also have a coat of insulation, comprising an outer layer of fat and fur to prevent
the escape of body heat during cold weather.
Other distinguishable mammalian features include four-chambered
hearts, a single bone in the lower jaw, highly differentiated teeth, and three
small ear bones that migrated from the jawbone backward as the brain grew
larger to improve hearing greatly. Mammals have live births. They possess
mammary glands that provide a rich milk to suckle their young, which gen-
erally are born helpless. Mammals have the largest brains, capable of storing
and retaining impressions. Therefore, they lived by their wits, which explains
their great success. They conquered land, sea, and air and are established, if
only seasonally, in all parts of the world.
The drifting of continents isolated many groups of mammals, and these
evolved along independent lines. For example, Madagascar broke away from
Africa about 125 million years ago. As such, it has none of the large mammals
that live on the adjacent continent except the hippopotamus.The hippos mys-
teriously landed on the island after it had drifted some distance from the
African mainland.
For the last 40 million years or so, Australia has been an island continent,
without a land link to other landmasses. It is home to many strange egg-laying
mammals called monotremes. These include the spiny anteater and platypus,
which should rightfully be classified as surviving mammal-like reptiles. Marsupi-
als are primitive mammals that suckle their tiny infants in belly pouches. They
originated in North America around 100 million years ago, migrated to South
America, crossed over to Antarctica when the two continents were still in con-
tact, and landed in Australia before it broke away from Antarctica.
Today, 13 of the world’s 16 marsupial families reside only in Australia.
The Australian marsupials consist of kangaroos, wombats, and bandicoots, with
opossums and related animals occupying other parts of the world.The largest
marsupial fossil found is that of diprotodon (Fig.176),which was about the
size of a rhinoceros. Many large marsupials,including giant kangaroos, disap-
peared soon after early humans invaded the continent some 60,000 years ago.
Camels,which originated about 25 million years ago, migrated out of
North America to other parts of the world by connecting land bridges. Horses
originated in western North America during the Eocene when they were
only about the size of small dogs. As they became progressively larger, their
faces and teeth grew longer as the animals switched from browsing to graz-
ing,and their toes fused into hoofs.The giraffes shifted from grazing on grass
to browsing on shoots, twigs, and leaves. Their necks lengthened in response
to reaching the tall branches. Many types of hoofed animals, called ungulates,
ev olved in response to increasing grasslands throughout the world.
All major groups of modern plants were represented in the early Tertiary
(Fig. 177). The angiosperms dominated the plant world. All modern families
TERTIARY MAMMALS