Martin A. Whyte, Nature, 438 (12/01/2005)

Behemoth.
Tracks of Hibberopterus (bottom panel) pulling itself along muddy ground became fossilized in face of a sandstone outcrop (top, with the tracks shown in the middle panel).

Tracks of Man-Sized Crab Discovered

John is a Science contributing correspondent.

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A crablike creature as large as a man appears to have lumbered onto land over a quarter of a billion years ago. Researchers have recently discovered the fossilized tracks of the beast, which was previously thought to have lived exclusively underwater. The findings suggest the animal would have been the biggest creature on land at the time by a long shot.

Crawling out of the water was becoming a popular pastime around 300 million years ago in the Carboniferous period. But the discovery last year of some odd tracks fossilized into a 330 million-year-old sandstone outcrop in Scotland puzzled Martin Whyte, a paleontologist at the University of Sheffield, U.K. For one thing, the markings--including a sizable central groove--indicated the track maker must have had three pairs of legs and a very long tail; no terrestrial creature like that is known from the Carboniferous. And judging by the width of the tracks, Whyte thinks the animal towered over its compatriots: It was about 1 meter wide and 1.6 meters long, while land creatures at that time were smaller than today's house cats. Whatever the creature was, it wasn't speedy: its leg pairs were of different lengths and its strides were relatively short, indicating a very slow crawl.

All these details pointed to Hibberopterus, a gigantic member of an extinct species commonly known as the "water scorpions." But Whyte, who was working alone, knew that water scorpions were considered strictly aquatic, scrounging through the muddy waters with a pair of raking appendages. The paleontologist, who reports his findings 1 December in Nature, thinks the creature's gills, like those of today's crabs, "would probably have operated in air as long as they remained wet." Higher oxygen levels in the atmosphere during the Carboniferous may have enabled it to stay out longer, he says. Hibberopterus's large size may have also helped prevent it from drying out during land visits.

It's "a surprise that an aquatic animal of this size came out onto land," says Mike Romano, a paleontologist at the University of Sheffield, U.K., considering how difficult it must have been to support its full weight out of the water. But the bigger question, he says, is why it bothered. Perhaps it was trying to escape from overcrowded pools or looking for a mate, says Romano. "Sadly, we can only geo-fantasize."

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