Dinosaurs are a large yet very specific group of creatures. To qualify, an animal has to satisfy a number of criteria:
0 It must have lived during the Mesozoic era (see page 10).
0 It must be a reptile, although not all reptiles are dinosaurs. For example, lizards are reptiles, but they are not dinosaurs.
0 Its legs must be located below its body, giving it an erect stance, as opposed to sticking out from the sides, like those of a crocodile.
0 It must have lived on land, not in the air like pterosaurs, or in the water like swimming reptiles.
Remember that not all of the dinosaurs were alive at the same time. Different species appeared and disappeared during the reign of the dinosaurs. For example, Tyrannosaurus rex (see page 140) never saw an Allosaurus or a Stegosaurus (see pages 24 and 128).
The history of the earth is divided into different times, or 'eras', each with its own name. The time when the dinosaurs were alive is called the Mesozoic era. The Mesozoic is divided into three periods, called the Triassic, Jurassic and Cretaceous.
The Triassic period lasted from 245 million years ago (mya) to 208 mya, the Jurassic from 208 mya to 145 mya, and the Cretaceous from 145 mya to the end of the dinosaurs (65 mya).
The world inhabited by the dinosaurs looked vastly different from the one we know today. The countries we recognize now did not exist millions of years ago. The world is constantly changing: it always has and always will do. The land moves slowly but surely and, over time, this movement can make mountain ranges appear or whole continents vanish.
During the Triassic period, all of the world's continents were joined together in one huge landmass which palaeontologists call Pangea. As time progressed, this block began to split so, by the Jurassic period, there were two massive continents called Laurasia and Gondwana. These in turn started to break up and, by the time of the Cretaceous period, the continents looked much like they do today.
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