"I accept extinction as best explaining disjoined species. I see that the same cause must have reduced many species of great range to small, and that it may have reduced large genera to so small, and of families."
― Asa Gray
The term extinction
refers to an occurrence, wherein a particular biological species and/or the entire taxonomic group disappears from the face of the Earth. According to scientists, about 98% of the documented biological species are now extinct. This means that extinction of species, just like speciation (process of formation of new biological species), plays a rather prominent role in maintaining and changing the Earth's biodiversity. An extinction event
, also known as mass extinction
or biotic crisis
, takes place when, in a given period of time, there is a rapid decline in the number of living organisms, not just in a particular region, but over a large geographical area. Notably enough, during an extinction event, the rate of speciation is always lesser than that of extinction. In other words, more species become extinct compared to those that are formed anew. This results in a drastic reduction in the quantity of life on Earth, thus, directly affecting its biodiversity.
Major Extinction Events
Lesser Extinction Events
Major Extinction Events
Paleontologists have arguably defined five major mass extinction events, which have occurred over a span of the last 542 million years (541.0 ± 1.0) on the Earth's geological timescale. It has been estimated that during these events, about 50% of the animal species on the Earth ceased to exist.
Ordovician-Silurian Extinction Event
: 450 - 440 million years ago
Also known as the Ordovician extinction
, this event of mass extinction was witnessed in two different phases, separated by a million-year gap.
This was a period when animal life was confined only to the marine environment, and as of today, there are no known terrestrial life forms from this geological epoch.
This event saw the extinction of over 60% of marine invertebrates, and is termed as the second-largest extinction event in the history of the Earth.
: According to the most widely accepted theory, the supercontinent of Gondwanaland
moved towards the south polar region during this period. This led to extreme glaciation and subsequent fall in the sea levels, which led to large-scale habitat loss of most marine species, especially along the continental shelves.
Late Devonian Extinction
: 375 - 360 million years ago
Like the Ordovician extinction event, the Late Devonian extinction was also seen in two distinct phases, viz. the Kellwasser Event
and the Hangenberg event
. However, the periodic span over which these events took place is still pretty obscure.
What we do know is that during this extinction event, about 19% of all the existent animal families and around 50% of all the existent genera died off. This was a rather big loss to the Earth's biodiversity.
By this time, terrestrial life had begun to evolve, but only the marine animals of this period seem to have been affected by extinction, where about 70% of the total marine species were wiped out.
: Fossil records indicate that the warm water species were the ones that were severely affected. Owing to this, some paleontologists suspect yet another phase of global cooling, leading to a sharp decrease in sea levels. Some have also proposed a possibility of a meteorite impact; however, the extinction did not take place all of a sudden, but was gradual.
Permian-Triassic Extinction Event
: 251 million years ago
Informally known as the Great Dying
, this is, by far, the largest extinction event in the history of the Earth. Moreover, it is the only recorded mass extinction of insects, alongside other life forms.
During this extinction event, about 70% of the species of terrestrial vertebrates and about 96% of all species of marine organisms vanished. This means that a vast majority of the biodiversity of the Earth was completely destroyed during this period.
Owing to the fact that most of the animal species were lost during this extinction event and new species were being formed at a comparatively slower rate, it might have taken as many as 10 million years to recover the lost biodiversity.
: Evidence recovered from the stratigraphic data of this period indicates that there were a series of tragedies and natural calamities, which may have led to the biotic crisis. Global warming and climate change, which may have followed these catastrophic events, might have resulted in the extinction of most animal species.
Triassic-Jurassic Extinction Event
: 200 million years ago
This extinction event occurred over a relatively short period of time, spanning less than 10,000 years. The importance of this extinction event lies in the fact that after it ended, dinosaur became a dominant species on Earth.
Both terrestrial as well as aquatic species were hardly hit during this event, but in the marine environment, an entire class of species―the Conodonta
―disappeared. On land, most of the large species of amphibians went extinct.
The fossil evidence recovered from this extinction event suggests that about 23% of all animal families, and about 75% of the existent species became extinct during this period.
: No particular cause has been established for the occurrence of this extinction event. However, theories which are most widely accepted pertain to climate change, large-scale volcanic eruptions, and a huge asteroid impact.
