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Planet Earth

Ancient Invertebrates May Have Formed Chains for Strength in Numbers

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A new fossil discovery shows that tiny, shrimp-like invertebrates living 525 million years ago linked up into formations that resemble daisy chains, and researchers say this could be the earliest example yet of animals engaging in group behavior. The fossilized creatures were found in closely interlocked chains of up to 20 individuals, with the tail of one animal inserted into the carapace of the next.

The ancient arthropods, a category of animals that includes insects, crustaceans and spiders, lived in open water rather than remaining on the sea bed. When they died, possibly as a result of moving into water loaded with toxins or short of oxygen, they sank to the seabed, where they were covered in sediment [The Times].

Researchers can't be certain why the arthropods joined together into chains, but their best guess is that the animals were in the middle of a migration when they perished. The fossils, described in Science [subscription required], date from the Early Cambrian Period when life on earth was diversifying rapidly; they were discovered in a fossil-rich area in southern China. The discovery took researchers by surprise, as examples of collective behavior are rare in modern-day invertebrates. There are a few examples: study coauthor Derkek 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 [Science News].

Jellyfish-like salps also form chain-like colonies as part of their reproductive cycle. But the newly discovered arthropods (which will be named shortly) weren't likely to get a boost in feeding or reproduction through ganging together, researchers say.

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 [National Geographic News].

And no arthropods are thought to have a reproductive cycle that resembles the salps'. So Siveter turned to another explanation:

"The simplest explanation for this is that it is some kind of collective behaviour coming together for migration, perhaps associated with defence in numbers," he says [New Scientist].

Image: Derek Siviter

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