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Discover dinosaurs;Discovery series

They ruled the Earth for 160 million years, and then they died. But the fascination lives on. Gerald Haigh explains why

Remember that scene in Jurassic Park when the dinosaurs first appear? The director cannily makes us wait a moment or two before we see them, pausing instead to focus on the lead characters. One by one their faces register the astonishing revelation that they are gazing across an awesome 65-million-year gulf. There in front of them, in the flesh, are the most exciting and dramatic creatures ever to have walked the Earth.

This really is the point about dinosaurs - that although they were once once every bit as real as the animals that share our world today, they remain tantalisingly out of reach. By the time the evolutionary process began on our own ancestors some four million years ago, the last dinosaur had been dead for more than 60 million years. But they had dominated the earth for 160 million years - 40 times longer than man has been around. We have a long, long way to go to equal their success.

Dinosaur facts never cease to amaze. Brachiosaurus measured more than 20 metres from nose to tail - one of them, the largest mounted skeleton in existence, towers up to the glass roof of the Humboldt Museum in Berlin. Allosaurus weighed two tons and had huge and powerful hinged jaws with 60 dagger-like, saw-edged teeth. It operated by picking a slow-moving, often much bigger, victim and running at it full tilt with its mouth wide open. Having latched on with its teeth and claws it dragged off chunks of flesh to be swallowed whole.

Velociraptor was smaller - about six feet long - but it hunted in fast-moving packs, and was armed with claws and needle-like teeth. Euplocephalus had a two-and-a-half-metre tail which ended in a 30-kilogram club made of solid bone which it thrashed around at leg level. (Go down with a broken leg in dinosaur land and you were finished).

Deinonychus, another fast runner, had teeth designed to slice flesh. It also had big claws on its front feet which, although fearsome, pale into insignificance next to the huge swivelling killing claw on each of its back feet. Its skeleton is the stuff of nightmares.

Not that it was all blood and guts. Many dinosaurs, including the biggest ones of all, such as brachiosaurus, were plant eaters, consuming up to a ton of vegetation a day, and, lacking any chewing-type teeth, swallowing stones - gastroliths - to help grind the stomach contents. Others, such as baryonix, found for the first time in Surrey in 1983, were fish eaters.

Dinosaurs made up a complete land animal system of hunters, scavengers and plant eaters, in balance with itself and the rest of nature. Coming to terms with this fact - that for a significant span of Earth time the animal kingdom was about reptiles, with mammals very much in the background - is a huge part of understanding the dinosaurs. As long as the reptiles were around, populating every usable part of the planet, mammals were going nowhere - those that developed were small scurrying creatures that came out while the dinosaurs were sleeping.

Dinosaurs were reptiles - they laid eggs, precisely spaced in nests. But don't think crocodile or lizard. Crocodiles have sprawling legs and drag themselves along with a side-to-side motion. Dinosaurs had the upright, straight-legged stance of today's mammals and birds. This was the key to their success. Without it there would have been no giant dinosaurs and no high-speed, smaller ones either. Dinosaurs were divided into many species, each with unique identifying characteristics. Even with modern animals, the science of identification and classification is complicated, so working with dinosaurs must be far more difficult. However, a huge amount of work has been carried out, and elaborate family trees identify a wide range of groups by body shape and bone structure.

We know a remarkable amount about these long-extinct creatures, given that all our knowledge is built on often incomplete collections of fossilised bones. But much remains unknown, colour, for instance - a huge disappointment given the public demand for colour models. Social behaviour, too, is a matter for conjecture - we can make an educated guess on whether a particular dinosaur was a lone operator or a pack animal, but proving it is something else.

Thanks to Dr Angela Milner of the Natural History Museum; Jenny Bradbury of the Dinosaur Society; Edith Lowe of Tourwest; John Martin of Leicester City Museum; Steve Huff of the Museum of Geology, Isle of Wight

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