How Snails, Clams Are the Ultimate Survivors

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The most catastrophic mass extinction in the history of Earth had little effect on the lifestyles of the lowliest of beasts living in the “benthic” (bottom-dwelling) muck on the sea floor, according to a new study. This could explain the puzzling fact that even after the worst extinction event of all time, very few new groups of benthic animals -- like snails and clams -- sprung from the wreckage.

Despite the fact that 96 percent of marine species perished in the end-Permian extinctions, some 252 million years ago, only one of the 25 benthic lifestyles that then existed actually disappeared when life struggled on into what's called the Triassic period. That made it harder for new groups of benthic animals to evolve in the aftermath, said William Foster and Richard Twitchett of Plymouth University in the U.K. Their study appears in the Feb. 23 issue of the journal Nature Geoscience.

We examine the dire state of perhaps the most interesting and diverse part of our planet.
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Put in theatrical terms, it's as if the roles in a play survived despite many changes of actors, making it very hard for the overall plot to change much.

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“Globally, the Early Triassic benthic ecosystem functioned much like a ship manned by a skeleton crew. Each post was occupied, but by only a few individual taxa (groups),” Foster said.

That skeleton crew appeared to be enough, however, to keep intact the lifestyles that have defined benthic animals ever since.

To come to that conclusion, Foster and Twichett had the daunting task of sorting through the worldwide benthic fossil record from that pivotal time in Earth's history.

Previous studies have looked at the groups that went extinct, of course, as well as how new lifestyles “went viral” and spread through the world, but this is the first to look at the greatest extinction solely terms of animal roles -- or the “ecospace” -- that animals filled.

“We sort of ignored (the animals') names for this study,” explained Foster. One advantage of this approach, he said, is that when the fossil record lacks evidence of a particular animal for a period of time, you can sometimes follow the lifestyle to fill in the blanks.

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“The ecospace occupied by a marine organism is characterized by three ecological variables: mobility, feeding mechanism and living location,” explained Martin Aberhan of the Museum für Naturkunde, Leibniz Institute for Research on Evolution and Biodiversity in Germany.

None of this means to suggest that the extinction event, which was accompanied by a severe bout of global warming, went entirely unnoticed by sea-floor inhabitants.

“The number of lifestyles didn't change, but that doesn't mean nothing happened,” Foster told Discovery News. In fact there was a turnover in the signature bottom dwellers.

In the late Permian the seas were dominated by crinoids and bachiopods. These signature species, which can be found fossilized around the world, were utterly wiped out. That set the stage for the early ancestors of today's burrowing clams and snails to take over.