A group of solitary horn corals, an extinct group of corals that belong to the Paleozoic evolutionary fauna. They are preserved in carbonate and are Ordovician in age (~450 million years old). They were found in southwestern Wisconsin. [See related image Here.]
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Recent research 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 during the past 500 million years.
A National Science Foundation supported-study by Shanan Peters, a University of Wisconsin-Madison assistant professor of geology and geophysics, found that 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 life began on Earth 3.5 billion years ago, scientists believe there may have been as many as 23 mass extinction events, and, during 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. To learn more, see the UW-Madison news release, "Ebb and flow of the sea drives world's big extinction events." (Date of Image: 2008)