Mass extinctions are devastating events. The largest, called the Great Dying, brought the Permian era to an end. In the process, it wiped out over 80 percent of the genera on the planet, and over 95 percent of those in the oceans. The event itself has been linked to a massive volcanic eruption, big enough to cause global ecological disruptions. But what actually kills the organisms that go extinct? A new study uses a database of over 50,000 fossils of marine organisms to answer that question, and comes up with a very simple answer: basic physiology.

The authors look at a collection of factors that could influence a species' survival, such as its geographic range and the habitats it occupied. But they also include information about its basic physiology. Some organisms, like sponges, allow their environment to strongly influence their chemistry—if the environment is acidic, for example, their cells will be, too. Others, like many mollusks, are more buffered from environmental changes; they actively control how their cells respond to different environments, and which chemicals get let inside the cell.

The authors plugged all that information into their database, and then ran a regression analysis, which examined how each individual trait affected the organism's survival while controlling for the impact of the rest of the factors. They tracked survival across six mass extinctions, culminating with the Great Dying at the end-Permian.

Most of the factors had no consistent impact. But for at least two of the extinction events, including the Great Dying, an organism's ability to keep itself separated from its environment had a large influence on its survival. In short, when the environment went bad, organisms that don't buffer themselves from the environment tended to end up dead.

How does this physiological stress actually kill anything? The authors propose that this is an indirect result of the volcanic eruptions that triggered the event: they released massive amounts of CO 2 , which can interfere with respiration and create oxygen-poor dead zones in the oceans. It will also lower the pH (called ocean acidification), which can dissolve carbonate shells and skeletal elements. For the Great Dying, having a carbonate skeleton also turned out to be bad news, which suggests ocean acidification can be a problem.

This isn't the first paper to suggest that mass extinctions aren't quite the random killing events that we often picture them to be, but it is the most comprehensive yet. And, given that ocean acidification and expanding dead zones are occurring in today's environment, the authors suggest that it might provide some relevant information.

Geology, 2011. DOI: 10.1130/G32230.1 (About DOIs).