Fossils Tell of Mass Exodus From Sea to Land

Sept. 24, 2008 -- New fossils of the first land animals reveal that ancient shores were alive with more crawling, slithering creatures than anyone previously thought.

In the late Cambrian era, nearly 500 million years ago, the seas were teeming with life. Food was abundant, but so were predators. Paleontologists believe animals fled the marine environment for the safe confines of tide pools, and ultimately dry land, where they could live without fear of being eaten.

From fossil tracks found in Ontario, Canada, researchers know that a group of insect-like creatures called arthropods were crawling on sand dunes around this time. But how they migrated from the ocean to the dunes -- and when -- is still a mystery.

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"They'd first have to cross an intertidal zone -- a tidal flat," James 'Whitey' Hagadorn of Amherst College in Massachusetts said. "So we went exploring, looking for rocks of similar Cambrian age representing coastal sandy settings."

In the fossil-laden rocks of Wisconsin, New York, and Missouri, they struck it rich -- arthropod tracks, left after the critters crawled through mud flats, were in abundance. Alongside the tracks, fossilized mud cracks and impressions from rain drops proved the mud was dry as they scampered past.

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Surprisingly, they also found evidence that slug- or snail-like mollusks and worm-like annelids had slithered through the tidal flats as well.

"We knew arthropods should be there, but didn't know what else," Hagadorn says. "In a way the mollusks are more interesting because they weren't carrying a big shell around, and they had to deal with all of the problems of being on land."