Biology Now, 2e

The First Bird ■ 263

this has happened—that is, the number of species

that have gone extinct during a given period—has

varied over time, from low to very high. At the

upper end of this scale, the fossil record shows

that there have been five mass extinctions,

periods of time during which great numbers of

species go extinct (Figure 14.15).

Although difficult to determine, the causes of

the five mass extinctions are thought to include

such factors as climate change, massive volcanic

eruptions, changes in the composition of marine

and atmospheric gases, and sea level changes.

The Cretaceous extinction event occurred

the earliest bird.” There are likely to be more

fossils that will shake up the bird tree, says Xu,

but that’s a good problem to have in science.

“There are so many new species, it just makes it

difficult for us all to agree.”

Xu, Godefroit, and others are sure to continue

to dig up dinosaur fossils for some time yet. Dino-

saurs arose about 230 mya and dominated the

planet from about 200 mya to about 65 mya.

Then the majority of them went extinct, except

for those that evolved into birds. As the fossil

record shows, species have regularly gone extinct

throughout the history of life. The rate at which

Cambrian Ordovician SilurianDevonian Carboniferous Permian Triassic Jurassic Cretaceous Paleogene/Neogene

540 490 445 415 360 300 250 200 145 65 2.60.0

mya

Extinction

Extinction

Extinction

Extinction

Extinction

Ordovician: 50% of animal

families, including many trilobites.

Devonian: 30% of animal families,

including many fishes and trilobites.

Triassic: 35% of animal families—in

Permian: 60% of animal families, particular, many reptiles.

including many marine species, insects,

amphibians, and all remaining trilobites.

Cretaceous: 50% of animal families,

including the last of the non-avian

dinosaurs and many marine species.

Quaternary

Groups experiencing

mass extinction

Bar width represents

number of living families

Period

Figure 14.15

Mass extinctions and biodiversity

In addition to the marine and terrestrial animal groups shown here, plant groups were severely affected by the five mass

extinctions that have occurred in Earth’s history. After each extinction, life again diversified.

Q1: What extinction event occurred about 200 mya? What animal groups were most affected by this event?

Q2: Which of the mass extinctions appears to have removed the most animal groups? How long ago did this extinction occur?

Q3: The best studied of the mass extinctions is the Cretaceous extinction. Why do you think it has been better studied

than the other extinctions?