What has eight legs, 46-inch claws, and the size of a professional basketball player? Jaekelopterus rhenaniae, that’s what, if a recent Bristol University (England) find is to be believed. Called a sea scorpion (although it didn’t live in the sea and is not technically a scorpion) this mammoth monster (2.5 meters/8 feet long) stalked modern Germany about 390 million years ago.

"This is an amazing discovery," said Bristol researcher Simon Braddy. This pre-dinosaur polypheme didn’t leave behind a complete fossil; Braddy and his colleagues found only a fossilized claw. But the claw (at actual size, our graphic wouldn’t fit on your jumbo flatscreen monitor) was enough to extrapolate the full beast.

The better to crush you with. (Credit: Biology Letters)

Past studies had placed prehistoric arthropods at about a foot and a half smaller. In 2005, for example, Blair Hedges, an evolutionary biologist at Pennsylvania State University in University Park, described a recently-discovered 5.2-footer as “huge, absolutely giant.” But faced with this new discovery, Braddy, who talks like a Monster-Watch writer, said, “We have known for some time that the fossil record yields monster millipedes, super-sized scorpions, colossal cockroaches, and jumbo dragonflies but we never realised, until now, just how big some of these ancient creepy-crawlies were.”

Theories on why such size evolved vary. Some scientists believe that higher levels of oxygen in ancient atmospheres allowed the arthropods to grow, grow, grow. (End pollution, create giants?) Oxygen levels, however, would have had a lesser impact on marine creatures such as the sea scorpion. Others theorize that arthropods were in an arms race with their prey: armored fish. Another evolutionary explanation argues that huge arthropods had no competition in the predator department; said Braddy, "The backboned animals hadn't stepped up a gear yet in their evolution, so they weren't any real threat to the arthropods. When they did, that's when the arthropods had to downsize." Giant arthropod fossils seem to disappear quickly at the same time as the rise of jawed fish, so those noble nibblers may have stolen the behemoths’ prey.

So what about that prey? Well, as this elephantine eurypterid trundled about the swamplands of Germany, it devoured not only armored fish but also fellow sea scorpions: "They were cannibals but they also would eat some of the early armored fish that were around at the time," said Braddy. Opined paleontologist Paul Selden of the University of Kansas, "It would pretty much have eaten anything smaller than itself.” With two huge, toothed claws on the ends of long, folding arms, the sea scorpion likely ambushed its prey: "I think [the claws] were designed for shooting out when close to prey, like the arms of a praying mantis," Selden said.

And did that prey include our own ancestors, soft-bodied four-leggers first tottering onto land? Short answer: no. Although previous finds, such as the 2005 fossil, had indicated an ability to walk on land, “there’s no way this monster bug would have been able to do that, because it was just too big,” said Braddy. Smaller sea scorpions likely came ashore to mate or molt, but not this mammoth. “Its legs were relatively flimsy compared to the size of its body,” Braddy explained, and without water buoying it, the beast would have crushed its own little legs. So our forefathers only had to contend with five-footers, not eight-footers. Whew.

To read the original article, check out Braddy, Simon J., Markus Poschmann, and O. Erik Tetlie. "Giant claw reveals the largest ever arthropod." In Biology Letters. Online date: Tuesday, November 20, 2007.

(Illustration: Simon Powell, Bristol University)