Although the Cretaceous-Tertiary (or K-T) extinction event is the most well-known because it wiped out the dinosaurs, a series of other mass extinction events has occurred throughout the history of the Earth, some even more devastating than K-T.
Mass extinctions are periods in Earth’s history when abnormally large numbers of species die out simultaneously or within a limited time frame.
The most severe occurred at the end of the Permian period when 96% of all species perished.
This along with K-T are two of the Big Five mass extinctions, each of which wiped out at least half of all species.
Many smaller scale mass extinctions have occurred, indeed the disappearance of many animals and plants at the hands of man in prehistoric, historic and modern times will eventually show up in the fossil record as mass extinctions.
Discover more about Earth’s major extinction events below.
Ordovician-Silurian mass extinction
The third largest extinction in Earth’s history, the Ordovician-Silurian mass extinction had two peak dying times separated by hundreds of thousands of years.
During the Ordovician, most life was in the sea, so it was sea creatures such as trilobites, brachiopods and graptolites that were drastically reduced in number. In all, some 85% of sea life was wiped out.
An ice age has been blamed for the extinctions – a huge ice sheet in the southern hemisphere caused climate change and a fall in sea level, and messed with the chemistry of the oceans.
This happened: 443 million years ago
Possible causes of this event: Climate change
Earth’s climate is not constant. Over geological time, the Earth’s dominant climate has gone from ice age to tropical heat and from steamy jungles to searing deserts.
When such climate change occurs abruptly – either in the form of a global warming or cooling – animals and plants have no time to adapt so mass extinctions can occur.
Late Devonian mass extinction
Three quarters of all species on Earth died out in the Late Devonian mass extinction, though it may have been a series of extinctions over several million years, rather than a single event.
Life in the shallow seas were the worst affected, and reefs took a hammering, not returning to their former glory until new types of coral evolved over 100 million years later.
In fact, much of the sea bed became devoid of oxygen, rendering it effectively out of bounds for anything except bacteria.
Changes in sea level, asteroid impacts, climate change and new kinds of plants messing with the soil have all been blamed for these extinctions.
This happened: 359 million years ago
Possible causes of this event: Impact events
Impact events, proposed as causes of mass extinction, are when the planet is struck by a comet or meteor large enough to create a huge shockwave felt around the globe.
Widespread dust and debris rain down, disrupting the climate and causing extinction on a global, rather than local, scale.
The demise of the dinosaurs at the end of the Cretaceous has been linked to an impact that left a crater in the seabed off the Yucatan peninsula of Mexico.
Impacts have also been blamed for other mass extinctions, but the timing and links between cause and effect for these is still debated by scientists.
Permian mass extinction
The Permian mass extinction has been nicknamed The Great Dying, since a staggering 96% of species died out. All life on Earth today is descended from the 4% of species that survived.
The event turns out to have been complex, as there were at least two separate phases of extinction spread over millions of years. Marine creatures were particularly badly affected and insects suffered the only mass extinction of their history.
Many causes have been proposed for the event: asteroid impact, flood basalt eruptions, catastrophic methane release, a drop in oxygen levels, sea level fluctuations or some combination of these.
This happened: 248 million years ago
Possible causes of this event: Catastrophic methane release; Flood basalt eruptions; Climate change; Impact events
Catastrophic methane release has been suggested as a possible cause of mass extinction. Methane clathrate is an ice-like substance formed from water and methane in the sea bed, arctic lakes and permafrost.
It forms where the temperature is at freezing or a little above and where the pressure of overlying water and sediment creates the right conditions. A temperature rise causes the methane in the clathrate to be released as gas.
Global warming results and causes further clathrate heating and methane release. The resultant soaring temperature causes such stress to plant and animal life that mass extinction follows.
Flood basalt eruptions are a type of large-scale volcanic activity, both in terms of extent and duration, that can occur on land or on the ocean floor.
A flood basalt may continue to erupt for tens of thousands – possibly millions – of years and the lava can cover hundreds of thousands of kilometres. Large plateaux and mountains can result from the huge volume of newly surfaced rock.
The huge volume of lava is accompanied by a similarly large release of volcanic gases such as sulphur dioxide and carbon dioxide. These can affect climate and cause acid rain, so flood basalts are thought to be a potential cause of mass extinctions.
Triassic-Jurassic mass extinction
During the final 18 million years of the Triassic period, there were two or three phases of extinction whose combined effects created the Triassic-Jurassic mass extinction event. Climate change, flood basalt eruptions and an asteroid impact have all been blamed for this loss of life.
Many types of animal died out, including lots of marine reptiles, some large amphibians, many reef-building creatures and large numbers of cephalopod molluscs.
Roughly half of all the species alive at the time became extinct. Strangely, plants were not so badly affected.
This happened: 200 million years ago
Possible causes of this event: Flood basalt eruptions; Climate change; Impact events
Cretaceous-Tertiary mass extinction
The Cretaceous-Tertiary mass extinction – also known as the K/T extinction – is famed for the death of the dinosaurs.
However, many other organisms perished at the end of the Cretaceous including the ammonites, many flowering plants and the last of the pterosaurs. Some groups had been in decline for several million years before the final event that destroyed them all.
It’s suggested that the decline was due to flood basalt eruptions affecting the world’s climate, combined with drastic falls in sea level. Then a huge asteroid or comet struck the seabed near the Yucatan Peninsula in Mexico and was the straw that broke the camel’s back.
This happened: 65 million years ago
Possible causes of this event: Flood basalt eruptions; Impact events
What was killed by this event: Reptiles
What the Earth was like
Death of a dynasty – the end of the Cretaceaous
At the end of the Cretaceous Period, 65 million years ago, all the dinosaurs died out. Why this happened is one of the most frequently asked – and intriguing – questions in palaeontology.
There have been many different ideas put forward to explain why the dinosuars died out. The two most likely are that their habitat slowly changed, and that a meteor impact triggered their extinction.
The gradualist hypothesis points to declines in the numbers and diversity of different groups of land and marine animals.
It suggests that the extinction of these groups was due to climate change. The climate at the end of the Cretaceous was cooling – and a fall in sea level reduced dinosaur and shallow water marine animal habitats.
The impact hypothesis gets a lot of press coverage because it is spectacular. There is good geophysical evidence for the occurrence of an asteroid impact at the end of the Cretaceous.
A band of clay rich in the mineral iridium was deposited at the end of the Cretaceous and has been found at many places in the world. This mineral is rare on Earth but more common in meteorites.
It has been suggested that the impact would have triggered a nuclear winter scenario that would have caused the death of the dinosaurs as well as the pterosaurs, several families of birds and mammals and also marine animals such as the plesiosaurs and ammonites.
At the end of the Cretaceous there were a lot of volcanic eruptions, at least in some parts of the world.
The Deccan Traps, huge flood basalts, were deposited at this time, and the dust and gases erupted at the same time would have had caused environmental changes over a wide area.
Will we ever know?
Unfortunately, while these hypotheses are plausible and they can both explain how many animals went extinct, neither can explain why certain animals died out while others survived.
Why did the dinosaurs, which were so successful, die out, while other animals such as frogs, which we know are environmentally sensitive, survive?
Although it is usually assumed that the dinosaurs all went extinct all at the same time all over the world, the truth of the matter is that we only have high resolution data for North America. In other parts of the world there is either no terrestrial record or we do not have good enough age resolution.
It is likely that as China and other countries outside of Europe and the US are studied more intensively we will be able to gather more data and build up a more comprehensive picture of what was going on in the world at the end of the Cretaceous period.