Planet Earth

#71: First Ground Animals Borrowed Shells

In the harsh dry air, the hermit crab-like animals needed shields to keep their gills warm.

By Jeremy LabrecqueDec 24, 2009 12:00 AM

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Some of the first animals to venture onto land commandeered empty seashells for protection, according to an April report in Geology. Amherst College geologist Whitey Hagadorn came to this realization while studying imprints in a 500-million-year-old sandstone formation in central Wisconsin. These markings resembled other tracks thought to be made by an early arthropod, Protichnites, as it dragged itself across an ancient beach. But the impressions exhibited curious diagonal notches between the leg prints that puzzled Hagadorn. He showed them to Yale geologist Dolf Seilacher, who noted that the pattern resembled the tracks created by modern hermit crabs as they drag their shells.

Hermit crabs carry shells on their backs for protection and to store water. Protichnites probably used shells to keep their abdominal gills moist, Hagadorn speculates. That would have allowed the animals to breathe and forage on land for longer periods. “Shells increased the range of conditions they could withstand,” he says. Hagadorn is following up on a related development, the discovery of a fossil of the lobsterlike creature that may have created the tracks.

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