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.
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Courtesy of Shanan Peters, University of Wisconsin-Madison
The Mystery of Mass Extinction is No Longer Murky
News story originally written on June 17, 2008
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.
Last modified March 30, 2009 by Lisa Gardiner.
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