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only half had lived in the proceeding Cretaceous while almost two-thirds are
still living today.
The evolution of the mammals following the dinosaur extinction,
however, was not gradual. It progressed in fits and starts. The early Tertiary
was characterized by an evolutionary lag, as though the world had not yet
awakened from the great extinction. By the end of the Paleocene epoch,
about 54 million years ago, when temperatures were on the rise, mammals
began to diversify rapidly. About 37 million years ago, a sharp extinction
event took many archaic mammal species, which were large, peculiar look-
ing animals (Fig. 174). Afterward, most of the truly modern mammals began
to evolve.
The extinction coincided with changes in the deep-ocean circulation
and eliminated many species of marine life in the shallow seas that flooded the
European continent. The separation of Greenland from Europe might have
allowed frigid Arctic waters to drain into the North Atlantic, significantly low-
ering its temperature and causing most types of foraminifers (marine proto-
zoans) to disappear. The climate grew much colder and the seas withdrew
from the land (Fig. 175), as the ocean dropped 1,000 feet to perhaps its low-
est level of the last several hundred million years.
Much of the drop in sea level resulted from the accumulation of massive
ice sheets atop Antarctica, which had drifted over the South Pole. A large fall
in sea level due to a major expansion of the Antarctic ice sheet led to another
extinction about 11 million years ago.These cooling events removed the most
vulnerable of species. So those living today are more robust, having withstood
Figure 174An extinct
Eocene five-horned, saber-
toothed,plant-eating
mammal.237TERTIARY MAMMALS