Extintions That Occur Continuously but a Very Low Rate

Throughout the 4.6 billion years of Earth's history, there have been five major mass extinction events that each wiped out an overwhelming majority of species living at the time. These five mass extinctions include the Ordovician Mass Extinction, Devonian Mass Extinction, Permian Mass Extinction, Triassic-Jurassic Mass Extinction, and Cretaceous-Tertiary (or the K-T) Mass Extinction.

Each of these events varied in size and cause, but all of them completely devastated the biodiversity found on Earth at their times.

Defining 'Mass Extinction'

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Before learning more about these different mass extinction events, it is important to understand what can be classified as mass extinction and how these catastrophes shape the evolution of species that happen to survive them. A "mass extinction" can be defined as a time period in which a large percentage of all known living species go extinct. There are several causes for mass extinctions, such as climate change, geologic catastrophes (e.g. numerous volcanic eruptions), or even meteor strikes onto Earth's surface. There is even evidence to suggest that microbes may have sped up or contributed to some of the mass extinctions known throughout the Geologic Time Scale.

Mass Extinctions and Evolution

The tardigrade (water bear) has survived all 5 major mass extinction.

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How do mass extinction events contribute to evolution? After a large mass extinction event, there is typically a rapid period of speciation among the few species that do survive; since so many species die off during these catastrophic events, there is more room for the surviving species to spread out, as well as many niches in the environments that need to be filled. There is less competition for food, resources, shelter, and even mates, allowing the "leftover" species from the mass extinction event to thrive and reproduce rapidly.

As populations separate and move away over time, they adapt to new environmental conditions and are eventually reproductively isolated from their original populations. At that point, they can be considered a brand new species.

The First Major Mass Extinction: The Ordovician Mass Extinction

Fossil trilobites from the Ordovician age.

John Cancalosi / Getty Images

The Ordovician Mass Extinction

• When: The Ordovician Period of the Paleozoic Era (about 440 million years ago)
• Size of the Extinction: Up to 85% of all living species eliminated
• Suspected Cause or Causes: Continental drift and subsequent climate change

The first known major mass extinction event occurred during the Ordovician Period of the Paleozoic Era on the Geologic Time Scale. At this time in the history of Earth, life was in its early stages. The first known life forms appeared about 3.6 billion years ago, but by the Ordovician Period, larger aquatic life forms had come into existence. There were also even some land species at this time.

The cause of this mass extinction event is thought to be the shift in the continents and drastic climate change. It happened in two different waves. The first wave was an ice age that encompassed the entire Earth. Sea levels lowered and many land species could not adapt fast enough to survive the harsh, cold climates. The second wave was when the ice age finally ended—and it was not all good news. The episode ended so suddenly that the ocean levels rose too quickly to hold enough oxygen to maintain the species that had survived the first wave. Again, species were too slow to adapt before extinction took them out completely. It was then up to the few surviving aquatic autotrophs to increase the oxygen levels so new species could evolve.

The Second Major Mass Extinction: The Devonian Mass Extinction

This limestone is full of bryozoa, crinoid, and brachiopod fossils from the Devonian period.

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The Devonian Mass Extinction

• When: The Devonian Period of the Paleozoic Era (about 375 million years ago)
• Size of the Extinction: Nearly 80% of all living species eliminated
• Suspected Cause or Causes: Lack of oxygen in the oceans, quick cooling of air temperatures, volcanic eruptions and/or meteor strikes

The second major mass extinction in the history of life on Earth happened during the Devonian Period of the Paleozoic Era. This mass extinction event actually followed the previous Ordovician Mass Extinction relatively quickly. Just as the climate stabilized and species adapted to new environments and life on Earth began to flourish again, almost 80% of all living species—both in the water and on land—were wiped out.

There are several hypotheses as to why this second mass extinction occurred at that time in geologic history. The first wave, which dealt a major blow to aquatic life, may have actually been caused by the quick colonization of land—many aquatic plants adapted to live on land, leaving fewer autotrophs to create oxygen for all of the sea life. This led to mass death in the oceans.

The plants' quick move to land also had a major effect on the carbon dioxide available in the atmosphere. By removing so much of the greenhouse gas so quickly, temperatures plummeted. Land species had trouble adapting to these changes in climate and went extinct as a result.

