Scientists have worked out how one unusual species of trilobite — an ancient, sea-dwelling relative of spiders and lobsters — was able to defend itself against predators and survive a bumpy ride as Earth’s oxygen levels fluctuated.

The seas were filled with trilobites for nearly 300 million years starting in the Cambrian Period, some 520 million years ago. During their time on Earth, which lasted much longer than the dinosaurs, they survived two major episodes of mass extinctions and dominated ocean floor ecosystems.

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