elements that enabled the advanced plants to cope with drought conditions.
Before such vessels appeared, plants were restricted to moist areas such as the
wet undergrowth of rain forests.
The angiosperms were distributed worldwide by the end of the Creta-
ceous. Today, they include about a quarter-million species of trees, shrubs,
grasses, and herbs. All major groups of modern plants (Fig. 164) were repre-
sented by the early Tertiary.The angiosperms dominated the plant world, and
all modern families had evolved by about 25 million years ago.Grasses were
the most significant angiosperms, providing food for hoofed mammals called
ungulates. Their grazing habits evolved in response to the widespread avail-
ability of grasslands, which sparked the evolution of large herbivorous mam-
mals and ferocious carnivores to prey on them.
Near the end of the Cretaceous, forests extended into the polar regions far
beyond the present tree line. The most remarkable example is a well-preserved
fossil forest on Alexander Island,Antarctica.To survive the harsh conditions, trees
had to develop a means of protection against the cold since plants are more sen-
sitive to the lack of heat than the absence of sunlight. They probably adapted
mechanisms for intercepting the maximum amount of sunlight during a period
when global temperatures were considerably warmer than today.
Figure 163The three
most prominent petrified
tree stumps on North
Scarp of Specimen Ridge
in Yellowstone National
Park,Wyoming.
(Courtesy National Park
Service)CRETACEOUS CORALS