Mass Extinctions 

    There have been several major mass extinctions of life recorded in the fossil record on the planet Earth. The extinction processes occurs on both the land and the sea. On the land, while animal life is eliminated repeatedly, plants tend to be more highly resistant to mass extinctions. There has been preferential disappearance of tropical life forms during previous mass extinctions.  There has been a suggested (but unsubstantiated, as of yet) equal spacing, or periodicity over geological time (occurring about every 26 million years).

    The agents that perpetuated the disappearances (mass extinction) of species are currently divided into two types:
            1.  Catastrophic Agents - such as meteorite impacts and comet showers and 
            2. Earth Agents - such as volcanism, glaciation, variations in sea level, global 
                              climatic changes, and changes in ocean levels of oxygen or salinity
    Although these agents can help explain mass extinction, the exact causes of mass extinction events remains unknown. Each agent of mass extinction is presumed to have caused massive changes in the environment that resulted in extinctions of species and life.
TOP FIVE    MASS EXTINCTIONS           
Cretaceous-Tertiary - Chicxulub Event
Date: About 65 million years ago
                  the so-called End-Cretaceous (K-T) extinction
Death Toll: Up to 75% of marine genera; 18 percent of land vertebrates, & the dinosaurs
Possible Causes and current research: Meteor impact; severe volcanism
           http://fig.cox.miami.edu/~cmallery/150/astrobiol/meteorites.htm
           http://www.lpl.arizona.edu/SIC/impact_cratering/Chicxulub/Chicx_title.html

Late Triassic
Date: About 200 million years ago
Death Toll: 52% of marine genera
Possible Causes: Severe volcanism; global warming

Permian-Triassic
Date: 286-248 million years ago;  
                  Terrestrial faunal diversification occurred in the Permian.
Death Toll: 84% of marine genera; 90-95% of marine species; 70% of land species.
Possible Causes: Asteroid or comet impact; severe volcanism; 
                   dramatic fluctuations in climate or sea level

Late Devonian
Date: About 365 million years ago [Devonian period ranged from 408-360 mya]
Death Toll: 55 percent of marine genera
Possible Causes: Global cooling; loss of oxygen in oceans; impact

Late Ordovician
Date: About 440-450 million years ago;  [Ordovician period was from 510-438 mya]
Death Toll: 60 percent of marine genera
Possible Cause: Dramatic fluctuations in sea level
              - Ordovician extinction was second most devastating in earth history

 

CURRENT MASS EXTINCTIONS...

    The Holocene epoch is the geologically brief interval of time encompassing the last 10,000 years.
With the evolution of humans beginning in the Neogene, humans have evolved into a significant agent of extinction. For example, David Western of the New York Zoological Society, has speculated that for the destruction of every two hundred square kilometers of tropical forest and one hundred thousand square kilometers of rangeland there is a resultant loss of hundreds, if not thousands, of species. Most of these have never been (or ever will be) documented by science.
    Deforestation, agricultural practices, pollution, overhunting, and numerous other human activities result in numerous species being threatened everyday. However, more information is required to see if the level of extinctions being experienced today is a harbringer of a mass extinction or merely reflect natural background levels of species replacement.

 

 

 

reference:    http://www.sciam.com/2001/0501issue/0501scicit3.html

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 Chicxulub Event    

    Sixty-five million years ago an asteroid 15 km in diameter struck a shallow sea on what is now the peninsula of Yucatan. The impact released more than 100 million megatons of energy and brought the warm, pleasant Cretaceous era to a violent end. Within hours of the impact the Earth's ecosystems were ravaged by global firestorms. The shorelines of Texas and Mexico were devastated by giant tsunamis. A pall of sulfuric acid aerosols filled the stratosphere, causing temperatures to drop dramatically for decades. Many groups of animals, including all of the dinosaurs, became extinct as a result of this catastrophe. This lecture will describe the evidence for these events and the mechanisms by which large impacts can affect life on the Earth.
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