Mysterious shrimp-like creatures from 525 million years ago have been identified as the first to exhibit collective behaviour.

All but one of dozens of the fossilised creatures found by researchers were linked together in chains that had survived even sudden death. The reason for linking in chains is uncertain but researchers believe it is most likely the animals sought safety in numbers while migrating.

Other animals, including trilobites, are known from their distribution patterns to have migrated during the Early Cambrian period but the chain fossils are the earliest to preserve a migration taking place.

The closely interlocked chains were formed when one of the animals inserted its tail beneath the shell at the front of another. 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.

Researchers said that the migration they had embarked on could have been to reach a neighbouring area much as modern animals seek out winter feeding ground. Equally, it could have been a vertical migration, perhaps at night when the creatures may have travelled to the surface to feed in comparative safety.

Researchers found 22 chains comprising up to 20 individual animals in the Chengjiang Lagerstätte in Yunnan, China, which is one of the world’s richest sources of Cambrian fossils.

How, or even if, they swam when part of a chain has mystified the Anglo-Chinese research team because none of the limbs or antenna, assuming the creatures had them, have survived the fossilisation process.

They might have simply drifted in the current or perhaps had a pulsating movement, similar to modern day salps, which are also found in chains. Salps are soft-bodied animals similar to jellyfish, but during an asexual reproduction stage they form long chains.

Professor Derek Siveter, of the University of Oxford, said: “It’s still a bit of a mystery and there doesn’t seem to be a direct comparison with any living animal.” The research, published in the journal Science, was carried out by scientists from Oxford, Leicester and Yunnan.

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