Evolution, 4th Edition

496 CHAPTER 19

variety of different modes of life and associated adaptations is far greater now than

in the Cambrian (FIGURE 19.6). The evolution of ecologically novel life forms, such

as dinosaurs, snakes, birds, and bats, likewise accounts for much of the increasing

diversity of tetrapods on land [8].

Rates of origination and extinction

The increase in diversity during the Mesozoic and Cenozoic tells us that on aver-

age, the rate of origination of marine animal taxa has been greater than the rate

of extinction. However, both rates have fluctuated throughout Phanerozoic his-

tory (FIGURE 19.7). Some lineages have become extinct during every geological

time interval—so-called normal or background extinction. But the fossil record

reveals several dramatic crashes in diversity, mass extinctions, when a great many

or even most species became extinct. What caused these global catastrophes is

one of the most fascinating questions in paleontology. Five mass extinctions are

generally recognized (see Chapter 17): at the end of the Ordovician, in the late

Devonian, at the Permian/Triassic (P/Tr) boundary (the end-Permian extinction),

at the end of the Triassic, and at the Cretaceous/Paleogene (K/Pg) boundary (the

K/Pg extinction).

David Raup and Jack Sepkoski discovered that the background extinction rate

has declined during the Phanerozoic [57], a conclusion supported by subsequent

studies (see Figure 19.7A). The rate of origination of new genera and families (see

Figure 19.7B–E) also declined after the Paleozoic, although it increased at certain

times, including after the “big five” mass extinctions, when diversity recovered,

usually within 10–15 My [1].

Futuyma Kirkpatrick Evolution, 4e

Sinauer Associates

Troutt Visual Services

Evolution4e_15.03.ai Date 02-02-2017

Q: Should x-axis be changed to (My) for consistency with other gures?

Should error bars be deleted to simplify?

Pg Ng

0

0

100

200

300

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500

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Diversity at start of interval

C O S D C P Tr J K

500 400 300 200 100

Time (Mya)

Paleozoic Mesozoic Cenozoic

FIGURE 19.5 Numbers of skeletonized marine animal genera over time, corrected

for biases such as temporal differences in rock volume and the pull of the Recent. The

smooth curve, a running average from the late Ordovician to the mid-Cenozoic, sug-

gests that, aside from mass extinctions and subsequent recoveries, diversity showed a

stronger increasing trend in the Mesozoic and Cenozoic than in the Paleozoic. (After [21].)

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