Evolution, 4th Edition

THE HiSToRy oF LiFE 451

end-Permian mass extinction, one of the most significant events in the history of

life. It is estimated that in this, the most massive extinction event in the history of

Earth, at least 56 percent of the genera and more than 80 percent of all species of

skeleton-bearing marine invertebrates became extinct within less than 200,000

years [7, 83]. Groups such as ammonoids, stalked echinoderms, brachiopods,

and bryozoans declined greatly, and major taxa such as trilobites and several

groups of corals disappeared entirely. Some orders of insects and many families

of amphibians and mammal-like reptiles became extinct, and the composition of

plant communities changed greatly [63]. The extinction was probably triggered

by vast volcanic eruptions in Siberia that covered 7 million km^2 (2.7 million mi^2 )

with layers of basalt as much as 6500 m (4 mi) deep [14, 46]. These eruptions

are thought to have released poisonous gases such as hydrogen sulfide and vast

quantities of carbon dioxide, which in turn caused global warming, aridity, and

increased acidity of ocean water (which interferes with the formation of calcium

carbonate shells and skeletons). The temperature change may have caused turn-

over in the water column and the reduced oxygen level that is evident in the

geological record from this time. On land, there were massive wildfires, defor-

estation, and soil erosion [27, 50, 83].

Futuyma Kirkpatrick Evolution, 4e

Sinauer Associates

Troutt Visual Services

Evolution4e_17.19.ai Date 01-05-2017

(D) Oligocene

30 Mya

(C) Late Cretaceous

90 Mya

(B) Late Jurassic

160 Mya

(A) Early Triassic

240 Mya

Mountainous highlands (>1500 m)

Other land masses

Shallow oceans (<200 m)

Ocean basins (>200 m)

Gondwana

Pangaea

FIGURE 17.19 Distribution of land masses in geological time. The outlines of the

modern-day continents are shown in maps C and D.; other black lines delineate

important plate boundaries. (A) In the early Triassic, most land was aggregated into

a single mass (Pangaea). (B) Eurasia and North America were separated by the late

Jurassic. (C) Gondwana had become fragmented into most of the major southern land

masses by the late Cretaceous. (D) By the Oligocene, the land masses were close to

their present positions. (Maps © 2004 by C. R. Scotese/ PALEOMAP Project.)

17_EVOL4E_CH17.indd 451 3/22/17 1:37 PM