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Fossils tell of mass exodus from sea


The researchers found fossil evidence that slug-like molluscs had slithered through the tidal flats towards land (Source: iStockphoto)





But how and when they migrated from the ocean to the dunes is still a mystery.

"They'd first have to cross an intertidal zone - a tidal flat," says Assistant Professor James 'Whitey' Hagadorn of Amherst College in Massachusetts. "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, fossilised mud cracks and impressions from rain drops proved the mud was dry as they scampered past.


Surprisingly, they also found evidence that slug- or snail-like molluscs 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 molluscs 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."



Modern-day arthropods include hermit crabs, horseshoe crabs and insects - all characterised by a hard, protective exoskeleton.


"But how did molluscs deal with this? They weren't carrying around a big shell," which made them vulnerable to drying out, says Hagador.


Hagadorn stresses that his team's interpretation of the fossils - especially the presence of annelids - are far from certain; fossils yet to be discovered could change the picture.

"You're just a detective in deep time, putting pieces of the puzzle together," he says, "Until someone comes and strikes it down. When it stands for a long time it becomes a theory, but I don't think we're anywhere near that yet."

"This is very exciting - it implies that the diversity of life on land at the time was higher than we thought," says Dr Simon Braddy of the University of Bristol.

"It's all filling in the picture of when life on land first occurred -- a breathtaking moment in the evolution of life on earth."

Tags: oceans-and-reefs, evolution, palaeontology, invertebrates