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point where the cryosphere expanded (and it has never returned to its previous state). Another example is the abrupt shift of meltwater drainage and the resulting change in thermohaline circulation patterns that may have triggered and terminated the Younger Dryas.

Ecological limitations of organisms yield thresholds of biotic response to environmental change. For example, global circulation models suggest that during the Cretaceous Period, surface water temperatures and salinities across much of the Tethyan Ocean may have come to exceed the tolerance of most modern reef-building corals (30°C and 3.7% salinity). A shift past critical limits may explain the corals' relatively sudden loss of dominance to rudist bivalves in the central Tethyan reefs during Albian time, about 113 to 98 m.y. ago (see Barron, Chapter 6).

At the end of the Westphalian age of the Pennsylvanian Period, drier climates swept across North America and Europe, with profound consequences for vegetation (see DiMichele and Phillips, Chapter 8). As loss of swamp habitat reached a critical threshold, the arborescent lycopods that had been a conspicuous feature of landscapes for tens of millions of years disappeared rapidly. When climates amenable to swamp formation returned, ferns and tree ferns rose to dominance in these environments. The paleobotanical record shows repeated evidence for the adaptation of plants to particular environments, followed by extinction when habitats were disrupted.

Thresholds appear to have been crossed for antelopes, micromammals, and members of the human family close to 2.5 m.y. ago in Africa, with the rapid shrinkage of forests during the onset of the modern ice age. Forest-dependent species within all three groups disappeared over a broad area during an interval on the order of 10,000 years, and many species adapted to open, grassy habitats made their first appearances (Vrba, 1985). Within the human family, it appears that gracile australopithecines died out because they had depended on forests for food and refuge (see Stanley, Chapter 14). In the manner of modern chimpanzees, these animals presumably slept in trees and fled into them when threatened by predators. The characterization of paleofloras at fossil hominid sites using carbon isotopic ratios of paleosols reveals an acceleration at about 2.5 m.y. ago in the long-term Neogene shift from closed forests toward grassy habitats (Cerling, 1992). Apparently no fossil site supported a closed canopy forest after this time.

Although the preceding examples highlight the role of environmental thresholds in promoting extinction, at times environmental change has opened up new evolutionary possibilities. For example, increases in the partial pressure of oxygen must have crossed crucial thresholds for the evolution of life during Archean and Proterozoic time (see Knoll and Holland, Chapter 1). Increased partial pressure of oxygen to 1, 10, and essentially 100% of present-day levels would have cleared the environmental path for the evolution of aerobic bacteria and mitochondria-bearing eukaryotes, obligately photosynthetic eukaryotes, and large animals, respectively. In addition, an increase to levels much closer to those of today may have permitted the evolution of large animals. (As animals evolved complex circulatory and respiratory systems, they would have been able to expand into less oxygen-rich regions of the ocean and to enclose their tissues within thick mineralized shells.)

PATTERNS OF BIOTIC RESPONSE

An adverse change in the environment can cause species to migrate, or if migration to a suitable habitat is impossible, it can lead to their extinction. Patterns of migration and extinction for ancient biotas are of particular interest because they yield predictions as to how modern communities may respond to future global change. Environmental changes of the past have also had positive effects on certain surviving forms—especially ecological opportunists—or have triggered adaptive radiations within an impoverished ecosystem or a newly expanding habitat. Interactions between species intensify biotic responses to



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