Mass Extinctions
There have been at least 10 mass extinctions over the life of the earth.

Period Years Ago (M) % of Species Cause
Precambrian 650 M 70% Astroids fracture earths surface creating more volcanoes. Dust clouds cause cooling and an ice age.
Vendian 523-543   Glaciation or cooling and depletion of oxygen in marine waters
Ordovician 443 66% Gamma ray blast from Supernova. (1)
Devonian (F-F) 360 60 Glaciation or Meteorite Impact
Permian-Triassiac (P-T) 245-250 75% land
95% ocean
Killed Gorginopsians - mammal-like reptiles.
Caused by: 1. Declining sea oxygen levels
or 2. Reduction of shallow continental shelves
or 3. Global Warming
or 4. Huge volcanic lava flows in Siberia causing co2 and hydrogen sulfide increases
or 5. Comet or Asteroid (2)
Triassic 220 50%  
Triassic-Jurassic 200 50% killed the last of the mammal-like reptiles that once roamed the Earth and left mainly dinosaurs. (3)
Cretacious-Tertiary (K-T) 65 50-85% Asteroid (4)
Late Pleistocene 11,000 yrs ago   Saber-toothed cats, mammoths, and mastodons. Cause ? Hunting or Climate change at end of ice age. (5)
Quaternary 2050 AD 20% Global Warming (6)
Future 500,000,000 AD all Heat from expanding sun, which will swallow the earth 5 B years from now.
(1) In a presentation at the American Astronomical Society on Jan 7, 2004, NASA Scientists presented a theory that the Ordovician extinction 440 million years ago was caused by a gamma ray burst from a supernova exploding within 10,000 light years of the earth. The gamma rays broke up molecules in the stratosphere, causing the formation of nitrous oxide and other chemicals that would destroy the ozone layer and shroud the planet in a brown smog. The loss of ozone would allow intense ultraviolet radiation from the sun to reach the surface of the earth killing exposed life. The brown smog caused a ice age that lasted over half a million years. They said you can expect a gamma ray burst every few 100 million years.

(2) Several theories exist for the Permian-Triassiac (P-T) extinction 250 MYA: 1. Glaciation on Supercontinent of Gondwana, or on the north and south poles. or 2. Reduction of shallow continental shelves due to the formation of the super-continent Pangea or 3. Global warming or 4. Huge volcanic lava flows from large cracks in the ground known as flood basalts in the Siberian Traps.
In 2006 Ohio State geoscientist Ralph von Frese discovered evidencce of an impact crater below the antartic ice sheet. Thi Siberian Traps would have been at the antipode (otherside of the earth) from this and severe shaking as a reslult could have trigered tdhe volcanic lava flows.

(3) A 2001 report indicated that this event happened over a short (10,000 yrs which is short in gelogical time) period of time which supports the theory of something like an asteroid hit.

(4) Cretacious-Tertiary (K-T) 65 3% marine reptiles non-avian (non-bird) dinosaurs and pterosaurs had 100% extinction from this event.

The idea of a K-T impact, first put forth by Luis and Walter Alvarez more than two decades ago, came under heavy criticism from paleontologists who had always thought the creatures who perished at the end of the Mesozoic were wiped out slowly over millions of years, not in a instant's time by a fiery meteorite.

* In 2004 A group of researchers led by Gerta Keller of Princeton University contends that the impact that caused the Yucatan crater occurred 300,000 years before the extinction 65 years ago.

* In 2004 A group of researchers led by Gerta Keller of Princeton University contends that the impact that caused the Yucatan crater occurred 300,000 years before the extinction 65 years ago.

The latest fossil evidence argues that the mass extinction of dinosaurs resulted from a protracted crisis, one that built over tens of thousands or hundreds of thousands of years before pushing earth over an ecological precipice. The fossil record of large animals in South Africa looks more consistent with extinction by a millennia-long volcanic eruption than by an asteroid or comet impact. Details of the research are presented.
See Science Jan 21, 2005

(5) Mostly mammals larger than approximately 44 kg (about 100 pounds). Some of the animals that went extinct are well known (like saber-toothed cats, mammoths, and mastodons). Others were less well known animals (like the short-faced skunk and the giant beaver). Some animals went extinct in North America but survived elsewhere (like horses and tapirs).

Before this extinction the diversity of large mammals in North America was similar to that of modern Africa. As a result of the extinction, relatively few large mammals are now found in North America.

See: Late Pleistocene Extinctions at Illinois State Museum

(6) In the Jan. 2004 issue of "Nature" a team of international scientists says global warming would drive more than a third of the wildlife in the worlds most ecologically sensitive areas to extinction by 2050. They say that increasing temperatures will make it impossible for many plants and animals to fight for shrinking habitat in the Amazon, Australia, Africa and Mexico.

In 1982, David Raup and John Sepkoski, both of the University of Chicago, examined marine invertebrate biodiversity in the Phanerozoic. They divided this 242-million-year sequence into 39 equal-length intervals, and defined an "extinction event" as an interval where at least 2% of all known marine invertebrate families became extinct. Raup and Sepkoski observed that such extinction events occur about every 26 million years.

In 1983, Marc Davis and Richard Muller, of the University of California at Berkeley, proposed that the Sun has a yet-undetected companion star with an eccentric orbit. The "unseen companion" is generally about 2 light years away from the Sun, according to Davis. As the companion star passes through the Oort cloud of comets surrounding the Sun, it launches many of these comets in the general direction of the earth
It was lster shown that the 26 M yr. cycle could be explained by other statistical means. The debate is still open.
See: Catastrophic Events in the History of Life: Toward a New Understanding of Mass Extinctions in the Fossil Record - Part II, David B. Weinreb, Yale

According to Mikhail Medvedev and Adrian Melott, researches at University of Kansas, cosmic rays produced at the edge of our galaxy have devastated life on Earth every 62 million years. In addition to rotating around the galactic center, our sun moves toward and away from the Milky Way's center, and also up and down through the galactic plane. One complete up-and-down cycle takes 64 million years, suspiciously similar to Earth's biodiversity cycle. As the solar system rises above the central plane it sticks out like a cherry on top of the flying galactic pie, closer to the source of the cosmic radiation. See: Ancient Mass Extinctions, Cyclic and Caused by Cosmic Radiation? SoftPedia.

See Also:
Geological Time Periods
Human Clades: A Look at a Complex Phylogeny: October 2011
Early Paleozoic
Geological Time
The History of Life at UC Berkeley

last updated 10 Mar 2008