The Earth has not experienced five, but six times, a massive extinction wave, American and Chinese scientists have discovered. Around 260 million years ago, the concentration of greenhouse gases increased as a result of intense volcanic eruptions. The warming that followed, large numbers of plant and animal species became too much.
The best known mass extinction is that at the end of the chalk, 66 million years ago. The dinosaurs then died out as a result of a comet impact.
Other extinction waves known in science come from a much deeper earthly past, some 200 to 450 million years ago. Scientists discovered them when studying rock layers that they discovered that in a younger rock all kinds of previously general fossils no longer occur 'suddenly'.
And that happened during perm (300 to 250 million years ago) not once, but twice, write researchers in the journal Historical Biology . They point to deposits in the western US, which contain a good historical archive of events around 260 million years ago.
The disappearance of entire families and genders
Already in the nineties it was discovered that many species had disappeared during that period, but further study shows that at the same time at least as much biodiversity was lost as with the other five large extinction waves.
According to the usual definition, at least three quarters of all larger species die out during a mass extinction. The new research shows that 260 million years ago, a large proportion of families and genera also disappeared - these are layers in biology, in which species are classified. The researchers therefore think that this period has been the third worst mass extinction in the history of the earth.
See also: According to scientists, Earth is in the middle of mass extinction species
Biggest extinction wave two-stage rocket with one cause
Afterwards, life barely recovered. In fact, 'only' 10 million years later, the largest mass extinction followed, which experts also call The Great Dying.
If you look at the graph of historical species richness on Earth, you could also merge the two mass extinctions as a two-stage rocket. The second blow made life on earth almost fatal. There are scientists who think that due to all sorts of domino effects it had made little difference to the environment or that the environment had become too toxic and unsuitable for all complex life.
That just didn't happen. In the oceans, 4 percent of the species survived. And in the 250 million years that followed, those species eventually produced the largest species wealth on earth.
There is another good reason to link the two mass extinctions together. They have the same (first) cause: so-called flood basaltes. This is a rare and intense form of volcanic eruption, during which large amounts of CO2 are released.
There is evidence of this left in southwest China: 300,000 cubic kilometers of petrified lava flows. Such outbursts were repeated ten million years later on another ten times as large a scale in Siberia.
The amount of biodiversity depends on plate tectonics
Other studies also show that the amount of biodiversity on earth depends on plate tectonics. These are phases in geology in which continents collide or break open, or in this specific case: moving over a very hot so-called mantle plume, a hot spot of volcanic eruptions. This hotspot is nowadays located in a safer place, under the Atlantic Ocean near Iceland.
In volcanic troubled periods, where continents are densely pressed together, the species diversity declined further and further, while in quiet periods it increases.
Two exceptions are known. These are the comet at the end of the chalk and the current biodiversity crisis, which, according to ecologists, also threatens to result in mass extinction. Not the 'sixth', as it is often called, but the seventh.
See also: 'Restoring current mass extinction of animal species takes millions of years'
Understanding historical extinctions important for current loss of biodiversity
The authors in Historical Biology say that studying very rare catastrophic events from the past provides insight into the current biodiversity loss.
"To properly investigate the causes, it is crucial that we know the number of serious extinction waves and their precise timing," said New York University lead author Michael Rampino.
"Massive volcanic eruptions brought large amounts of greenhouse gases into the atmosphere, especially CO2 and methane. That causes severe climate change, with warm and low-oxygen oceans that are unsuitable for marine life."