Science

Explained: What we know about mass extinctions

Scientists have sounded an alert on the possibility of a sixth mass extinction event after measuring the rate at which species are going extinct or tending to go extinct. In a study published in the journal Science this month, the researchers had reported that large sized marine mammals faced a greater threat of extinction. Till date, Earth has witnessed five big mass extinction events. One reassuring factor is that these “events” take place over tens of thousands of years...

How do they happen?

Mass extinctions have happened earlier in geologic time. Five major events have been established and studied by geologists. Anything, from a meteor collision to climate change or a volcanic eruption can trigger the extinction of a large number of species. These events demonstrate the perseverance and tenacity of life. Life seems to take on various forms and diversify and change, but it does bounce back eventually. Evolutionary niches created by the extinction of one species or family leads to the diversification of another.

Here is what happened during five major mass extinction events in the past.

Click on the green dots in the interactive above to learn more about each event. [Graphic by Dhivya E.]

The end-Ordovician Hirnatian mass extinction (444 Ma)

The Ordovician period spanned nearly 45 million years, from 488.3 million years ago (Ma) to 443.7 Ma. The world then was very different from what it is now – the northern hemisphere almost entirely consisted of water as the continents had not yet formed. The entire landmass – Gondwana – was stuck together in the south. Gondwana was a huge supercontinent believed to have formed between 570 and 510 Ma.

Living organisms mostly populated the sea and there was hardly anything on the rocky land. Studies of fossil records have pieced together the life forms of this time, which include what are called trilobites, eel-like conodonts (toothed animals), blastoids, corals, snails, clams, cephalopods, etc. The fish that existed at this time were mostly jawless and some of the earliest vertebrate fossils belong to this period. Jawed fishes are a minority among fossils from this period.

A sudden intense cooling followed by a fall in sea levels, coinciding with rapid glaciation led to the habitat destruction and extinction of a majority of genera, especially of marine invertebrates. This extinction is said to be equivalent in scope to the end-Permian and end-Cretaceous extinctions. Nearly 85 per cent of marine life came to an end with this mass extinction that is believed to be the third largest.

Late Devonian mass extinction

By the Devonian period land on Earth was being colonised by bacteria and algae.

It was during the Devonian period (419.2 Ma to 358 Ma) that life forms first hopped on to land from the seas. Plants and insects first spread out on the land masses during this time. Slowly plants evolved and during the end of this period, seed-bearing plants had evolved.

The land masses had split into three – present-day Europe and North America were near the equator, portions that later formed South America, Africa, Antarctica, India and Australia were joined and formed a mass in the south and what is known today as Siberia was a piece of land in the north.

Two events in this period have been heavily investigated - the Kellwasser event (374.5 Ma) which led to the loss of 60 per cent of genera and up to 82 per cent of species and the later, Hangenberg event (359 Ma). Between these two, the Hangenberg event seems to show all the signs of a mass extinction.

Many causes have been attributed to these – glaciation and lowering of sea levels being one and meteorite impacts being the other.

At the end of this period, the first vertebrates climbed on to the land.

End Permian event

The Permian period (299 – 251 Ma) witnessed the most massive extinction event in which most of life was demolished.

At this time, the tectonic action had brought together most of the land masses to form one great mass called Pangea. This stretched from the North Pole to the South Pole. Asia was separate. There was a single huge ocean known as Panthalassa.

About 96 per cent of marine species and 70 percent of those on land were wiped out in this extinction event known as “the great dying” which took place approximately 250 Ma. Even insects were not spared in this. The minority that survived lived on to proliferate, and the rest told their story through fossil remains!

Volcanic eruptions, meteor impacts and climate change due to the release of huge amounts of methane are said to be responsible for this event.

Triassic-Jurassic event

This event marks the boundary of the Triassic age and the Jurassic age and took place about 200 Ma. In the beginning of this age, land existed as Pangea, as a supercontinent that incorporated almost all the landmasses on Earth (Encyclopaedia Britannica). Around the mid-Triassic, land masses started breaking away. With this event, nearly half the species living at the time became extinct: reef-building animals, marine reptiles, some amphibians, etc.

This event created ecological niches, leading to the growth of dinosaurs, for example. There are several reasons for this event such as gradual climate change, ocean acidification, meteor hit and volcanic eruptions. One clue is that the event lasted less than 10,000 years ago, and hence was rather sudden.

Cretaceous-Tertiary mass extinction

This event was the one that destroyed the dinosaurs, a mere 65 Ma. Apart from dinosaurs many other organisms, even flowering plants died during this event. It is believed that a massive comet or asteroid impact was what caused this sudden event.

Due to the ecological niches created by this event, many species, including mammals, diversified to a large degree.

(sources: Wikipedia, BBC Nature and University of California Museum of Paleontology)


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