Why did both the jawed placoderms and the jawless ostracoderms develop such heavily fortified bodies? What was it about those Palaeozoic seas that demanded such formidable protection? The presumed answer is equally formidable predators, and the obvious candidates, apart from other placoderms, are the eurypterids or sea scorpions, some more than two metres in length—the largest arthropods that ever lived.
