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5-Foot Giant Water Scorpion Once Roamed U.K. Shores

John Roach
for National Geographic News
November 30, 2005
 
If you think scorpions are scary, try this on for size: a six-legged water scorpion the size of a human. Newly discovered tracks reveal that about 330 million years ago, just such a creature lumbered along the riverbanks in present-day Scotland.

The fossilized track is the largest of its kind ever found and shows these now extinct creatures could walk on land, according to Martin Whyte, a geologist at the University of Sheffield in England.

"There's been lot of debate about this particular [species of] water scorpion—whether it could only live in water or if it could come out. … What the track shows is they could come out at least for short intervals," he said.

The geologist describes the find in tomorrow's issue of the science journal Nature.

Blair Hedges, an evolutionary biologist at Pennsylvania State University in University Park, agreed that the tracks looks like they were made on solid ground.

"That tell us that about this time our ancestors, the first vertebrates with four limbs instead of fins, were confronting very large arthropods on land," he said. "That's the neat thing it brings up."

Slow Crawl

The footprints were made by a species of Hibbertopterus, a family of water scorpions that are among the largest arthropods—a group that includes insects and crustaceans—ever known.

Whyte estimates the individual who made the track was 5.2 feet (1.6 meters) long and 3.2 feet (1 meter) wide.

The track itself is 20 feet (6 meters) long and about 3 feet (1 meter) wide.

"It's huge, absolutely giant," Hedges said. "To consider an arthropod made the thing, that's really impressive. A lot of people don't realize the arthropods of that time period, including dragonflies, millipedes, and centipedes, were much larger than today."

Whyte's analysis of the tracks suggests that the ancient water scorpion took its sweet time as it lumbered along the shoreline.

He said the creature walked in-phase, with each pair of limbs moving forward at the same time rather than alternating, like a human gait. Also, the scorpion's stride averaged 10.6 inches (27 centimeters) long, short enough that they overlapped.

The track also features a central groove left by the water scorpion's dragging tail, leaving indications of jerky movements.

"The whole thing adds up to fairly look as though the body [was] heavy and the animal was moving quite slowly," Whyte said. "For that reason I think it was out of the water. Had it been in [the water], water would've buoyed up the tail."

Simon Braddy, an earth scientist at the University of Bristol in England, specializes in ancient water scorpion tracks. He said the evidence is "unclear" that these newly discovered tracks were made on land.

"Sometimes you find mud cracks on a surface that are a sure sign, but these are lacking in this case," he said in an e-mail interview.

Hedges, the Pennsylvania State University biologist, disagrees, saying the analysis "makes sense."

"I don't know for absolute certain, but it made a track, so it was walking on a mucky surface. Maybe it was right at the water's edge, which is where the first tetrapods would have occurred as well," he said.

Tetrapods are four-limbed creatures with backbones, a group that includes humans. The earliest ones looked like giant salamanders, measuring about four feet (1.2 meters) long, Hedges said.

Landward Ho

According to Hedges, by about 360 million years ago the transition of lobe-finned fish—prehistoric fish with fleshy fins—to four-limbed tetrapods was nearly complete.

The newly discovered trackway, which is dated to 330 million years ago, therefore suggests that some of the earliest tetrapods would have confronted these giant water scorpions as they scurried about the shore.

"All kinds of things are brought out by discoveries like these," Hedges said. "For example, what kind of interactions would have occurred between these giant water scorpions and the early tetrapods?"

According to Whyte, even though water scorpions shared the same environment with tetrapods, the six-legged giants likely left our ancestors alone.

"They didn't have large pincers or aggressive weaponry that would have made them a danger to the early tetrapods," he said. "If anything the tetrapods were more a danger to them than the reverse."

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