Life on earth began 3.7 billion years ago in the worldwide ocean. It was microbes but today, earth’s life is so different and so advanced. Now 8.7 million species live on earth. They occupied the surface, ocean, sky, and underground. But the journey of life was not a straight curve. It has spent tough times like mass extinctions.

How many mass extinctions have there been in the past 500 million years?

According to scientists, there were five mass extinction events have happened on earth in an observed 500 million years. All these happened to a diverse set of species who live forms at various times. So, what are the five mass extinctions?

  1. Ordovician-Silurian Extinction (440 million years ago)
  2. Devonian Extinction (365 million years ago)
  3. Permian-Triassic Extinction (250 million years ago)
  4. Triassic-Jurassic Extinction (210 million years ago)
  5. Cretaceous-Tertiary Extinction (65 million Years Ago)

Define mass extinctions

Those species who can’t adapt to the environment they live in, go extinct every time. So how can we define mass extinction? Mass extinctions are events in which more than 50% percent of existing species at that time died off within 2.8 million years. For more clarification, a species is a group of living organisms consisting of similar individuals capable of exchanging genes or interbreeding.

Most mass extensions are not catastrophic events for our regular measuring of time. But from a geological scale, it is a fraction of the time. The maximum time for a mass extinction event is still 1/2000 of the total age of life on earth.

Ordovician-Silurian extinction

445.2 million years ago life on earth faced an event they never faced before. That is the first known mass extinction the Ordovician-Silurian extinction. It happened in a 1.4 million years period between 445.2 million to 443.8 million years ago. In the age, we know as the Hirnantian age.

After the Permian period, it was a time of animals are rioting in the world. This extinction is happening in the ocean because almost all life was marine life at that time. But it was also the movements of plants beginning their journey on land.

With marine life including arthropods like trilobites, vertebrates like conodonts who resemble eels, snails, and cephalopods life was rich and diverse.

The Land was divided into two supercontinents. The southern supercontinent Gondwana was on its way toward the southern pole. Laurentia and Baltica were other continents. But most of the north was the ocean.

What caused the first mass extinction?

Scientists believe climate change caused this mass extinction. It’s like the shift of Gondwana toward the southern pole change the earth’s climate significantly. A huge ice cap was created and the sea levels dropped drastically. The extinction even came in two pulses. First on is because of the cold environment. The southern hemisphere was in an ice age. The second pulse came later when the environment became warmer.

Another reason given for this event is a gamma-ray burst from super-novae. Such an event can happen 1 in a billion years. Radiation effects on life forms and the ozone layer could be the cause of the world’s second large one of all mass extinctions.

Approximately 27% of marine families and 60% of marine genera went extinct. 85% of species that lived on the earth at that time died. The first mass extinction event ended like that cleaning rooms for survivors. Trilobites, brachiopods, corals, crinoids, and graptolites.

Late Devonian extinction

375 million years ago the second mass extinction has happened affecting both marine and landscape life. It happened for 15 years (375 million to 360 million years ago). The Devonian period is between 419.2 million and 358.9 million years ago.

At that time 85% of the earth’s surface was covered by water. The land was split into three continents. Gondwana, a supercontinent in the south consists of today’s South America, Africa, Antarctica, India, and Australia. Then we had Laurussia-Euramerica consists of today’s  North America and Europe. Small continent Siberia was also present at the time.

After the Ordovician-Silurian mass extinction, life on earth had slightly changed. On the ocean, there was new and old marine life. Brachiopods, the hard-shelled animals were dominating the vast ocean. Crinoids and echinoderms could be popular too. Coral species also took the spot like Rugose corals and Tabulata. Rugose corals managed to make large reefs in shallow waters. New fishes also starting to emerge such as Benthic, jawless fish, armored fish, and lobe-finned fish.

On the landscape, the first tetrapods (land-living vertebrates) appeared such as early amphibians. There were many arthropods. Wingless insects were all over the place. The earliest arachnids (Sorpians, spiders) and mites could be seen.

Early terrestrial vegetation that triggered the previous extinction event, now growing and slowly occupying the land. They now have developed roots-like structures. However, they were still nothing like today’s trees. Instead, they are like few centimeters tall. Zosterophylls and Trimerophytes are two categories the plant life in Devonian life can be divided into.

What caused the late Devonian extinction?

The causes of the mass destruction of life had made the earth’s climate colder. It resulted in changing sea levels. Ocean hyperoxia is another fuel to this fire. The causes are still uncertain but many scientists believe it was super volcanic eruptions that makes the temperature go down. Another hypothesis says about an asteroid or comet impact.

Cosmic rays from an explosion of a star could have done that. Some spores in the fossil record have corrupted DNA which could be a result of radiation due to cosmic rays. They also could harm the ozone layer which further increases the radiation levels.

We cannot forget the vegetation on the land. In the late Devonian period, they were tall and some of them were flowering plants. They reduced the carbon dioxide levels and as a result, oxygen is also increased. Their roots break down rocks into minerals which later flowed to oceans. That caused a massive increase in algae then results in oxygen-fed bacteria. So, the oxygen in the water was decreased and many species died out.

Despite what happened, it was not very good for living beings. Half of the genres vanished. A quarter of marine animals died out. 70% of total species including 85% of animal species also extinct. Some stomatopods, Corals, and brachiopods could not see ever again.

Permian-Triassic extinction

The Permian period is the foundation of modern life. At the beginning of the period, 23% of the earth’s atmosphere was oxygen. Carbon dioxide was 900 parts per million. With those atmospheric conditions, the average temperature on earth was 16 degrees of celsius only. The ice age began in the Carboniferous period still going on. Sea-level was 60% lower than it is today.

