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Dinosaurs and people can coexist only in books, movies and cartoons. The last dinosaurs - other than birds - died about 65 million years ago, while the first humans did not evolve until about 2 million years ago.

Tiny mammals lived in the shadow of the dinosaurs for more than 150 million years. The ancestors of mammals were animals called synapsids, which evolved before dinosaurs but lost the evolutionary competition to the newcomers. By 295 millions years ago, the remaining synapsids had shrunk to become mammals as small as two grams, surviving by eating insects and worms - rather like today's rodents. Mammals remained small until 65 million years ago, when the death of the dinosaurs left them room to grow.

Dinosaurs coexisted with mammals for 150 million years. Although dinosaur nests were undoubtedly vulnerable, the most dangerous predators were probably small dinosaurs. Most mammals of the time were too small to eat the eggs of large dinosaurs.

A layer of iridium-rich rock marks the impact 65 million years ago of a 10-kilometre asteroid in shallow water covering what is now the Yucatan peninsula, forming a crater 180 kilometres across called Chicxulub. There is no convincing evidence that any non-avian dinosaurs survived the aftermath of the impact. Yet we are still not sure how or why the dinosaurs died. The impact itself could have killed only the dinosaurs in the immediate vicinity of the crater. But it also produced devastating after-effects, including giant tsunamis, powerful shock waves, rain that may have been as acidic as battery acid, and clouds of dust that darkened the globe for months or years. A combination of those effects probably did for the dinosaurs. Another theory suggests that before the impact, dinosaurs were already gradually dying out as falling sea levels and volcanic eruptions took their toll.

Dinosaurs survived for more than 150 million years, so they cannot be considered unsuccessful. Humans have lived for only a couple of million years, and Homo sapiens date back no more than 200,000 years. Dinosaurs out-competed other animals of their era, but they couldn't out-compete the effects of an asteroid impact.

Birds evolved about 150 million years ago. Most dinosaur specialists believe they evolved from small predatory dinosaurs, which would make them "dinosaurs" according to modern methods of grouping animals. These avian dinosaurs probably suffered some losses after the asteroid impact 65 million years ago, but they soon rebounded and came to occupy many niches originally occupied by larger "non-avian" dinosaurs.

Some terrestrial reptiles reached between 2 and 5 metres in length before the first dinosaurs evolved 230 million years ago. Large marine reptiles called plesiosaurs, pliosaurs and ichthyosaurs were related to dinosaurs, but were not true dinosaurs. Crocodiles are also related to dinosaurs, but they're not true dinosaurs either.

All true dinosaurs - other than birds - were terrestrial animals. Several types of marine reptiles evolved during the dinosaur age, but none was a true dinosaur. Marine crocodiles, like other crocodiles, were closely related to the dinosaurs. So were large, extinct marine reptiles called plesiosaurs, pliosaurs and ichthyosaurs. Mosasaurs were large marine lizards which evolved about 100 million years ago, and which died out at the same time as the dinosaurs.

Flying reptiles called pterosaurs first appeared slightly after the dinosaurs, and the last pterosaurs vanished at the same time as the dinosaurs. The largest grew to be the size of a small airplane. However, while they were close relatives of the dinosaurs, like crocodiles and ichthyosaurs, they were not true dinosaurs.

Early palaeontologists thought dinosaurs must have been slow and sluggish to have lost the evolutionary race to birds and mammals. Modern studies find no sign that they were laggards, lazily dragging their tails behind them. Most dinosaurs were probably as mobile as large, modern mammals. Like lions, meat-eating dinosaurs were active predators that probably lay down and relaxed after eating their fill.