April 8, 2009 -- Long before there was life on land, hermits from the sea crawled ashore toting their homes on their backs, according to a new study.
Half a billion years ago Earth's warm oceans were brimming with life, but the continents were bare. It would still be a hundred million years before the first terrestrial creatures lived.
Now fossil tracks discovered on an ancient Cambrian-period beachhead suggest an intrepid group of aquatic scorpion-like creatures commandeered empty mollusk shells, much like modern day hermit crabs.
Researchers think they used the shells as protection against the harsh dry air, and stole ashore under cover of darkness to graze on mats of algae exposed during low tide.
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James 'Whitey' Hagadorn of Amherst College noticed the unusual tracks while studying the Elk Mound sandstone in Wisconsin. A trace fossil reveals much about the animal that left them, like how many legs it had, and whether it had a tail. But these trails had something unusual -- an impression that looked like a tail, but always pointed to the left.
"You know from dinosaur tracks that if the animal turns left, its tail swings right," Hagadorn said. "With these tracks, the animals are turning all over the place, almost doing figure eights in the sand. But their 'tail' impression is always to the left."
He and Adolf Seilacher of Yale University reason that the tracks -- dubbed Protichnites eremita -- were left by an arthropod, a distant cousin of the horseshoe crab and scorpion, that scurried along using between 6 and 13 pairs of legs. They estimate the animal measured close to 20 centimeters (7.9 inches) long.
If the ancient arthropod did use a shell, it differed from today's hermit crabs in one major way -- it was too small for the animal to retreat fully inside. Instead, the shell served to protect the creature's gills, keeping them moist while it spent hours scavenging among the tidal flats.
"These are very unusual trackways," Mark Erickson of St. Lawrence University said. "But I don't think the shell that could've made this mark had evolved yet."
To get a left-leaning impression in the sand, the shell must've been asymmetrical, like a modern-day conch shell. But twisted shell fossils don't appear until the early Ordovician, tens of millions of years after the arthropod traipsed through the sand.
Hagadorn admits the notion of innovative hermit arthropods crawling in the open 500 million years ago is speculative. And all he and Seilacher have to go on right now are a series of tracks -- no body fossils have been discovered.
"I am confident the animal that made these tracks was an arthropod. I am not at all confident it was a hermit arthropod," Hagadorn said. "It's the best idea we've got, though. It's kind of like solving a crime without the murder weapon or DNA evidence."
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