These fossil crinoids lived approximately 460 million years ago in a shallow ocean that covered the area where Ontario, Canada is today. Relatives of sea urchins and starfish, crinoids attached themselves to the sea floor and filtered food from the water with their feathery “arms.” While there are still crinoids alive today they are not nearly as common as they were during the Paleozoic. Click on image for full size (616 Kb) Courtesy of Shanan Peters, University of Wisconsin-Madison

If you are curious about Earth's periodic mass extinction events such as the sudden demise of the dinosaurs 65 million years ago, you might consider crashing asteroids and sky-darkening super volcanoes as culprits.

But a new study, published online June 15, 2008, in the journal Nature, suggests that it is the ocean, and in particular the epic ebbs and flows of sea level and sediment over the course of geologic time, that is the primary cause of the world's periodic mass extinctions over the past 500 million years.

"The expansions and contractions of those environments have pretty profound effects on life on Earth," says Shanan Peters, a University of Wisconsin-Madison assistant professor of geology and geophysics and the author of the new Nature report. In short, according to Peters, changes in ocean environments related to sea level exert a driving influence on rates of extinction, which animals and plants survive or vanish, and generally determine the composition of life in the oceans.

Since the advent of life on Earth 3.5 billion years ago, scientists think there may have been as many as 23 mass extinction events, many involving simple forms of life such as single-celled microorganisms. Over the past 540 million years, there have been five well-documented mass extinctions, primarily of marine plants and animals, with as many as 75-95 percent of species lost. For the most part, scientists have been unable to pin down the causes of such dramatic events. In the case of the demise of the dinosaurs, scientists have a smoking gun, an impact crater that suggests dinosaurs were wiped out as the result of a large asteroid crashing into the planet. But the causes of other mass extinction events have been murky, at best.

"No matter what the ultimate driving extinction mechanisms might be at any one time, Professor Peters brings the repeated and resultant extinction on oceanic shelves front and forward where it belongs," says National Science Foundation (NSF) Program Manager Rich Lane. "This breakthrough speaks loudly to the future impending modern shelf extinction due to climate change on Earth."

Paleontologists have been chipping away at the causes of mass extinctions for almost 60 years, according to Peters, whose work was supported by NSF. "Impacts, for the most part, aren't associated with most extinctions. There have also been studies of volcanism, and some eruptions correspond to extinction, but many do not."

Arnold I. Miller, a paleobiologist and professor of geology at the University of Cincinnati, says the new study is striking because it establishes a clear relationship between the tempo of mass extinction events and changes in sea level and sediment: "Over the years, researchers have become fairly dismissive of the idea that marine mass extinctions like the great extinction of the Late Permian might be linked to sea-level declines, even though these declines are known to have occurred many times throughout the history of life. The clear relationship this study documents will motivate many to rethink their previous views."

Peters measured two principal types of marine shelf environments preserved in the rock record, one where sediments are derived from erosion of land and the other composed primarily of calcium carbonate, which is produced in-place by shelled organisms and by chemical processes. "The physical differences between these two types of marine environments have important biological consequences," Peters explains noting differences in sediment stability, temperature and the availability of nutrients and sunlight.

In the course of hundreds of millions of years the world's oceans have expanded and contracted in response to the shifting of the Earth's tectonic plates and to changes in climate. There were periods of the planet's history when vast areas of the continents were flooded by shallow seas such as the shark and mosasaur infested seaway that neatly split North America during the age of the dinosaurs.

As those epicontinental seas drained, animals like mosasaurs and giant sharks went extinct, and conditions on the marine shelves where life exhibited its greatest diversity in the form of things like clams and snails changed as well. The new Wisconsin study, Peters says, does not preclude other influences on extinction such as physical events like volcanic eruptions or killer asteroids, or biological influences such as disease and competition among species. But what it does do, he argues, is provide a common link to mass extinction events over a significant stretch of Earth history.

"The major mass extinctions tend to be treated in isolation by scientists," Peters says. "This work links them and smaller events in terms of a forcing mechanism, and it also tells us something about who survives and who doesn't across these boundaries. These results argue for a substantial fraction of change in extinction rates being controlled by just one environmental parameter."

Text above is courtesy of the National Science Foundation

These fossil crinoids lived approximately 460 million years ago in a shallow ocean that covered the area where Ontario, Canada is today. Relatives of sea urchins and starfish, crinoids attached themselves to the sea floor and filtered food from the water with their feathery “arms.” While there are still crinoids alive today they are not nearly as common as they were during the Paleozoic. Click on image for full size (616 Kb) Courtesy of Shanan Peters, University of Wisconsin-Madison

About 250 million years ago, almost all of the life in the sea became extinct – about 95% of species. This was during a huge mass extinction. Mass extinctions happen when the number of species decreases fast.  Why do they happen?  Scientists may have found the answer.

Over about the past half a billion years there have been five large mass extinctions. A new study suggests that changes in sea level over geologic time is the main reason that mass extinctions happen. Changes in ocean environments related to sea level have a big impact on living things that call those places home. As sea level changes, some animals and plants survive while others go extinct.

Over hundreds of millions of years, the world's oceans have grown and shrunk because of moving tectonic plates and because of climate change. There were times in the past when sea level was high and huge areas of the continents were flooded by shallow seas - the shark and mosasaur-infested sea that was in the middle of North America 100 million years ago, for example. When sea level fell and the water drained off the continent, the mosasaurs and giant sharks went extinct.

Sea level change may not be the only reason for mass extinctions. There can be other causes too - events like volcanic eruptions, killer asteroids, or disease. In the case of the extinction that killed off the dinosaurs 65 million years ago, for example, a huge impact crater suggests that the extinction might have been started when a giant asteroid crashed into Earth.  Sea level change is, admittedly, much less dramatic than a crashing asteroid. It happens so slowly you can not watch it happen.  But over geologic time it is a powerful force and it appears to be the cause of mass extinction events over much of Earth history.

Today, sea level is rising as climate warms. Given the link between sea level change and mass extinction, there may be a change in the species that live in the sea.

These fossil crinoids lived approximately 460 million years ago in a shallow ocean that covered the area where Ontario, Canada is today. Relatives of sea urchins and starfish, crinoids attached themselves to the sea floor and filtered food from the water with their feathery “arms.” While there are still crinoids alive today they are not nearly as common as they were during the Paleozoic. Click on image for full size (616 Kb) Courtesy of Shanan Peters, University of Wisconsin-Madison

About 250 million years ago, almost all of the life in the sea became extinct. This was during a huge mass extinction. Mass extinctions are when the number of different living things shrinks.  Over about the past half a billion years there have been five large mass extinctions. Now, scientists may have figured out why they happen.

Scientists have found that changes in sea level are the main reason that mass extinctions happen. Sea level rises and falls have a big impact on living things that call the ocean home. As sea level changes, some animals and plants survive while others go extinct.

Over hundreds of millions of years, the world's oceans have grown and shrunk because of plate tectonics and climate change. There were times in the past when sea level was high and huge areas of the continents were flooded by shallow seas. For example, 100 million years ago there was a sea in the middle of North America. Sea level dropped and the North American sea dried up. The mosasaurs and giant sharks that had lived in the sea because extinct.

Sea level change may not be the only reason for mass extinctions. There can be other causes too. When the dinosaurs became extinct 65 million years ago, for example, a giant asteroid crashed into Earth. Scientists think that the asteroid was large enough to change the climate which caused the dinosaurs to die.  Sea level change is not as dramatic as a crashing asteroid. It happens so slowly you can not watch it happen.  But over geologic time it is a powerful force and it appears to be the cause of mass extinction events over much of Earth history.