Why the Ordovician–Silurian Extinction Event Occurred

A new study reveals some of the most important missing details about how the Ordovician–Silurian extinction event (known simply as the Ordovician extinction) occurred, about 450 million years ago.

One of the most interesting aspects of this event was that it occurred in two stages. This is partially why it was so devastating. The second wave of destruction hit species that had not yet fully recovered from the first wave. It was nature's equivalent of hitting someone when they're down.

The two “extinction bursts” were separated by about one million years, investigators say. Scientists have always suspected that climate change played the most important role in the Ordovician extinction, but exactly how this happened was largely unknown.

Experts do know that an intense, yet brief, ice age occurred around the same time, and that massive glaciers developed all around the planet, at the expense of sea levels. California Institute of Technology (Caltech) scientists now believe they figured out how the ice age caused the Ordovician extinction.

In the new study, they were able to put together a framework for analyzing the factors that may have contributed to the event. The specialists were able to determine that falling sea levels led to significant habitat loss for countless species.

At the same time, the tropical portions of all oceans cooled down significantly, further contributing to habitat loss. Most species were unable to adapt to the new conditions in time, and simply perished.

Caltech investigators, working with colleagues at the University of Wisconsin-Madison (UWM), published details of the research in the latest early online issue of the esteemed journal Proceedings of the National Academy of Sciences (PNAS).

Around 75 percent of marine species living in the Late Ordovician were lost during the extinction. “What we did [in this study] was essentially the same thing you'd do if confronted with a disease epidemic,” expert Seth Finnegan explains.

“You ask who is affected and who is unaffected, and that can tell you a lot about what's causing the epidemic,” adds the scientist, a Caltech postdoctoral scholar, and the lead author of the PNAS study.

“This is the first really attractive demonstration of how you can use multivariate approaches to try to understand extinctions, which reflect amazingly complex suites of processes," adds Woodward Fischer.

“As Earth scientists, we love to debate different environmental and ecological factors in extinctions, but the truth is that all of these factors interact with one another in complicated ways, and you need a way of teasing these interactions apart,” he further says.

Fischer is an assistant professor of geobiology at Caltech, and the principal investigator on the study.