The eurypterids or sea scorpions were the invertebrate kings of the seas 400 million years ago. For a while, before the armored fish known as placoderms got to be the size of small buses, they were unchallenged. The largest species ever had spiked claws a half-meter long and could slice and dice anything it was likely to meet. (This was also before the nautilus-like ammonites started putting on weight - the monster Parapuzosia seppenradensis, 2 meters across, didn't show up until the Cretaceous and was not seen again until 1957, when something that might have been its much-evolved descendant starred in the delightful grade-B creature film The Monster That Challenged the World.)
Anyway, we have a new entry in the "scariest giant invertebrate with killer claws" category. The whole family tree of its larger group, the carcinosomatoids, is accordingly screwed up. That, however, is how science works. For palentologists, the cool things is that they now know that looking in older rocks for sea scorpions is well worth their time. There are discoveries to be made!
Yale University: Used under educational/journalistic "Fair Use)