"In this case, the right type of rock would have a high phosphate content," he says. "And that would represent the right environmental conditions. Such rocks would have likely formed in areas of productivity in the ocean where the phosphates would have been provided by decomposing organic material buried on the seafloor."

"Deposits of such rock would have to do with how the ancient ocean was moving and that's harder to reconstruct for 600 million years ago." Other processes, unlike anything taking place today, such as certain kinds of microbial activity, may also have aided in the preservation of these fossils, he adds.

The scientific group decided to explore the Doushantuo Formation in Guizhou Province, a Precambrian deposit where Chen and others had already found tiny fossilized eggs and embryos.

The task, funded by NASA, the Chinese Academy of Sciences and the National Natural Science Foundation of China, involved searching for the proverbial needle in a haystack. The rock samples they collected were sliced into cross sections so thin they were translucent. After mounting them on slides, the team found 10 examples of Vernanimalcula out of a pool of more than 20,000 slides.

Their discovery established more than just the minimum age of bilaterians. Since fossils of larger bilaterians appear in the fossil record soon after Vernanimalcula's, it indicates that internal complexity in animals came before increased size.

"It's significant because it probably means that Vernanimalcula's environment constrained its ability to increase in size," says Dornbos. "That gives some clues to the potential cause of its later evolution into larger animals."

The fossils also suggest that the Cambrian "explosion" of higher animals may have been much more gradual than traditionally thought.

For more on Dornbos's research, log on to http://www.uwm.edu/~sdornbos/Pages/Research.html.

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