> Top 10: Dinosaur Myths

1. Humans lived alongside dinosaurs

Dinosaurs and people coexist only in books, movies and cartoons. The last dinosaurs - other than birds - died out dramatically about 65 million years ago, while the fossils of our earliest human ancestors are only about 6 million years old.

2. Mammals only evolved after dinosaurs died out

Tiny mammals lived in the shadow of the dinosaurs for more than 150 million years, occupying ecological niches as small, nocturnal animals weighing as little as 2 grams. The ancestors of mammals, animals called synapsids, actually appeared before dinosaurs.

Mammals remained relatively small until 65 million years ago, when the demise of the dinosaurs left a mass of niches for larger mammals to fill. Most of the types of mammals we know today evolved after this time.

3. Dinosaurs died out because mammals ate their eggs

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

4. An asteroid impact alone killed the 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 Mexico's Yucatan peninsula. That impact formed the 180 kilometre-wide Chicxulub crater. There is no convincing evidence that any non-avian dinosaurs survived the aftermath of the impact. Yet we are still not totally sure how the dinosaurs died.

The impact itself could only have killed the dinosaurs in the immediate vicinity of the crater. But it also produced devastating after-effects including giant tsunamis, rain that may have been as acidic as battery acid, and clouds of dust that darkened and cooled the globe for months or even decades.

Another theory suggests that before the impact, dinosaurs were already dwindling as falling sea levels and volcanic eruptions took their toll. A combination of those effects probably wiped out the dinosaurs.

5. Dinosaurs died out because they were unsuccessful in evolutionary terms

Dinosaurs survived for more than 150 million years, so they cannot be considered unsuccessful. Hominids have lived for only 6 million years, and Homo sapiens date back no more than 200,000 years. Dinosaurs out-competed other animals of their era, but they lost the battle to survive the effects of the asteroid impact.

6. All dinosaurs died out 65 million years ago

Birds evolved about 150 million years ago. Most experts believe they evolved from small predatory dinosaurs, which would classify them as dinosaurs according to modern methods of grouping animals. These avian dinosaurs probably suffered some losses after the asteroid impact, but they soon rebounded.

7. Dinosaurs were slow and sluggish animals

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 rested after eating their fill.

One study in 2000 of an exceptionally well-preserved hadrosaur fossil, found in a South Dakota riverbed, suggested that dinosaurs had powerful hearts more like those of birds or mammals than modern reptiles. Researchers argue that the fossilised, four-chambered heart points to an active, bird-like metabolism.

8. All large land reptiles from prehistoric times were dinosaurs

Terrestrial reptiles reached 5 metres in length before the first dinosaurs evolved 230 million years ago. Some - such as sail-backed Dimetrodon, which flourished in North America during the Permian period (290 to 240 million years ago) - were related to dinosaurs, but were not true dinosaurs.

9. Marine reptiles - for example, plesiosaurs and ichthyosaurs - were dinosaurs

Several types of marine reptiles evolved during the dinosaur age, but all true dinosaurs were terrestrial animals. Marine crocodiles, like other crocodiles, were closely related to the dinosaurs. So were large, extinct marine reptiles called plesiosaurs, pliosaurs, mosasaurs and ichthyosaurs.

10. Flying reptiles were dinosaurs

Flying reptiles called pterosaurs first appeared just after the dinosaurs, and then died out at the same time as the dinosaurs. The largest grew to the size of a small aeroplane. However, while they were close relatives, they were not true dinosaurs.

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