Dinosaurs lived on earth for over 140 million years. They lived during the
TRIASSIC， JURASSIC and CRETACEOUS periods. Suddenly， about 65
million years ago， they vanished.
The first animals were invertebrates (animals without backbonesl， which
lived in the sea. About 400 million years ago， invervebrates evolved into fishes
and then amphibians. Amphibians are the simplest forms of vertebrates (ani
reals with backbones). They are cold-blooded animals that live in the sea in
their early years and then live on land when they mature. Reptiles came next
and they were the ancesvors of all dinosaurs， birds and mammals. (See Geologi-
cal Time Chart.) Dinosaurs are the direct descendants of the THECODONTS，
heavy， four-legged， flesh-eating reptiles. Later， some Thecodonts became the
first upright-mowng animals， using their long， strong tails for balance.
~ saurs evolved from the Thecodonts. They are
Many different types of.dino . ，. j ~_Abird.hinned. The lizard-hipped
divided into two basic kinds: lizarct-nlppeu m\"~ r~
group includes all the meat-eaters as
well as the giant-sized plant-eaters.
The horned， armored， plated and
bird-footed (such as Iguanodon) dino-
saurs belong to the bird-hipped group.
{See Basic Dinosaur Classification--
p. 15.) So far， about 340 genera
(closely related groups) of dinosaurs
have been discovered. They range
from the little， two-pound， meat-
eating SALTOPUS to the 150-ton，
plant-eating ，，ULTRASAURUS.\" L
The first dinosaur fossils were ~ ~1
found in England in the early 1800s. Ww
Since then they have been found on ~ ~l%a k g
east Hl~\"
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