Time periods in the history of life on Earth during which exceptionally large numbers of species go extinct are called mass extinctions. These extinctions are quite different from the rate of extinction, which occurs even when the diversity of life is increasing. Many species vanished in five cataclysmic mass extinctions and today, 99.9 percent of all species that have existed on Earth are extinct.
The Ordovician-Silurian extinction
occurred about 439 million years ago due to a drop in sea levels as
glaciers formed followed by rising sea levels as glaciers melted. During
this extinction 25 percent of marine families and 60 percent of marine
genera (the classification above species) were lost.
The Late Devonian extinction
took place somewhere around 364 million years ago. To this day its
cause is unknown. However, evidence supporting the Devonian mass
extinction suggesting that warm water marine species were the most
severely affected in this extinction event, has lead many
paleontologists to believe that an episode of global cooling, similar to
the event which that may have resulted in the Ordovician-Silurian mass
extinction, may have lead to the Devonian extinction. Thus this theory
suggests that the extinction of the Devonian was triggered by another
glaciation event on Gondwana, which is evidenced by glacial deposits of
this age in northern Brazil.
Similarly to the late Ordovician crisis, agents such as global cooling
and widespread lowering of sea-level may have triggered the late
Devonian crisis. Scientists have also suggested that meteorite impacts
may have been possible agents for the Devonian mass extinction, but the
data in support of a possible extra-terrestrial impact remains
inconclusive, and the mechanisms responsible for the Devonian mass
extinction are still under debate. What is know, however, is that this
mass extinction killed 22 percent of marine families and 57 percent of
The Permian-Triassic extinction
happened about 251 million years ago and was Earths worst mass
extinction. 95 percent of all species, 53 percent of marine families, 84
percent of marine genera, and an estimated 70 percent of land species
such as plants, insects and vertebrate animals were killed during this
catastrophe. Direct evidence for this period has not been found but many
scientists believe a comet or asteroid impact led to this extinction.
Others think that volcanic eruption, coating large stretches of land
with lava from the Siberian Traps, which are centered around the
Siberian City of Tura, and related loss of oxygen in the seas were the
cause of this mass extinction. Still other scientists suspect that the
impact of the comet or asteroid triggered the volcanism.
The End Triassic extinction,
taking place roughly 199 million to 214 million years ago, was most
likely caused by massive floods of lava erupting from the central
Atlantic magmatic province triggering the breakup of Pangaea and the
opening of the Atlantic Ocean. The volcanism may have led to deadly
global warming. Rocks from the eruptions now are found in the eastern
United States, eastern Brazil, North Africa and Spain. 22 percent of
marine families, 52 percent of marine genera, and an unknown percentage
of vertebrate deaths were the result.
Finally, the Cretaceous-Tertiary extinction
occurred about 65 million years ago and is thought to have been
aggravated, if not caused, by impacts of several-mile-wide asteroid that
created the Chicxulub crater now hidden on the Yucatan Peninsula and
beneath the Gulf of Mexico. Yet, some scientists believe that this mass
extinction was caused by gradual climate change or flood-like volcanic
eruptions of basalt lava from the Deccan Traps
in west-central India. During this extinction, 16 percent of marine
families, 47 percent of marine genera, and 18 percent of land vertebrate
families including the dinosaurs.