Millions and millions of years ago, the earth was overrun with oversized arthropods, the phylum that includes spiders, scorpions, crabs, centipedes and barnacles. They crawled through the undergrowth, flew through the air and swam in the sea. The biggest of all may have been Jaekelopterus rhenaniae, a fearsome-looking sea scorpion. The 390-million-year-old creature has no modern equivalent; horseshoe crabs are its closest living relatives.The creature had been known to paleontologists for decades, but the description of a huge claw in 2007 catapulted the obscure creature to fame. According to paleontologist Simon Braddy and colleagues, the 18-inch long claw indicates that the aquatic predator may have been more than eight feet long.