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Chapter 26 The Tree of Life: - page 9 / 18





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Its habitat may have been destroyed, or its environment may have changed in a direction unfavorable to the species.

Biological factors may change, as evolutionary changes in one species impact others.

On a number of occasions, global environmental changes were so rapid and major that the majority of species went extinct.

Such mass extinctions are known primarily from the loss of shallow-water, marine, hard-bodied animals, the organisms for which the fossil record is most complete.

The Permian mass extinction defines the boundary between the Paleozoic and Mesozoic eras.

Ninety-six percent of marine animal species went extinct in less than 5 million years.

Terrestrial life was also affected.

The Cretaceous extinction of 65 million years marks the boundary between the Mesozoic and Cenozoic eras.

More than half of all marine species and many families of terrestrial plants and animals, including the dinosaurs, went extinct.

The Permian mass extinction happened at a time of enormous volcanic eruptions in what is now in Siberia.

These eruptions may have produced enough carbon dioxide to warm the global climate.

Reduced temperature differences between the equator and the poles would have slowed the mixing of ocean water.

The resulting oxygen deficit in the oceans may have played a large role in the Permian extinction.

A clue to the Cretaceous mass extinction is a thin layer of clay enriched in iridium that separates sediments from the Mesozoic and Cenozoic.

Iridium is a very rare element on Earth that is common in meteorites and other objects that fall to Earth.

Walter and Luis Alvarez and their colleagues at the University of California proposed that this clay is fallout from a huge cloud of debris that was thrown into the atmosphere when an asteroid or a large comet collided with Earth.

The cloud would have blocked sunlight and disrupted the global climate for several months.

A 65-million-year-old crater scar has been located beneath sediments on the Yucatán coast of Mexico.

At 180 km in diameter, it is the right size to have been caused by an object with a diameter of 10 km.

Much remains to be learned about the causes of mass extinctions.

Lecture Outline for Campbell/Reece Biology, 7th Edition, © Pearson Education, Inc. 26-9

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