The second wave of the Devonian mass extinction is more of a mystery. It could have included mass volcanic eruptions and some meteor strikes, but the exact cause is still considered unknown.

The Third Major Mass Extinction: The Permian Mass Extinction

Dimetrodons went extinct in The Great Dying.

Stephen J Krasemann / Getty Images

The Permian Mass Extinction

• When: The Permian Period of the Paleozoic Era (about 250 million years ago)
• Size of the Extinction: An estimated 96% of all living species eliminated
• Suspected Cause or Causes: Unknown—possibly asteroid strikes, volcanic activity, climate change, and microbes

The third major mass extinction was during the last period of the Paleozoic Era, called the Permian Period. This is the largest of all known mass extinctions with a massive 96% of all species on Earth completely lost. It is no wonder, therefore, that this major mass extinction has been dubbed "The Great Dying." Aquatic and terrestrial life forms alike perished relatively quickly as the event took place.

It is still much of a mystery what set off this greatest of the mass extinction events, and several hypotheses have been thrown around by scientists who study this time span of the Geologic Time Scale. Some believe there may have been a chain of events that led to so many species disappearing; this could have been massive volcanic activity paired with asteroid impacts that sent deadly methane and basalt into the air and across the surface of the Earth. These could have caused a decrease in oxygen that suffocated life and brought about a quick change in the climate. Newer research points to a microbe from the Archaea domain that flourishes when methane is high. These extremophiles may have "taken over" and choked out life in the oceans as well.

Whatever the cause, this biggest of the major mass extinctions ended the Paleozoic Era and ushered in the Mesozoic Era.

The Fourth Major Mass Extinction: The Triassic-Jurassic Mass Extinction

About half of the known species on Earth perished during the Triassic-Jurassic Mass Extinction.

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The Triassic-Jurassic Mass Extinction

When: The end of the Triassic Period of the Mesozoic Era (about 200 million years ago)

Size of the Extinction: More than half of all living species eliminated

Suspected Cause or Causes: Major volcanic activity with basalt flooding, global climate change, and changing pH and sea levels of the oceans

The fourth major mass extinction was actually a combination of many, smaller extinction events that happened over the last 18 million years of the Triassic Period during the Mesozoic Era. Over this long time span, about half of all known species on Earth at the time perished. The causes of these individual small extinctions can, for the most part, be attributed to volcanic activity with basalt flooding. The gases spewed into the atmosphere from the volcanoes also created climate change issues that changed sea levels and possibly even pH levels in the oceans.

The Fifth Major Mass Extinction: The K-T Mass Extinction

The K-T extinction was responsible for the end of the dinosaurs.

Richard T. Nowitz / Getty Images

The K-T Mass Extinction

• When: The end of the Cretaceous Period of the Mesozoic Era (about 65 million years ago)
• Size of the Extinction: Nearly 75% of all living species eliminated
• Suspected Cause or Causes: Extreme asteroid or meteor impact

The fifth major mass extinction event is perhaps the best-known, despite it not being the biggest. The Cretaceous-Tertiary Mass Extinction (or K-T Extinction) became the dividing line between the final period of the Mesozoic Era—the Cretaceous Period—and the Tertiary Period of the Cenozoic Era. It is also the event that wiped out the dinosaurs. The dinosaurs were not the only species to go extinct, however—up to 75% of all known living species died during this mass extinction event.

It is well-documented that the cause of this mass extinction was a major asteroid impact. The huge space rocks hit Earth and sent debris into the air, effectively producing an "impact winter" that drastically changed the climate across the entire planet. Scientists have studied the large craters left by the asteroids and can date them back to this time.

The Sixth Major Mass Extinction: Happening Now?

A. Bayley-Worthington / Getty Images

Is it possible that we are in the midst of the sixth major mass extinction? Many scientists believe we are. A number of known species have been lost since humans' evolution. Since these mass extinction events can take millions of years, perhaps we are witnessing the sixth major mass extinction event as it happens. Whether or not humans will survive has yet to be determined.

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Source: https://www.thoughtco.com/the-5-major-mass-extinctions-4018102

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