There was an interesting phenomenon happening with the landscape. Two supercontinents Gondwana and Lurussia were combined and made one supercontinent known as Pangea. The formation of the Pangea affects the environment and life significantly. Since clouds can’t reach the central sides of the Pangea most area dried out creating large desert areas. They were bigger than any dessert today. The areas were Gondwana and Lurussia (created mountains)

Now terrestrial plants are living where water can be found. Ferns, seed ferns, and lycophytes are some of the terrestrial plants at that time. Animal habitats changed to rely on plants. Insects did that well and they evolved rapidly. You could see the largest insects ever lived on the planet if you can travel back to the Permian time. Predator griffin flies and dragonfly-like insects were common.

However, the most special thing about the life of the Permian is the rise of reptiles. The reptile population increased throughout the period with diversifying species. The dry condition in Pangea must have been an opportunity for them. Mammal-like dimetrodon, scutosaurus, dicynodonts, and Gorygonopsids are some of the common reptiles in the Permian period.

What happened in the third mass extinction?

All of this started to change radically 252 million years ago. We don’t know the reason behind the mass extinction event. But the most believed hypothesis is a volcanic eruption. Siberian traps had a massive eruption exact time of the start of the Permian extinction. It is one of the largest if not the largest volcanic eruptions in the last 500 million years.

Volcanos ejected an unseen amount of magma to the surface. It was significant enough to cover up the entire United States mainland. This event increased global temperature by 15 degrees of celsius. This rapid change paves the way for many species to extinction. But it was only the beginning.

Volcanic asses spread to the earth’s atmosphere. They brought super dioxide. Suhulper dioxide and water mixed in the atmosphere and acid rain s starts to fall. Much terrestrial vegetation die because of that and it destroyed the animal life that relied on plants. Carnivores also started to die because now their food is gone. But the worst part is yet to come.

More heat

Carbon dioxide released from the Siberian traps was trapped in the atmosphere. Since most vegetation is gone there was no way to reduce them. So another 15 degrees of Celsius temperature hike thanks to the greenhouse. Oceans were warmed and algae were the only survivors. Some parts of oceans started to get a pink color because of an algae species. Algae activities release more carbon dioxide temperature increases by 6 Celsius.

Earth’s average temperature is now 42 degrees of Celsius. 90% of earth’s bellowed species are now gone. That includes 96% marine life and 70% terrestrial life. Due to these massive extinctions, we call the end of Permian extinction ‘The great death’. It took about 2 million years for the environment, 5 million years for vegetation, and 10 million years for biodiversity to recover. So, the largest mass extinction of all mass extinctions ended giving dinosaurs the first place.

Triassic-Jurassic Extinction

The Triassic period began 248 million years ago. Afterlife faced many losses, new species started to roam on the supercontinent Pangea. Pangea started to split up in the middle of this period. It was the period of the beginning of dinosaurs. Reptiles start to dominate the world but life has to face another disaster at the end of the Triassic period.

It started 201.3 million years ago. Scientists are still not sure what caused the Triassic-Jurassic mass extinction. However, amniote fossils, conodont fossils, and mercury in sediments prove this extinction event. There are three causes they assume could have happened.

Climate change could do a good job of extracting species. Rising sea levels realized extra carbon dioxide in the atmosphere causing global warming is a well-known phenomenon. Scientists also found high methane activity. It can affect the climate more than carbon dioxide.

The main hypothesis is volcanic activities. Rifting of Pangea could trigger such events. Carbon dioxide released from volcanos must have increased the acidity of the ocean, destroying marine life.

Extraterrestrial triggers could be the cause. Space objects like meteors, asteroids, and comets can impact life on earth.

Whatever the cause could be a lot of species suffered the extinction. 23% of marine genera vanished including fascinating species like brachiopods, shelled cephalopods, sponges, and corals. The landscape also suffered many losses. Phytosaurs and crocodile-like animals are extinct. 76% of marine and terrestrial species vanished and 60% of pollen assemblages are gone. That gave an opportunity for corolina who dominated them in the Jurassic period. We lost all archosaurs in the Triassic-Jurassic extinction.

Cretaceous-Tertiary Extinction

66 million years ago, when the last mass extinction happened, the great era of dinosaurs came to an end. Many hypotheses were introduced trying to explain how that happened. But it was about an asteroid impact that took the spotlight.

It could be a meteorite or comet. When this asteroid hit the earth a massive amount of heated rock debris ejected into the atmosphere. The ejecta cloud spread around the world increasing temperatures and turning the earth into hell. Some of those who tried to escape the ejecta cloud were attacked by the melted rock debris.

Small animals like mammals took shelter underground. All non-avian dinosaurs vanished from the earth. After all, 76% of all species have gone extinct. Cretaceous-Tertiary extinction is important to humans because it is one of the mass extinctions that pave the way for our kind. Read more about the dinosaurs and their extinction here.

Are we in a mass extinction?

Over 75% percent species in 2 million years. That is not so fancier. Over the past 700 years, over 1000 mammal species and over 45000 amphibian species have become extinct. This is way faster than the five mass extinctions. So, it is not wrong to assume that we are in the 6th mass extinction. In this time, we humans are the asteroid. Who could survive the 6th mass extinction? That is another topic to discuss.

So, all of you are survivors. It is your responsibility to gain more knowledge about life on earth. We have plenty of stuff that you may find interesting. Stay with us.

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