Super-Sized Scorpion

The reconstructed fossil claw of ancient sea scorpion, Jaekelopterus rhenaniae (e) and its size relative to a human male and to the sea scorpion (a), the trilobite Isotelus rex (b), the dragonfly Meganeura monyi (c), and the millipede Arthropleura armata. Braddy, et al; Royal Society Biol. Lett.

Thursday, November 29th, 2007--

Think ordinary scorpions are scary? Imagine one larger than you, that swims. 400 million years ago, these aquatic arthropods, which include spiders, crabs, insects--anything with a segmented body, jointed limbs and a hard exoskeleton, roamed the sea. And a recently-discovered fossil claw indicates that ancient sea scorpions (Jaekelopterus rhenaniae) were even larger than previously thought, researchers recently reported in the Royal Society Biology Letters.

Markus Poschmann, of the University of Bristol, dug up the scorpion claw (called a chelicera) in a quarry near Prum, Germany. The claw alone was 18 inches. Based on the proportions of other fossilized sea scorpions, the sea scorpion to which the claw belonged was likely over eight-feet-long (not including claws), making it the biggest arthropod on record.

Sea scorpions are composed of a head, tail, twelve body segments, six legs and two claws.

"The massive claws must have been used for dismantling other animals," says Erik Tetlie, a post doctoral associate at Yale University and an author on the study. Despite the big claws, Tetlie says that other fossils indicate sea scorpion legs "are very, very puny. We don't think the legs could actually carry the heavy body." The twiggy limbs suggest the scorpion was designed for swimming: "It kind of looks like a flattened submarine, with claws in front and a rudder in the back," says Tetlie.

It's not clear why sea scorpions grew so big, but the fact that they did may complicate one theory explaining arthropod gigantism. The other super-sized arthropods were land-dwellers--like the ancestors of millipedes that were over eight feet long and dragonfly relations with wingspans of two and half feet. Their size has been linked to the elevated oxygen in the atmosphere during the Carboniferous period. Researchers say the limiting factor for the size of some arthropods may be the size of their tracheal tubes. With more oxygen in the air, the tracheal tubes could be thinner, allowing the bug to grow bigger. (Read more: Science Friday, "Beetle Breath".)

The sea scorpion, which lived underwater, got gigantic for some other reason, Tetlie says. Sea scorpions may have evolved to be giant as they competed with other top predators--particularly cephalopods, the ancestors of squid. "It's speculated to be competition, but we don't really know," says Tetlie. The sea scorpions went extinct around 250 million years ago--Tetlie says no one is sure why--but it might be related to the rise of the jawed fishes, he says.

--Flora Lichtman