How Snails, Clams Are the Ultimate Survivors

Bottom-dwellers survived because of their specific undersea roles, even when almost every other marine species died out, a new study finds.

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.

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.

"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.

"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.

Gastropods (snails) from the Early Triassic representing the ecospace called "slow-moving, epifaunal grazers." It's just one of the two dozen lifestyles that survived the worst mass extinction in Earth's history.

Oct. 23, 2012 --

For 37 years the Nikon Small World photography competition showcases the beauty and extreme details captured using light microscopes. Every year the world below the waves provides many interesting subjects for the Small World competition. Most examples of aquatic life are entered by professionals in, or students of marine biology, but many of these images were provided by entrants from other disciplines who simply find marine life fascinating. Here, photomicrographer Arlene Wechezak of Anacortes, Wash., and 10th place winner of the 2009 Small World Contest, magnified 10x an Obelia species of hydrozoa with extruded medusae as a fresh sea water mount using a darkfield.

Here we see the same species as before only this time magnified 40x its original size. The extruded medusae of the Obelia hydrozoa contain tentacles that sting and capture the animal's prey.

In this image, biologists magnified 20x a species of bryozoan of the genus Membranipora found on seaweed and kelp using stereomicroscopy.

Almost transparent tissue covers the hard skeleton "cups" of each polyp in this brightfield photo of brain coral in the genus Goniastrea magnified 25x.

Marine biologist Alvaro Migotto of the University of São Paulo in Brazil used stereomicroscopy and a darkfield to capture this photo of a brittle star magnified 8x.

Photographer David Maitland of Feltwell, UK, zoomed in with 100x magnification on coral sand over a brightfield.

Surface of shark skin tanned with a chromium compound magnified 40x with reflected light.

ANALYSIS: US Shark Fin Soup Appalling and Widespread

On its side this marine ciliate (Rhabdonella spiralis)looks like a trumpet; when the image is turned vertical it looks like a champagne flute - a fine photo from French oceanographer John Dolan, who used a differential interference contrast technique and 40x magnification.

A freshwater Bryozoan, or moss animal, Cristatella mucedo in a darkfield magnified 6.5x.

BIG PIC: Rock Vomit: A Sea Squirt Infestation

Biologist Diana Lipscomb of George Washington University in Washington, used Nomarski Interference Contrast and a magnification of 400x the original size to showcase a suctorian ciliate, Acineta tuberosa, in the phylum Ciliophora.

Using Nomarski Interference Contrast and a magnification of 400x the original size this photo shows a ciliate in the genus Sonderia that preys upon various algae, diatoms, and cyanobacteria.

PHOTOS: Evolution Before Your Eyes

Algae magnified 400x using a confocal technique. (Algae genus: Staurastrum, Kirchneriella, Akistrodesmus and Microthamnion)

PHOTOS: Small Worlds Never Before Seen