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