Geology Reference
In-Depth Information
Diversifi cation continued during the Eocene when sev-
eral more types of mammals appeared, but if we could go
back and visit this time, we would probably not recognize
many of these animals. Some would be vaguely familiar, but
the ancestors of horses, camels, rhinoceroses, and elephants
would bear little resemblance to their living descendants.
By Oligocene time, all the orders of existing mammals were
present; however, diversifi cation continued as more familiar
families and genera appeared. Miocene and Pliocene mam-
mals were mostly mammals that we could readily identify,
although a few types were unusual (
evolution was during the Mesozoic Era (see Chapter 22).
However, following the Mesozoic extinctions, mammals began
to diversify and soon became the most abundant land-dwelling
vertebrates. Now, more than 4000 species exist, ranging from
tiny shrews to giants such as elephants and whales.
When people consider mammals, they think mostly of
larger species such as elephants, horses, deer, dogs, and cats, but
they fail to realize that most mammals are small, weighing less
than 1 kg. In fact, with few exceptions, rodents, insectivores,
rabbits, and bats fall into this category and they constitute
fully 75% of all mammal species. These animals adapted to the
microhabitats unavailable to larger mammals, or in the case of
bats, became the only fl ying mammals. With this in mind, we
now turn our attention to some of the larger ones, especially
hoofed mammals, carnivores, elephants, and whales.
◗
Figure 23.22).
Mammals evolved from mammal-like reptiles known as
cynodonts during the Late Triassic, so two-thirds of their
Hoofed Mammals
Ungulate
is a general
term referring to several types of hoofed
mammals, but especially the orders
Artiodactyla and Perissodactyla. About
220 living species of antelope, cam-
els, giraffes, deer, goats, peccaries, pigs,
and several others are even-toed hoofed
mammals, or
artiodactyls
, the most
common living ungulates. In marked
contrast, only 18 species of horses, rhi-
noceroses, and tapirs are
perissodactyls
,
or odd-toed hoofed mammals. As even-
and odd-toed imply, artiodactyls have
two or four toes, whereas perissodactyls
have one or three (
Figure 23.23a).
Some ungulates are small and de-
pend on concealment to avoid predators;
others, such as rhinoceroses, are so large
that size alone is enough to discourage
predators, at least for adults. But many
of the more modest-sized ungulates are
speedy runners. Adaptations for running
include elongation of some of the limb
bones, as well as reduction in the num-
ber of bony limb elements, especially toes.
Accordingly, the limbs of speedy ungu-
lates are long and slender (Figure 23.23b).
All ungulates are herbivores, but
some are
grazers
, meaning that they feed
on grasses, and others are
browsers
, feed-
ing on the tender shoots, twigs, and leaves
of trees and bushes. When grasses grow
through soil, they pick up tiny pieces of
sand that are quite abrasive to teeth, so
the grazing ungulates developed high-
crowned chewing teeth resistant to abra-
sion (Figure 23.23c). Browsers, on the
other hand, never developed these kinds
of chewing teeth.
Rabbit-size ancestral artiodactyls
of the Early Eocene differed little from
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Image not available due to copyright restrictions
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