What Is A Dinosaur

Dinosaurs were among the most amazing and successful animals ever. From ancestors no bigger than dogs, they evolved into gigantic killers as heavy as elephants, plant-eaters several bus-lengths long, and nimble little creatures the size of chickens. While they ruled the land, no mammal larger than a domestic cat survived. Dinosaurs first appeared about 230 million years ago and flourished for an astonishing 165 million years. Then, 65 million years ago, they suddenly and mysteriously disappeared. By comparison, modern humans have inhabited the Earth - .. " . for only about 100,000 years.

Richard owen

Discovering dinosaurs

People have been finding dinosaur fossils for thousands of years, but the first to be identified as a giant extinct reptile was the fanged jawbone of Megalosaurus ("great lizard"), named in 1824 by William Buckland, a British naturalist. The term dinosaur ("terrible lizard") was coined by the British scientist Richard Owen in 1842.

Wk mm

Key features

The dinosaurs were a group of mainly large, land-living reptiles. Like reptiles today, most had scaly skin (although some had feathers), a long tail, teeth, and claws on the fingers and toes. But while modern reptiles walk with their legs splayed sideways, dinosaurs walked upright with their legs directly below them, as mammals do. This key feature made many swift and agile on land.

I Large tail for balance

Dinosaurs had an upright stance, with straight legs directly below their bodies.

Lizards have a sprawling stance. Their legs are held sideways, and their elbows and knees bend at right angles.

every m0ming in order to become active.

Keep the heat

Birds and mammals are warm-blooded, which means their body temperature is always the same. In contrast, reptiles are cold-blooded - they heat up and become active only when it is warm, and they cool down and become sluggish when it is cold. Were the dinosaurs warm- or cold-blooded? Most scientists think at least some flesh-eating dinosaurs were warm-blooded and that all big dinosaurs stayed warm because their bodies were too big to cool down at night.

Crocodiles have a semi-sprawling stance, with their knees and elbows slightly bent.

Some dinosaurs had a backward-pointing toe a little like the reversed toe of a bird's foot.

IMuscular hindlegs

, Birdlike feet ifc. -Xv

Not dinosaurs

While dinosaurs ruled the land, flying reptiles such as Pteranodon ruled the skies. Many people mistakenly think these animals were dinosaurs, but they formed a different branch of the reptile family tree. Likewise, the large seagoing reptiles, such as ichthyosaurs and plesiosaurs, belonged to other branches of the reptile family tree. Like most of the dinosaurs, these animals were an evolutionary dead-end. Eventually they died out, and birds and mammals evolved to take their place.


Two-legged dinosaurs leaned forward with their backs horizontal. The weight of the tail balanced the front part of the body.

Some dinosaurs had a row of bony spines running from the head to the tail.

Outer ear






Dinosaurs today

Most scientists now believe that not all dinosaurs became extinct 65 million years ago. Some live on as birds -feathered descendants of small carnivorous dinosaurs. Evidence for this theory comes from the many striking similarities between the skeletons of birds and dinosaurs, and from recent discoveries of birdlike feathered dinosaurs such as Caudipteryx. If the theory is correct, living dinosaurs outnumber their extinct relatives by ten to one.

The predatory secretary bird uses its long tail feathers for balance, just as Giganotosaurus used its tail.

Scarlet ibis

Scarlet ibis

The predatory secretary bird uses its long tail feathers for balance, just as Giganotosaurus used its tail.




During the age of dinosaurs - the Mesozoic Era - Planet Earth was very different from today. The climate was hotter, and the land was covered by deserts or strange prehistoric vegetation. The plants that dominate the land today - flowering plants - did not exist. Instead of grasses, there were ferns. Instead of broadleaved trees, there were forests of conifers, palmlike cycads, and tall tree ferns. The coastlines were unrecognizable. At the start of the Mesozoic, the continents were all joined together. Over millions of years, they broke up and drifted apart, carried by currents in the semi-molten rock deep below the planet's crust.


Earth today

This satellite image of Earth shows the planet's continents as they are now. The continents are still moving around, just as they were during the Mesozoic, although the movement is too slow for us to notice during a human lifetime. Millions of years from now, the Earth will be unrecognizable again.

Earth timeline

The Mesozoic stretched from 248 to 65 million years ago - an unimaginably long period of time, yet only a small fraction of the Earth's history. Scientists divide it into three distinct periods: the Triassic, the Jurassic, and the Cretaceous.

4,600 million years ago (mya)

Triassic life conifer/

The first dinosaurs appeared in the Triassic Period, about 230 million years ago. They coexisted with crocodilians, lizards, pterosaurs (flying reptiles), and tortoises. Ferns and palmlike cycadeoids and cycads grew near streams, conifers on drier lands. But vast, hot deserts covered inland areas.


I Horsetail plants mm

I Horsetail plants

Jurassic life

As continents fragmented, moist sea air shed rain on inland deserts. Here, cycads, cycadeoids, ferns, and horsetails grew near water, conifers on drier ground. Immense plant-eating and predatory dinosaurs eventually shared the land with the first birds and mammals, and with crocodilians and pterosaurs.

Cretaceous life

There were now more kinds of dinosaur than ever. Sharp-toothed plant-eaters grazed the flowering plants that were replacing older kinds of vegetation. Conifers and broadleaved trees that looked like today's appeared, as well as modern-looking frogs, snakes, birds, and mammals. But prehistoric reptiles still ruled the land, sea, and air.

Formation of the Earth

Formation of the Earth

Origin of life (3,800 mya)


dk guide to dinosaurs

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