Fossils tell of mass exodus from sea
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.
Palaeontologists 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 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."
Despite the lack of predators, living on land was a dangerous proposition for creatures used to life at sea.
Suddenly they were faced with intense radiation from the sun, an atmosphere that would dry them out in a matter of hours, and no support from buoyancy.
Modern-day arthropods include hermit crabs, horseshoe crabs and insects - all characterised by a hard, protective exoskeleton.
Their Cambrian cousins also had hard exteriors, making them well-suited to the rigors of life out of water.
"But how did molluscs deal with this? They weren't carrying around a big shell," which made them vulnerable to drying out, says Hagador.
"Were they nocturnal? We see evidence of them burrowing beneath the surface, perhaps to avoid desiccation from the heat."
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."