Throughout the history of the Earth, there have been five major mass extinctions which almost ended all life on our little blue planet. A mass extinction happens when there is a sharp decrease in the diversity of macroscopic life. Because of years of research, we now know that around 98% of all the species that ever existed on Earth are now extinct, but these extinctions to not happen at an even rate. By studying the fossil record, we ca now definitely say that there have been five major mass extinction events, in which the fossil record shows that there was a sharp decrease in the number of species living on Earth, much more dramatic than normal background extinction rates. These major events are as follows:
1. The Ordivician-Silurian Extinction Event: 450 to 440 million years ago. 27% of families and 57% of all genera went extinct in two proximal evets which occurred at the transition between the Odivician and Silurian time periods. This is ranked by many scientists as the second-most dramatic extinction event. The immediate cause of this extinction seems to be the movement of the supercontinent Gondwanaland into the south polar region, causing global cooling, glaciation, and sea level fall. The most affected organisms were sea life, including brachiopods, echinoderms, bivalves, bryzoans, and corals.
2. Late Devonian Extinction Event: 375 to 360 million years ago. 19% of all families, 50% of all genera, and 70% of all went extinct during this period. Some scientists believe that it lasted up to 20 million years, and manifested as pluses of extinction instead of one prolonged one. At this time, Euramerica and Gondwana were beginning to converge to create what would become known as the supercontinent Pangea. The cause of these extinction pulses is unknown, but some theories have suggested changes in sea level and ocean anoxia, which could have been caused by global cooling or oceanic volcanic activity. Asteroid and comet impacts have also been discussed. This extinction seems to have only affected marine life, hitting brachiopods, trilobites, and reef-building oganisms the hardest.
3. Permian-Triassic Extinction Event: 251 million years ago. This extinction is also known as the Great Dying. 96% of all marine species and 70% of all terrestrial vertebrate species became extinct at this time. Every type of organism was hit hard and lost very significant numbers of species, from marine life to plants to terrestrial invertebrates and vertebrates. Because the loss of life was so enormous during this extinction, the Earth took a significantly longer period of time to recover than from other mass extinctions. It is possible that it even took 10 million to get back to a healthy biodiversity. The species that did not get completely wiped out by this extinction struggled for survival for a long time, and were significanly affected as well. Some researchers suggest that there were one to three pulses, or phases, of extinction. The earlier phase was most likely due to a gradual climate change, while the latter phase is believed to be due to a catastrophic event. This could include a meteor impact, coal/gas fires and explosions from the Siberian Traps, increased volcanism, and sudden methane release from the ocean floor (methane clathrate).
4. Triassic-Jurassic Extinction Event: 205 million years ago. 23%of all families and 48% of all genera went extinct. About half of the species o Earth were wiped out, including non-dinosaurian archosaurs (other than crocodiles), some therapids, and many large amphibians. Because of this, the reign of the dinosaurs officially began, most of their competition having been wiped out. This event happened fairly quickly on a geologic time scale (less than 10,000 years), right before the supercontinent Pangea began to break apart. There are several theories as to why this extinction happened, but nothing has been concretely found to determine the exact cause. The leading theories are gradual climate change, sea-level fluctuations, pulse of ocean acidity, asteroid impact, or massive volcanic eruptions (specifically from the Central Atlantic Magmatic Province, or CAMP). The reason could be any combination of these factors.
5. Cretaceous-Paleogene Extinction Event (K-T Boundary Extinction): 65.5 million years ago. 17% of families, 50% of all genera, and 75% of all species became extinct. About 33% of all sessile (stationary) marine life was wiped out, as well as most non-aviary dinosaurs. This extinction is now believed to have been caused by an asteroid impact whose mark on the earth can still be detected in Mexico, the believed impact point. Because the dominant terrestrial species on the planet became extinct, it gave mammals and birds the opportunity to become the prevailing land life. Before this extinction, mammals were few and small, most resembling field mice. As we all know, mammals are very diverse in modern times, with humans being the dominant species, and we may have never even evolved farther than mice had it not been for this mass extinction.
Category: Science by Numbers