Cretaceous-Paleogene Extinction Event
: 66 million years ago
This is the most recent extinction event to have occurred on Earth. This extinction event is important, in that it marked the end of the Age of Reptiles
, after which the current Age of Mammals
began, which continues to this day.
A huge variety of species is known to have perished during this event, especially the non-avian species of dinosaurs that had dominated the scene till then. Over three quarters of life on Earth, including both, plants and animals, vanished from the face of the Earth.
Sediments narrating the story of this mass extinction event can be found the world over, and paleontological evidence shows us that a lot of animals, which managed to survive extinction evolved a number of new adaptive features.
: The most established cause of this extinction event is that of the asteroid impact that may have made it impossible for the plants to be able to photosynthesize. This, in turn, may have resulted in the extinction of plants, thus, putting an abrupt halt to the entire food chain.
In spite of the fact that the above five have been widely accepted as the major extinction events, there are still arguments, owing to the fact that the fossil records may not be very accurate. The proponents of the five mass extinctions, however, opine that we may be in the middle of the sixth extinction event right now, with drastic climate changes and over 90% of the marine species disappearing over the last century.
Paleontologists have also listed a number of lesser extinction events, the impact of which may not be as visible as that of the major ones; nonetheless, these, comparatively shorter periods of time, did witness the mass extinction of certain species of plants and animals.
Great Oxygenation Event
: 2,400 million years ago
Also known as the Oxygen Catastrophe
or Great Oxidation
, this lesser extinction event occurred due to a sudden increase in the amount of free oxygen in the atmosphere, which is poisonous for the obligate anaerobes. Many species of these anaerobic microorganisms became extinct during this period; however, oxygen has been an important component of Earth's atmosphere ever since.
: 542 million years ago
This event is marked by the mass extinction of acritarchs
, known from the small organic fossils, dating back to approximately 1,400 to 3,200 million years ago. It is also known for the extinction of the Ediacara biota
, tubular sessile organisms, known from their trace fossils. The most widely accepted theory pertaining to the cause of this event relates to the exhaustion of oxygen from the oceans.
End-Botomian Mass Extinction
: 517 million years ago
Numerous small shelly fossils
(mineralized fossils, only a few millimeters in length, and so diverse that they haven't found a proper definition as yet) have been found from this period, suggesting that there was a mass extinction of these organisms during this lesser event. The cause for this extinction event is not known till date.
: 502 million years ago
Several marine fossils have been found pertaining to this period. The fossil evidence suggests that about 40% of the marine genera were eliminated during this extinction event. However, like the End-Botomian extinction event, the causes of this mass extinction are also not known.
Cambrian-Ordovician Extinction event
: 488 million years ago
This event is marked by the extinction of a large number of brachiopods
(marine organisms with hard shells on the upper and the lower surfaces) and conodonts
(chordates, known from their tooth-like microfossils). Possible causes of this extinction event include the reduction of oxygen level in the oceans and glaciation that resulted in dramatic climate change, owing to falling temperature levels.
: 428 million years ago
The event gets its name from the place where it has clear records―Ireviken in the Gotland Province of Sweden. Paleontological evidence from this period indicates that more than half of the trilobite
(marine arthropods, known from their fossils) species vanished, along with about 80% of the conodont
species. The most widely accepted theory with regard to the cause of this event points towards the depletion of oxygen levels in the ocean depths.
: 424 million years ago
This is a relatively less-studied event, owing to the thin traces in the marine rock sediments. What we do know about this period is that there was a dramatic drop in the levels of sea water throughout the world, owing to which, a lot of marine organisms died out, as fossil evidence suggests.
: 420 million years ago
Also known to have occurred due to a major drop in the levels of sea water, the Lau event also impacted the conodont
species, the world over. The event gets its name from the parish of Lau in Sweden.
End-Silurian Extinction Event
: 416 million years ago
During this minor extinction event, the free-swimming marine organisms were mainly struck. However, certain geological records also show us that sessile organisms, such as corals also faced extinction, resurfacing again after a long interval. The exact cause of this event is yet to be known.
