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New arthropod species really stuck together

Fossilized chain formations reveal community behavior

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12:52pm, October 9, 2008
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A new fossil find reveals that in an ancient arthropod species, no animal was an island.

The discovery of 525-million-year-old fossils belonging to a new species of arthropod shows that these animals formed communal chains never before seen in fossilized invertebrates, scientists report in the Oct. 10 Science.

These arthropods, a phylum that includes lobsters, beetles and tarantulas, were found in sturdy chain configurations in southern China’s Chengjiang Lagerstätte, a treasure trove of fossils often compared to Canada’s Burgess Shale. In the chain formations, each animal faced the same way, with its tail latched onto the carapace of the individual behind. This unique fossil configuration provides evidence that these arthropods did not act as solitary beings, the authors report.

Nigel Hughes, a paleobiologist from the University of California, Riverside comments that these types of finds provide snapshot scenes of "normal" life.

“Of the millions of fossils, the chances of getting an occurrence where we can determine collective behavior is quite rare,” says coauthor Derek Siveter of the University of Oxford in England. He and his colleagues found 22 complete or partial chains, but only one solitary specimen.

“As far as I know, the group behavior exhibited by these fossils is nearly unknown in modern invertebrates, and has never been demonstrated in fossil invertebrates,” says geobiologist Anthony Martin of EmoryUniversity in Atlanta.

Modern-day animals are known to display collective behavior. Siveter points to spiny lobsters in the Caribbean that form trains in which the antennae of one lobster grasp the carapace of the next individual in line. This formation is thought to aid in feeding, molting or migration following a storm. But unlike those lobster trains, these fossil arthropod chains, dated to the early Cambrian, appear to have been almost unbreakable. The animals collectively show signs of twisting, turning, bending and telescoping, all without coming apart. 

Based on the twists and turns of these fossilized chains, Siveter and his colleagues postulate that the chains may have floated in the early Cambrian ocean.

But the researchers still don’t know why these arthropods linked themselves together. Feeding behavior is an unlikely reason since each individual’s mouth is covered by the tail of the preceding arthropod. And it is also unlikely that being in a chain made it easier to reproduce. Because other types of fossilized arthropods were known to migrate, the scientists’ best guess is that these chains may have provided members protection against predators while on a long migration — in other words, strength in numbers.

“When you’re dealing with 525-million-year–old animals, it’s not like math where five plus five is ten. There are a lot of interesting discussions to have,” explains Siveter. 

The purpose of these chains is something scientists will puzzle over for a long time, but one thing is clear: These ancient animals did not act alone.

“This find provides extraordinary insights into the early evolution of group behaviors, possibly representing a combination of reproductive and anti-predatory strategies,” says Martin. “In short, this find should cause all who are interested in fossil behavior to reconsider what we know about group behavior.”

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