12.36 Mass Extinction

What happened to the dinosaurs?

Most of the dinosaurs disappeared from Earth about 65 million years ago. This is probably the most famous example of a mass extinction. So how do you define a mass extinction?

Mass Extinctions

An organism goes extinct when all of the members of a species die out and no more members remain. Extinctions are part of natural selection. Species often go extinct when their environment changes, and they do not have the traits they need to survive. Only those individuals with the traits needed to live in a changed environment survive (Survival of the Fittest) (Figure below). 

Humans have caused many extinctions by introducing species to new places. For example, many of New Zealand’s birds have adapted to nesting on the ground. This was possible because there were no land mammals in New Zealand. Then Europeans arrived and brought cats, foxes, and other predators with them. Several of New Zealand’s ground nesting birds, such as this flightless kiwi, are now extinct or threatened because of these predators.

Mass extinctions, such as the extinction of dinosaurs and many marine mammals, happened after major catastrophes such as volcanic eruptions and earthquakes (Figure below).

The fossil of Tarbosaurus, one of the land dinosaurs that went extinct during one of the mass extinctions.

Since life began on Earth, there have been several major mass extinctions. If you look closely at the geological time scale, you will find that at least five major mass extinctions have occurred in the past 540 million years. In each mass extinction, over 50% of animal species died. Though species go extinct frequently, a mass extinction in which such a high percentage of species go extinct is rare. The total number of mass extinctions could be as high as 20. It is probable that we are currently in the midst of another mass extinction.

Two of the largest extinctions are described below:

• At the end of the Permian Period, it is estimated that about 99.5% of individual organisms went extinct! Up to 95% of marine species perished, compared to “only” 70% of land species. Some scientists theorize that the extinction was caused by the formation of Pangaea, or one large continent made out of many smaller ones. One large continent has a smaller shoreline than many small ones, so reducing the shoreline space may have caused much of the marine life to go extinct (Figure below).

The supercontinent Pangaea encompassed all of today’s continents in a single land mass. This configuration limited shallow coastal areas which harbor marine species. This may have contributed to the dramatic event which ended the Permian—the most massive extinction ever recorded.

• At the end of the Cretaceous Period, or 65 million years ago, all dinosaurs (except those which led to birds) went extinct. Some scientists believe a possible cause is a collision between the Earth and a comet or asteroid. The collision could have caused tidal waves, changed the climate, increased atmospheric dust and clouds, and reduced sunlight by 10-20%. A decrease in photosynthesis would have resulted in less plant food, leading to the extinction of the dinosaurs.

Evidence for the extinction of dinosaurs by asteroid includes an iridium-rich layer in the Earth, dated at 65.5 million years ago. Iridium is rare in the Earth’s crust but common in comets and asteroids. Maybe the asteroid that hit the Earth left the iridium behind.

After each mass extinction, new species evolve to fill the habitats where old species lived. This is well documented in the fossil record.

Further Reading

Human Causes of Extinction

Summary

• Extinctions, when a species entirely dies out, can happen when the environment changes, and the organisms do not have the traits they need to survive.
• Since life began on Earth, there have been at least five major massive extinctions.

Explore More

Use the resource below to answer the questions that follow.

1. What is a mass extinction? How many have occurred in the last 600 million years?
2. What are some of the reasons proposed for mass extinctions?
3. Which event wiped out 95% of animal species in the oceans? When did this occur?
4. How do scientists think the activity of the "Siberian Traps" changed the chemistry of the oceans?
5. How does the temperature of water affect how much gas can be dissolved in it?
6. How did the changing ocean environment lead to a mass extinction?

Review

1. Why do species sometimes go extinct?
2. What is a mass extinction?
3. What may have caused the mass extinction that killed the dinosaurs, and what is the evidence?