The continental mass, Pangaea, created through the collision of the continents in the Palaeozoic era, remained intact for about 100 million years. This gave rise to a new set of tectonic, climatic and biological conditions. Then in the Mesozoic era the process turned into its opposite. The super-continent began to break up. Vast glaciers covered the southern parts of Africa-America-Australia and Antarctica. During the Triassic (250-205 million years ago) dinosaurs evolved on the land and pleisiosaurus and ichthyosaurus in the sea, while the winged reptile pterosaurus later took to the air. Mammals evolved from the thraspid reptiles, but they developed very slowly. The explosive growth of the dinosaurs which dominated other vertebrate terrestrial life-forms did not permit a major development of mammals. They remained small both in size and numbers for millions of years, eclipsed by the shadow of their giant contemporaries, searching for food at night.
The Jurassic (205-145 million years ago) saw a major climatic change marked by the retreat of the glaciers, leading to a rise in global temperature towards the end of the period. The level of the seas rose by at least 270 meters during the Mesozoic, reaching almost double the present average level.
It takes a long time to fragment a supercontinent. The break-up of Pangaea began at the beginning of the Jurassic (180 million years ago) and the last continent was not separated until the early Cenozoic (40 million years ago). The first separation was on an east-west axis, where the creation of the Tethys Ocean split Pangaea into Laurasia in the North and Gondwanaland in the South. In turn, Gondwanaland split into three parts in the east – India, Australia and Antarctica. During the late Mesozoic a North-South split appeared, creating the Atlantic Ocean which separated North America from Laurasia and South America from Africa. India moved to the north and collided with Asia, while Africa also moved to the north and partly collided with Europe after the destruction of the Tethys Ocean. Of this mighty ocean, only a tiny part remained as the Mediterranean Sea. In the Pacific, Atlantic and Indian Oceans, periods of rapid expansion of the sea floor assisted the movement of the continental fragments.
Throughout the Mesozoic, dinosaurs were the dominant group of vertebrates. Despite the separation of the continents, they were firmly established all over the world. But at the end of this period – 65 million years ago – there was a new period of mass extinctions, in which the dinosaurs vanished from the face of the earth. Most of the terrestrial, marine and flying reptiles (dinosaurs, ichthyosaurs and pterosaurs) were wiped out. Of the reptiles, only the crocodiles, snakes, turtles and lizards survived. This spectacular elimination of species was not confined to the dinosaurs, however. In fact, about one-third of all living species became extinct, including the ammonites, bellemnites, some plants, bryozoa, bivalve molluscs, echinoids and others.
The remarkable success of the dinosaurs was a result of their perfect adaptation to the existing conditions. The total population was at least as big as that of mammals today. At present, everywhere in the world, there is a mammal, big or small, occupying every available ecological space. We can be sure that 70 million years ago, those spaces were occupied by an immense variety of dinosaurs. Contrary to the common impression of the dinosaurs as huge, lumbering creatures, they existed in all sizes. Most were relatively small, many walked upright on their hind legs, and could run very fast. Many scientists now believe that at least some of the dinosaurs lived in groups, looked after their young, and possibly even hunted in packs. The Mesozoic-Cenozoic boundary (65 million years ago) represents yet another revolutionary turning point in the evolution of life. A period of mass extinction prepared the way for a huge evolutionary leap forward, opening the way for the rise of the mammals. But before we deal with this process, it is worthwhile considering the question of why the dinosaurs disappeared.