Carboniferous Rainforest Collapse
: 305 million years ago
This was an extinction event that took place during the Carboniferous period, marked by the existence of dense forests in the equatorial region of Euramerica. Paleontological theories tell us that a dramatic change in climate destroyed the tropical rainforests, breaking them up into isolated 'islands', thus, resulting in the extinction of numerous important plant and animal species.
: 270 million years ago
The event gets its name from the American paleontologist, Everett C. Olson
, who discovered the presence of a lacuna, followed by a drastic change in the animal species of the Early Permian (299.0 ± 0.8 - 270.6 ± 0.7 million years ago) and Middle/Late Permian periods (270.6 ± 0.7 - 251.0 ± 0.4 million years ago). Later studies suggest that the lacuna did not pertain only to animals, but also to plants and other marine microorganisms. This gap and the advent of new species suggests that the earlier ones, which no longer existed, had gone extinct. The cause for this mass extinction hasn't been established as yet.
Carnian Pluvial Event
: 232 million years ago
This event was marked by a major change in the global climate. Various terrestrial and marine species went extinct during this event, including conodonts, ammonoids, and briozoa. Moreover, one of the earliest known species of dinosaur, the Eoraptor
, dates back to this period.
: 183 million years ago
Also known as the Toarcian extinction
or the Early Jurassic extinction
, this lesser extinction event is known for the global extinction of various marine mollusks, such as the ammonites
. Researches point towards the depletion of oxygen levels in the ocean depths as the probable cause of this event.
: 145.5 million years ago
This extinction event took place during the Tithonian
period (152.1 ± 4 - 145 ± 4 million years ago) of the Late Jurassic Age. A large number of marine reptiles and bivalves went extinct by the end of this event. Moreover, the Stegosaurus (armored stegosaurid dinosaur) also vanished after this event. This event continues to remain one of the lesser studied extinction events till date.
: 117 million years ago
This lesser extinction event can be easily witnessed in the marine fossil deposits of the Early Cretaceous period (125 - 100 million years ago). The probable cause that has been hypothesized for the event is associated with the volcanic activity in the Rajmahal Traps in the Bengal region of India.
Eocene-Oligocene Extinction Event
: 33.9 million years ago
Occurring during a transitional phase between the end of the Eocene (56 - 33.9 million years ago) and the beginning of the Oligocene (33.9 - 23 million years ago), this event was result of global cooling, alongside other factors, such as large-scale volcanic activity and meteorite impact. This event mostly affected aquatic organisms.
Middle Miocene disruption
: 14.5 million years ago
This event is marked by the extinction of several aquatic and terrestrial species of plants and animals during, as the name suggests, the middle of the Miocene period (between 14.8 - 14.5 million years ago). One of the possible causes of the event is associated with the growth of the East Antarctic Ice Sheet.
Pliocene-Pleistocene Boundary Marine Extinction
: 2 million years ago
The Pliocene-Pleistocene boundary is associated with a number of marine extinctions all over the world. The most accepted explanation for this event is associated with a supernova, which may have triggered a breakdown of the ozone layer of the Earth, thus, leading to climate change, owing to the increasing levels of carbon dioxide in the atmosphere.
Quaternary Extinction Event
: 50,000 years ago - till date
The Quaternary period on the Earth's geological timescale is essentially divided into two phases, viz. the Pleistocene (2,588,000 - 11,700 years ago) and the Holocene (11,700 years ago - till today). This extinction event is characterized by the disappearance of numerous megafauna (large or giant animals), such as the woolly mammoth. This event occurred in the transitional phase from the Pleistocene and the Holocene. Notably enough, humans appeared on the scenario by the middle of the Pleistocene period, and hence, alongside climate change, excessive human intervention in natural habitats of the animals has also been proposed as one of the major reasons for this extinction event.
The study of extinction events is very interesting―it tells us about the patterns of environmental dominance among species. One species becomes extinct in a particular ecological niche, and another group, that need not be always superior to the previous one, becomes dominant. This is often because the group that went extinct could not evolve according to the changed situations. Extinction events also tell us how the Earth has evolved over a period of time, and turns our attention towards the law of nature that nothing, no matter how powerful it may be, stays forever.