It is commonly said that the final fate of all
the species is Extinction,
that is, animal and plant species appear and eventually disappear they
do not longer exist on Earth. The cause of the extinction; however,
is variable. For example, in the process known as pseudoextinctionthe
species evolves into a new species that differs so much from its ancestral
group that the parent species can be considered as extinct although in
the strict sense no extinction event occurred. In other cases the species
dies out (disappear) without giving origin to anything else, this process
is know as anextinction.
are times or episodes in the geological past in which the extinction rates
are greatly accelerated resulting in the marked decrease in the diversity
of organism. Such extinction that involves the disappearance a number
of groups are known as mass
During geologic history, there have been times when large groups of organisms became extinct within a short period (a few million years)- same of these mass extinction have been used to define period breaks.
Main episodes of mass extinction in the history of the biosphere include:
1) Cambrian/Ordovician, 505 my ago: Many trilobites, sponges, and gastropods. About 52 percent of biota involved.
Ordovician-Silurian extinction, about 439 million years ago, caused by a drop in sea levels as glaciers formed, then by rising sea levels as glaciers melted. The toll: 25 percent of marine families and 60 percent of marine genera.
360 my ago: groups
of corals, trilobites, bryozoan and fish. About 30 percent of the biota
3) Permian/Triassic, 251 my ago: Rugose corals, trilobites, blastoids, brachiopods, and foraminifera became extinct. About 50 percent of the biota involved. Many scientists suspect a comet or asteroid impact, although direct evidence has not been found. Others believe the cause was flood volcanism from the Siberian Traps and related loss of oxygen in the seas. Still others believe the impact triggered the volcanism and also may have done so during the Cretaceous-Tertiary extinction. The Permian-Triassic catastrophe was Earth’s worst mass extinction, killing 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.
The End Triassic
extinction, roughly 199 million to 214 million years ago, most likely
caused by massive floods of lava erupting from the central Atlantic magmatic
province -- an event that triggered 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. The death toll: 22 percent of marine families, 52 percent of
marine genera. Vertebrate deaths are unclear.
65 my ago: Ammonites,
dinosaurs, marines reptiles become extinct, many types of marine invertebrate
were affected. 26 percent of biota involved. Cretaceous-Tertiary
extinction, about 65 million years ago, probably caused or accelerated
by impact of a several-mile-wide asteroid that created the Chicxulub crater
now hidden on the Yucatan Peninsula and beneath the Gulf of Mexico. Some
argue for other causes, including gradual climate change or flood-like
volcanic eruptions of basalt lava from India’s Deccan Traps. The extinction
killed 16 percent of marine families, 47 percent of marine genera (the
classification above species) and 18 percent of land vertebrate families,
including the dinosaurs.
The Causes of Mass Extinctions
Extraterrestrial Impacts: A controversial hypothesis for mass extinctions has been postulated, particularly for the extinction of the dinosaurs at the end of the Cretaceous--extraterrestrial meteor or asteroid impacts. While meteors and/or asteroids have certainly struck the earth In the past, the causal relationship between impacts and extinctions is still debated.
Humans and human activities should be listed
as a cause of extinction of species. As we know there is a long list of
endangered biota (animals and plants) due to human intervention on the
environment or by direct abuse of species.
FOSSILS AND THE GEOLOGIC TIME SCALE
The fossil record shows many evolutionary (new
appearance) and extinction (disappearance) events, From an examination
of the timing of these events, geologists have been able to divide geologic
time into major units based on the history of life. Phanerozoic time (from
about 700 million years ago to the present) is divided into three eras-Paleozoic,
Mesozoic, and Cenozoic the basis of the overall nature or predominant character
of fossils found in the strata. The -subdivision of eras into periods is
clone primarily on the basis of extinction events, In general, the Paleozoic
fossil record is water marine invertebrates characterized by shallow-water
marine invertebrates. Thus, the Paleozoic is sometimes called the Age of
Invertebrates. The Mesozoic rock record contains a relatively large percentage
of continental deposits with fossils of reptiles. hence, the Mesozoic has
been termed the Age of Reptiles. The Cenozoic fossil record contains a
large number of mammals and flowering plants that evolved during this time.
The Cenozoic is informally named the Age of Mammals.
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Other species did not cope with the presence of humans