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When giant scorpions swarmed the seas

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Sea scorpions thrived for 200 million years, coming in a wide variety of shapes and sizes. Over time, they developed a number of adaptations— from crushing claws to flattened tails for swimming. And some of them adapted by getting so big that they still hold the record as the largest arthropods of all time.

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Sea scorpions aren’t the only arthropods that swarmed the seas. It turns out evolution loves arthropods, making them the most diverse multicellular animal phylum today. The first arthropods appeared during the Cambrian Explosion because minerals released by rock weathering allowed complete encasing in armour called an exoskeleton, a characteristic feature of arthropods. The most notable members of this radiation are the popular trilobites which survived two mass extinction events and only disappeared in the Great Dying at the end of the Permian.

The anomalocarids were not so lucky and only made it to the Devonian period. But they resembled the first ferocious arthropod predators that could live up to the younger sea scorpions. Finally, in the Devonian period, crabs made a breakthrough and never looked back. Crab morphology is so successful that it arose independently several times, a special case of convergent evolution called carcinization. Evolution threw barnacles into the mix in the Carboniferous, weird arthropods that stumped scientists for a long time. Importantly, when crustaceans made their way onto land they went through the largest radiation in Earth history which resulted in insects.

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