EARTH TIMES

End of the Devonian
           The Devonian Period ended about 370 million years ago.  About half of all animals and plants were wiped out.
            As with the Ordovician, many of the species that became exinct were from the oceans, especially the shallower areas.  Sea levels may have been rising and falling, and many creatures could not adapt.
            More plants had begun growing on land during this time.  They were pulling in more carbon dioxide from the air.  Since carbon dioxide is a greenhouse gas, it warms the planet.  When plants took carbon dioxide out of the air, the planet may have cooled.  Glaciers probably grew during this time also.
End of the Permian
            Life on Earth almost ended at the end of the Permian Period, about 250 million years ago.  During this time, known as the Great Dying, more than 90 percent of all species became exinct.
            Experts believe several things combined to kill almost all life.  A string of volcanoes, known as the Siberian Traps, had been erupting for a million years.  They poured out poisonous gases, as well as carbon dioxide, which warmed up the planet.
            Global warming and volcano-caused acid rain probably changed the makeup of the ocean waters.  There is evidence that an asteroid fell into the ocean, releasing methane gas from the bottom of the seas.
            All land was joined into one big continent, Pangaea.  Because of that, when conditions turned dangerous, animals could not escape.
End of the Triassic
            When the Triassic Period ended about 200 million years ago, about half of all species disappeared.  Volcanoes erupting under the Atlantic Ocean for centuries probably helped cause this extinction.
            Again, harmful gases, including carbon dioxide, changed the climate.  All land was still joined.
End of the Cretaceous
            At the end of the Cretaceous Period, about 65 million years ago, most dinosaurs became extinct.*  Unlike other extinctions, this one happened quickly, over hundreds of years instead of thousands.  An asteroid the size of a mountain hit Earth, causing fast climate changes.

*Large theropod dinosaurs became extinct, while most small ones survived on as birds.