Fossils of over 800 genera of dinosaurs have been found and described - usually on the basis of only a few teeth or bones. More are identified every year. Most of these genera are based on a single species, which suggests that thousands more genera and species are waiting to be discovered. Numerous others may never have been fossilised. Yet the dinosaurs were only one of the major taxa of Mesozoic reptiles. This gives some indication of the plethora of reptile species that must have evolved and become extinct during the Era. Each of these must have been adapted to a specific habitat at a particular moment during a time span of about 185 my. The smallest known dinosaur was only 60 cm in length and weighed about 3-5 kg, the largest was over 30 m long and weighed some 60 tonnes,but all that these scaly-skinned reptiles left behind were fossilised skeletons and trackways. The feathered dinosaurs are with us still in the form of birds.
Our solar system began some 4,550 mya when a star collapsed and exploded, but life did not appear for another 1,000 my. Even so, the period during which vertebrates have existed represents only a tiny fraction of the time during which the world has supported life, and it was not until around 540 mya that the chordates arose. The first reptiles evolved during the Middle Cambrian and the pelycosaurs diversified in the Early Permian. The Mesozoic Era (250-65 mya) lasted for 185 my. Some idea of the length of time involved can be gained from the thickness of the Upper Cretaceous chalk deposits of southern England and Europe. These were formed from the shells of microscopic marine organisms over many millions of years. Crude oil, too, was formed from organic remains by the action of heat and pressure over comparable millions of years. Again, when Pangaea broke up and continental drift began, at a rate of a centimetre or so per year, organic matter in sedimentary rocks was forced downward by movement of the tectonic plates, eventually to be melted and finally expelled in the form of carbon dioxide in the gaseous emissions of modern volcanoes.
Many of the Mesozoic reptiles were more spectacular and exciting than any animals that had appeared before that time or have existed later. Different forms evolved in response to particular environmental conditions, and died out when these conditions changed. When similar conditions returned, they were replaced by ecological equivalents, usually not closely related to them. Such considerations enhance the fascination afforded by the study of ancient life in general and of Mesozoic reptiles in particular.
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