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About 453 million years ago, a trilobite might have escaped the claws of a hungry giant sea scorpion. That is according to palaeontologists who analysed an unusual fossil found in the Czech Republic.

Trilobites flourished in the oceans from around 522 million to 252 million years ago before going extinct. These hard-bodied arthropods resembled woodlice and ranged between about 1 millimetre and 70 centimetres in length. About 20,000 trilobite species are known from the fossil record.

Trilobite fossils with evidence of head injuries are rare, suggesting that most attacks to the head were fatal and that any trilobite attacked this way was then eaten by its attacker. But at Charles University in the Czech Republic and his colleagues studied an unusual eye trauma in the fossilised head of a common trilobite species called Dalmanitina socialis and believe it survived a predator鈥檚 attack.

Casts of the fossil鈥檚 inner and outer surfaces enabled the researchers to see that part of the eye was missing. They think either all or part of the eye was originally gouged away but repeated natural moulting of the animal鈥檚 exoskeleton helped the wound heal and allowed a much smaller eye to grow back. Other signs of injury and healing included scratches, a crescent-shaped scar and misshapen cheeks.

The researchers say the damage isn鈥檛 explained by some problem with the moulting process itself, which is often a cause of abnormalities seen in trilobite fossils. They also ruled out genetic mutation because of signs of healing.

The most likely explanation, the researchers think, is that the trilobite was attacked by a predator. In theory, that predator might have been a larger trilobite, an octopus-like cephalopod or a giant eurypterid 鈥渟ea scorpion鈥� 鈥� the fossil record shows that all were living in the region at the same time as the small trilobite.

However, the team eliminated the first two candidates because their method of inflicting damage didn鈥檛 tally with the trilobite鈥檚 injuries. That left just one contender. Giant sea scorpions were the largest arthropods known to have ever lived, with some species reaching 2.5 metres in length. They had arm-like appendages with toothed-claws for grabbing prey.

鈥淎 failed predatory attack with the trilobite surviving to regenerate the damaged eye and surrounding cheeks really is the most likely cause,鈥� says at the Natural History Museum in London. 鈥淚t must have been this guy鈥檚 lucky day.鈥�

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