Evolution, 4th Edition

THE HiSToRy oF LiFE 459

about 15 percent of the families and 47 percent of the genera of marine animals

[36]. Ammonoids, rudists, most marine reptiles, and many families of invertebrates

and planktonic protists became extinct.

The Cenozoic Era

The Cenozoic era, which started 66 Mya, marks the start of modern times (FIG-

URE 17.29). Both in the sea and on land, the flora and fauna have a more familiar

cast than in the Mesozoic. To be sure, many groups of animals that originated

and flourished in the Cenozoic have become extinct, but even these were closely

related enough to living forms that they seem less foreign to us than the plesio-

saurs, sauropods, and pterosaurs that they replaced.

By the beginning of the Cenozoic, North America had moved westward, becom-

ing separated from Europe in the east, but forming the broad Bering Land Bridge

between Alaska and Siberia (FIGURE 17.19D). The Bering Land Bridge remained

above sea level throughout most of the Cenozoic (see Figure 17.33A). Gondwana

broke up into the separate island continents of South America, Africa, India, and far

to the south, Antarctica plus Australia (which separated in the Eocene). About 18–14

Mya, during the Miocene, Africa made contact with southwestern Asia, India col-

lided with Asia (forming the Himalayas), and Australia

moved northward, approaching southeastern Asia.

During the Pliocene, the Isthmus of Panama arose, and

fully connected North and South America for the first

time about 2.8 Mya [54, 68].

This reconfiguration of continents and oceans con-

tributed to major climate changes. In the late Eocene

and Oligocene there was global cooling and drying;

extensive savannahs (sparsely forested grasslands)

formed for the first time, and Antarctica acquired

glaciers. Sea level fluctuated, dropping drastically in

the late Oligocene (about 25 Mya). In the Miocene,

cacti and other plant groups that were adapted to arid

conditions diversified in several parts of the world

[5]. During the Pliocene, temperatures increased

but then dropped again, and the Pleistocene epoch,

which started about 2.6 Mya, was marked by a series

of about 11 glacial-interglacial cycles. The most recent

such “ice age” ended only about 12,000 years ago.

The modern world takes shape

Over the course of more than 10 Ma, marine animal

diversity recovered from the end-Cretaceous extinc-

tion event, although the speed and amount of recov-

ery differed among animal groups and geographic

regions [37]. Some major groups, such as ammo-

noids, were gone forever, but groups such as gastro-

pods (snails, whelks) flourished, and marine diversity

eventually reached new heights [15]. Animals such

as crinoids (sea lilies) that stand on the ocean floor

were reduced, but the diversity and ecological impor-

tance of deep burrowers and especially of predators

increased. The Mesozoic marine revolution continued,

with more effective predatory crustaceans becoming

more prominent [93]. Marine mammals and a great

Futuyma Kirkpatrick Evolution, 4e

Sinauer Associates

Troutt Visual Services

Evolution4e_17.29.ai Date 02-16-2017

Cenozoic

Mya

2.58

23

66

Pleistocene

2.58–0.01 Mya

Quaternary

Holocene

(0.01 Mya–present)

Paleogene

Neogene

Pliocene

5.33–2.58 Mya

Miocene

23.03–5.33 Mya

Oligocene

33.9–23.03 Mya

Eocene

56.0–33.9 Mya

Paleocene

66.0–56.0 Mya

Mesozoic

Paleozoic

Proterozoic

Archean

Precambrian

Phanerozoic

FIGURE 17.29 Time line of the Cenozoic

era, with illustrations of some noteworthy

organisms.

17_EVOL4E_CH17.indd 459 3/22/17 1:37 PM