Evolution, 4th Edition

452 CHAPTER 17

Mesozoic Life

Now we come to history’s most romantic era, the one that most grips our imagi-

nation. Divided into the Triassic (252–201 Mya), Jurassic (201–145 Mya), and Creta-

ceous (145–66 Mya) periods, the Mesozoic era is often called the “Age of Reptiles,”

so named for some of the most extraordinary creatures of all time (FIGURE 17.20).

During the Mesozoic, Pangaea began to break up, beginning with the formation

of the Tethyan Seaway between Asia and Africa, and then the full separation of a

northern land mass, called Laurasia, from a southern continent known as Gond-

wana. Laurasia began to separate into several fragments during the Jurassic (FIG-

U R E 17. 19B), but northeastern North America, Greenland, and western Europe

remained connected until well into the Cretaceous. The southern continent, Gond-

wana, consisted of Africa, South America, India, Australia, New Zealand, and

Antarctica. These land masses slowly separated in the late Jurassic and the Cre-

taceous, but even then the South Atlantic formed only a narrow seaway between

Africa and South America (FIGURE 17.19C). Throughout the Mesozoic, sea level

rose, and many continental regions were covered by shallow seas. Although the

polar regions were cool, most of Earth enjoyed warm climates: Antarctica had

forests and dinosaurs. Global temperatures reached an all-time high in the mid-

Cretaceous, after which substantial cooling occurred.

MARiNE LiFE Extinctions continued during the earliest Triassic, but diversity

slowly recovered. Many of the marine groups that had been decimated during the

end-Permian extinction again diversified. Ammonoids, for example, increased from

2 to more than 100 genera by the middle Triassic (see Figure 17.12). Planktonic fora-

miniferans (shelled protists) and modern corals evolved, and bony fishes continued

to radiate. Another mass extinction occurred at the end of the Triassic, associated with

a massive release of carbon into the atmosphere and global warming [79]. Marine

biodiversity decreased by about half, and groups such as ammonoids and bivalves

were devastated, but then recovered and experienced yet another adaptive radiation.

The teleosts, today’s dominant group of bony fishes, evolved and began to diversify.

During the Mesozoic and continuing into the early Cenozoic, predation seems to have

escalated [35, 93]. During this so-called Mesozoic marine revolution, crabs and bony

fishes evolved mechanisms for crushing mollusc shells, and molluscs evolved pro-

tective mechanisms such as thick shells and spines (FIGURE 17.21).

During the Jurassic and Cretaceous, modern groups of gastropods (snails and

relatives), bivalves, and bryozoans rose to dominance; gigantic sessile bivalves (rud-

Futuyma Kirkpatrick Evolutionists) formed reefs; and the seas harbored several groups of large marine reptiles. , 4e

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Evolution4e_17.20.ai Date 01-17-2017

Precambrian

Phanerozoic

Cenozoic

Mya

66

145

201

252

Cretaceous

145–66 Mya

Jurassic

201–145 Mya

Triassic

252–201 Mya

Mesozoic

Paleozoic

Proterozoic

Archean

FIGURE 17.20 Time line of the Me-

sozoic era, with illustrations of some

noteworthy species. Triassic: Ginkgo

and phytosaur. Jurassic: cycad and

Apatosaurus. Cretaceous: magnolia

and Tyrannosaurus.

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