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When did the first dinosaurs appear?

A Triassic archosaur running on its back legsA Triassic archosaur running on its back legsA Triassic archosaur running on its back legsThe archosaurs were a group of reptiles that had powerful jaws and bony armour. Somehow they survived the Great Dying at the end of the Permian. Early archosaurs ran like modern lizards do. Some kinds evolved longer legs and ran faster. By the middle of the Triassic Period, 230 million years ago, a few could run on their two back legs all the time. The first dinosaurs had evolved.

This illustration compares how dinosaurs stand with modern reptiles and another prehistoric reptile. A modern reptile, the Komodo dragon (top), has legs to the side of its body. A thecodont (above, left), a reptile that lived before the dinosaurs, had partly bent legs, half-way between an ordinary reptile and a dinosaur. Compsognathus, a dinosaur (above, right), had straight, upright legs. Reptiles' standing positions comparedThis illustration compares how dinosaurs stand with modern reptiles and another prehistoric reptile. A modern reptile, the Komodo dragon (top), has legs to the side of its body. A thecodont (above, left), a reptile that lived before the dinosaurs, had partly bent legs, half-way between an ordinary reptile and a dinosaur. Compsognathus, a dinosaur (above, right), had straight, upright legs.

What is a dinosaur?

Dinosaurs were reptiles that lived on land in the Triassic, Jurassic or Cretaceous periods (230 to 66 million years ago). They walked upright like mammals or birds, not sprawling like lizards or crocodiles. Dinosaurs lived only on land. Neither marine reptiles nor flying reptiles (pterosaurs) were dinosaurs.

Triassic South America

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