When Did Dinosaurs Live? Triassic, Jurassic and Cretaceous Explained

“Jurassic” has become almost another word for dinosaurs, thanks to a famous film. But dinosaurs lived across a span of time so vast it is hard to imagine, and the animals in that film did not all live together. Here is a simple guide to the age of dinosaurs.

The Mesozoic Era

Geologists divide Earth’s history into eras and periods, based on changes in rocks and fossils. The age of dinosaurs is the Mesozoic Era, meaning “middle life”, which ran from about 252 to 66 million years ago. It has three periods:

• Triassic: about 252 to 201 million years ago.
• Jurassic: about 201 to 145 million years ago.
• Cretaceous: about 145 to 66 million years ago.

The Mesozoic began just after the largest mass extinction in Earth’s history, at the end of the Permian, which wiped out most species. It ended with the asteroid impact that killed the non-bird dinosaurs.

Triassic: small beginnings

The first dinosaurs appeared around 230 million years ago, late in the Triassic. Some of the earliest, such as Eoraptor and Herrerasaurus, come from rocks in Argentina. They were mostly small to medium-sized, lightly built animals that walked on two legs.

At first dinosaurs were not in charge. The Triassic world was dominated by other reptiles, including the ancestors of crocodiles, some of them large predators. Almost all land formed one huge supercontinent, Pangaea, with a hot, often dry climate.

Then, about 201 million years ago, a mass extinction at the end of the Triassic, probably linked to massive volcanic eruptions as Pangaea began to split, wiped out many competitors. The dinosaurs survived and took over.

Jurassic: the age of giants

In the Jurassic, as Pangaea broke apart and the climate became more humid, dinosaurs diversified dramatically. Giant long-necked sauropods such as Brachiosaurus and Diplodocus browsed on conifers and ferns. Plated Stegosaurus and predators like Allosaurus roamed North America. And late in the Jurassic, around 150 million years ago, the first birds, such as Archaeopteryx, appeared.

Cretaceous: variety

The Cretaceous was the longest period of the Mesozoic. Flowering plants appeared and spread, changing landscapes and food webs. Dinosaurs reached their greatest variety: horned Triceratops, armoured ankylosaurs, duck-billed hadrosaurs, the giant titanosaurs of South America, small feathered predators like Velociraptor, and finally Tyrannosaurus rex. The period ended abruptly 66 million years ago.

A gulf of time

The time spans are almost impossible to grasp. Stegosaurus lived about 150 million years ago and T. rex about 67 million years ago. That means more time separates Stegosaurus from T. rex than separates T. rex from us. And our own species, Homo sapiens, has existed for only about 300,000 years.

In short

• Dinosaurs lived in the Mesozoic Era, about 252–66 million years ago.
• It has three periods: Triassic, Jurassic and Cretaceous.
• The first dinosaurs appeared about 230 million years ago, in today’s Argentina.
• After the end-Triassic extinction, dinosaurs became dominant.
• Jurassic: giant sauropods, Stegosaurus, Allosaurus and the first birds.
• Cretaceous: Triceratops, Velociraptor, T. rex and flowering plants; it ended 66 million years ago.