Swarms of up to a thousand giant trilobites—extinct marine arthropods such as this 35-inch-long (90-centimeter-long) fossil specimen—roamed shallow prehistoric seas, new fossils show.

The 465-million-year-old fossils, found recently in northern Portugal, are of the largest trilobites ever discovered.

The trilobites may have clustered to mate and molt—shedding old exoskeletons as new ones grew in—as well as avoid predators, scientists say.

The benefits of swarming may explain why these distant relatives of horseshoe crabs were among the most widespread arthropods of the Paleozoic era (542 to 251 million years ago).

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