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"Daisy Chains" of Fossil Creatures Found in China

James Owen
for National Geographic News
October 9, 2008
 
"Daisy chains" of small fossil creatures recently discovered in southwest China reveal an extremely ancient and bizarre type of animal grouping, scientists say.

The shrimplike marine organisms, a previously unknown species that lived around 525 million years ago, were found linked head-to-tail at the fossil-rich Chengjiang site in Yunnan Province.

The fossils show 22 complete or partial chains containing up to 20 animals, a team led by Xianguang Hou of China's Yunnan University reports in this week's issue of the journal Science.

The fossil sample contained just one unattached individual, which was the longest specimen found at just under an inch (2.4 centimeters).

"It's a really fascinating finding—quite extraordinary, and a big puzzle," commented Nicholas Strausfeld, a regents' professor at the University of Arizona.

"The regularity of this row of animals is amazing," added Strausfeld, who wasn't part of the study team.

The extinct species, which is due to be named shortly, was a crustacean ancestor with a head shield—or carapace—and a segmented body, said team member Derek Siveter, professor of earth sciences at the University of Oxford in the U.K.

"If you found one on your plate at the local restaurant, you'd identify it as a shrimplike thing," Siveter said.

The interlinked fossil chains remained intact despite becoming stretched and twisted on the seabed.

"You can see quite clearly where individual animals have been bent at right angles, but the integrity of the chain is maintained," Siveter said.

Open Water

While it's possible the animals lived on the seabed, the study team thinks they swam or floated in the water column.

Feeding in this chained formation probably wasn't an option, since each animal's mouth opening would have been covered by the tail of the one in front, Siveter noted.

"The tail may have been grasped by the head appendages of the animal behind," he added.

Such groupings are otherwise unknown among living and extinct arthropods (the group that includes crustaceans, insects, and spiders), the study team said.

The only obvious living parallels for this kind of setup are salps, jellyfishlike ocean-drifters that are related to sea squirts, the researchers said.

Salps form lines of interlinked colonies as part of their reproductive cycle.

"But we don't find this type of reproductive mode in other arthropods," Siveter said. "So then we come back to a more simple explanation, that these animals congregated in this fashion for migration."

A loose analogy exists in today's spiny lobsters, which are found in tropical regions including the Caribbean, he said.

Though the lobsters don't link up in a chain during migration, "they march roughly in lines on the seabed at certain times of the year," Siveter said.

Migration does seem the most rational explanation for the fossils, agreed Strausfeld, of the University of Arizona.

"That's all you have to go on, quite frankly, based on the crustaceans tramping around on the planet today," he said.

Another possibility, Strausfeld said, is that the animals "may have been preserved before they hatched out of this long egg case."

The strength and precision of the chains, with their repeated head and tail insertions, suggests some kind of packaging was involved, he said.

"The egg case, if it's gelatinous, wouldn't be preserved," he added.

And the bigger, unattached specimen could represent an adult or a hatchling.

"Maybe the sac breaks down when the animal reaches a certain size," Strausfeld said.

Evolutionary Test?

The fossils date to the so-called Cambrian Explosion, a rapid evolutionary flowering when many of the major animal groups appeared.

"At that time in history there were some very strange things, and a lot of experimentation going on," Strausfeld said.

Although he knows no precedent for crustaceanlike animals producing egg cases that house numerous developing embryos, "it's possible [the fossils represent] strange ways of reproduction and development that are no longer with us," he added.

Siveter, of the University of Oxford, said the new discovery "gives us another window on this early proliferation of life, when all the major groups which sustain biodiversity to the present day were introduced to the fossil record."
 

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