[Photo] The fossil claw of the largest arthropod found yet, held by the scientist who discovered it, Markus Poschmann. Credit: Markus Poschmann

The giant fossil claw of the largest sea scorpion found yet has just been uncovered.

The 18-inch (46-centimeter) claw likely belonged to an 8-foot (2.46-meter) sea scorpion. Sea scorpions are thought to be the extinct aquatic ancestors of scorpions and possibly all arachnids.

"This is an amazing discovery," said researcher Simon Braddy at the University of Bristol in England. "We have known for some time that the fossil record yields monster millipedes, super-sized scorpions, colossal cockroaches and jumbo dragonflies, but we never realized, until now, just how big some of these ancient creepy-crawlies were."

This is the largest arthropod known yet, beating the last largest by 14 inches (36 centimeters). Arthropods include spiders, insects, crabs and others with a hard exoskeleton and jointed limbs.

The extinct sea scorpion (Jaekelopterus rhenaniae) was discovered in a quarry near Prum in Germany by researcher Markus Poschmann at the German Department for the Protection of Cultural Monuments of the Rhineland-Palatinate. The thick greenish-gray siltstone the claw was found in suggests the creature lived in a brackish lagoon or a flood plain lake.

"Most of the remaining fossils from this site are of very small animals???less than 10 centimeters (4 inches)???so I'm a bit puzzled by what this particular sea scorpion specimen would be eating," researcher Erik Tetlie, a Yale paleontologist, told LiveScience.

The fossil is roughly 400 million years old, dating back to the Early Devonian. Gargantuan arthropods were widespread in the distant past. The most common explanation for this gigantism among arthropods then was the higher level of oxygen in the atmosphere back then.

"There is no simple single explanation," Braddy explained. "It is more likely that some ancient arthropods were big because there was little competition from the vertebrates, as we see today."

This fossil dates roughly from the time when vertebrates first got their jaws, a prerequisite for becoming successful predators.

"We already knew that some arthropods and cephalopods???squids and their chums???had jaw-like parts and were predators long before the vertebrates became successful predators," Tetlie said. "The most exciting thing about the find is really that it shows that the arthropods were not giving over their role as top-level predators without a fight."

Still, the exoskeletons that arthropods are known for probably helped bring about their downfall, Tetlie added.

"The internal skeleton of vertebrates grows with the animal, but a hard outer skeleton like in the arthropods have to be shed for the animal to grow," he explained. "The process of shedding this skeleton, and breathing effectively, are probably two reasons why arthropods could not fully compete with the vertebrates."

Even bigger creepy-crawlies may eventually be unearthed. "I'm sure we have not yet found the largest sea scorpions," Tetlie said.

Braddy, Poschmann and Tetlie detailed their findings online Nov. 21 in the journal Biology Letters.