Page 106 - Ultimate Visual Dictionary (DK)

PREHISTORIC EAR TH

Mammals 1

MODEL OF A
MEGAZOSTRODON
SINCE THE EXTINCTION of most of the dinosaurs 65 million years
ago, mammals (along with birds) have been the dominant vertebrates
on land. This class includes terrestrial, aerial, and aquatic forms.
Having developed from the therapsids, the first true mammals—
small, nocturnal, shrewlike creatures, such as Megazostrodon-
TETRALOPHODON appeared over 200 million years ago during the Triassic period
CHEEK TEETH (250–200 million years ago). Mammals had several
features that differed from those of their ancestors: an efficient
four-chambered heart allowed these warm-blooded animals Long tail aids Insulating
balance
to sustain high levels of activity; a covering of hair helped them hair
maintain a constant body temperature; an improved limb
structure gave them more efficient locomotion; and the birth of live
young and the immediate supply of food from the mother’s milk aided their
rapid growth. Since the end of the Mesozoic era (65 million years ago), the
Neural
number of major mammal groups and the abundance of species in each spine
have varied dramatically. For example, the Perissodactyla (the group
Scapula
that includes Coelodonta and modern horses) was a common group
during the Early Tertiary period (about 54 million years ago). Cervical
Today, the mammalian groups with the most species include the vertebra
Rodentia (rats and mice), the Chiroptera (bats), the Primates
(monkeys and apes), the Carnivora (bears, cats, and dogs),
and the Artiodactyla (cattle, deer, and pigs), while the
Proboscidea group, which formerly included many genera,
such as Phiomia, Moeritherium, Tetralophodon, and
Mammuthus, now has only three species of elephant.
In Australia and South America, millions of years
of continental isolation led to increased diversity
of the marsupials, a group of mammals
distinct from the placentals (see p. 74)
that existed elsewhere.

Humerus

Nasal horn
Naris
Orbit
Radius
Mandible
Premaxilla bone
Ulna

Chisel-edged
molar Metacarpal
Phalanx

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