The Mesozoic Era – the ‘Age of Dinosaurs’

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The dinosaurs roamed the Earth during the Mesozoic Era, 248 to 65 million years ago, the second of three geological eras during which complex life forms with skeletons diversified. (The Paleozoic Era, meaning “ancient life”, is the first. The Cenozoic, meaning “recent life”, is the third and current era). As the Mesozoic Era was dominated by various reptilian creatures, it is often called the Age of Reptiles. Because the dinosaurs ruled the land, some call it the ‘Age of Dinosaurs’.

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“Generally in the Mesozoic, it was warm,” says Hans-Dieter Sues, vice-president of collections and research at the Royal Ontario Museum in Toronto. Climates were also more temperate than now. They still, however, must have adjusted to the varying seasons.

“We know from the Early Cretaceous dinosaurs from southern Australia that they lived inside the Antarctic Circle,” says Sues. “We know from the rock evidence that there were permafrost soils. And for some of them, we have indications they stayed active throughout the year because the bones don’t show the kind of arrested growth lines that you get in animals that hibernate.”

The Mesozoic Era is divided into three periods: Triassic (248 to 206 million years ago); Jurassic (206 to 144 mya); and Cretaceous (144 to 65 mya). The first dinosaurs appeared sometime in the Middle to Late Triassic. The earliest known dinosaurs, Herrerasaurus and Eoraptor, are dated to be 227.8 million years old. But since they were well developed, paleontologists believe there must have been many more before them.


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Dinosaurs first appeared during the Late Triassic period, about 230 million years ago.
At the start of the Mesozoic, all the continents were all stuck together as one supercontinent called Pangaea. The Earth’s lithosphere (or outer layer) is made up of large plates that move apart or collide with each other. Over millions of years, the plate movement – plate techtonics – causes the continents to join together and separate. When the first dinosaurs appeared, Pangaea was beginning to slowly break apart. So, the dinosaurs were able to roam freely over the whole Earth. By the Late Triassic about 228 million years ago, all the major dinosaurian lineages were established.


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Icthyosaurs in the seas grew up to nine metres long.
By the end of the Jurassic period, a narrow Atlantic Ocean had separated Europe from North America and the Turgai Sea was splitting Europe from Asia. In the sea, enormous marine reptiles, such as plesiosaurs and icthyosaurs, dominated. Primitive birds made their first appearance, but the pterosaurs – giant flying reptiles – still ruled the skies. On land, the dinosaurs grew in number and diversified. As a group, they also grew bigger.


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Ankylosaurs, like Ankylosaurus above, replaced the stegosaurs, which became extinct during the Cretaceous period.
During the Cretaceous, the continents continued to shift apart. For most of the dinosaurs’ reign, North America was split down the middle by a large body of water called the Western Interior Seaway, which extended from present-day Gulf of Mexico into the Canadian Northwest Territories. The Rocky mountains started to rise and the Earth started to look as it does today. In the sea, icthyosaurs declined as plesiosaurs prospered. Frogs and salamanders appeared, while sea turtles grew into giants. Giant marine lizards called mosasaurs were the killer predators in the water. Birds evolved, but the skies were still dominated by the pterosaurs, which also grew some to enormous sizes. Some were as big as a small plane with a wingspan of 15 metres. On land, the stegosaurs became extinct and were replaced by ankylosaurs. The duckbilled dinosaurs appeared and flourished. But by the end of the Cretaceous, 65 millions years ago, they all died out. Why? That has become one of the biggest unsolved mysteries of all time.

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