Age of Dinosaurs
North America about 300 million years ago The Triassic, Jurassic and Cretaceous periods, which lasted from 252 to 65.8 million years ago, are together known as the Mesozoic Era, but more commonly called the Age of Dinosaurs. The Palaeozoic, the previous era, ended with the greatest mass extinction of life the Earth has ever known, in which more than 90% of species were wiped out. The disappearance of large amphibians and many other types of reptiles opened the way for the archosaurs, the group of reptiles which dinosaurs, crocodiles and pterosaurs evolved from, to dominate life on Earth.
Before the dinosaurs
The story of the dinosaurs began long before they first appeared about 230 million years ago. Their evolution can be traced back to fish, the first vertebrates. About 360 million years ago, some kinds of fish developed the ability to crawl on to land, and so gave rise to amphibians. These animals still kept close to water, where they laid their jelly-covered eggs. It took another 50 million years for some creatures, the first reptiles, to find a way of laying their eggs on land. Gradually the reptiles spread across the world, and, being perfectly suited to the new drier conditions, they became the dominant type of animal on land.
Evolution of the dinosaurs
A panorama of the Permian and Triassic periods Cretaceous dinosaurs Dinosaurs were reptiles that had legs held upright beneath their bodies, rather than sprawling. They walked on either two legs or all fours. They lived on land (not in the air or water) during the Mesozoic Era. During that time, hundreds of different species of dinosaur evolved. Ranging from about the size of a duck to 100-tonne giants, the dinosaurs inhabited all the Earth's continents—including even Antarctica, then a subtropical land.
The dinosaurs evolved from the archosaurs, a group of reptiles with powerful jaws and bony armour. Early archosaurs had a low, sprawling gait, like that of modern lizards, but later some kinds began to adopt a more upright posture. By the late Triassic Period, some were standing on their two back legs all the time: the first dinosaurs. All flesh-eaters, they had powerful back legs for running and short arms with claws for grappling. The plant-eaters evolved from the flesh-eaters.
Over the course of Age of Dinosaurs, a time lasting about 165 million years, the continents gradually shifted their positions: continental drift. The drift took place as a result of plate tectonics, the movement of the vast slabs that make up the Earth's outer layer: tectonic plates.
At first, in the Triassic Period, all of the main continents were joined together in one vast landmass, the supercontinent of Pangaea. During the Jurassic Period, the continents began to move apart. Pangaea separated into two: Laurasia and Gondwana. This drift continued through the Cretaceous Period. Sometimes, sea levels rose and flooded the land, then they fell to leave it dry again. So the shapes of the continents changed too.
Range of dinosaur species
Jurassic and Cretaceous dinosaursMost of the story of the dinosaurs takes place during two geological periods: the Jurassic and Cretaceous. The Jurassic was the time of the truly gigantic sauropods, such as Diplodocus. Their very size helped protect them from fierce predators like Allosaurus. The birds evolved from a group of small, feathered theropods in the Jurassic Period. During the Cretaceous Period, many (but not all) sauropods went extinct, to be succeeded in many regions by other kinds of plant-eating dinosaurs, including those with armour or some other kind of protection from predators.
Palaeontologists divide the dinosaurs into two major groups, based on the shape and positions of their hip bones: saurischians (“lizard-hipped”) and ornithischians (“bird-hipped”). The saurischians were themselves divided into two major sub-groups: the meat-eating, bipedal theropods, and the plant-eating sauropodomorphs, many of which were quadrupedal—they went about on four legs. The ornithischians, all plant-eaters, were divided into several sub-groups: ornithopods, horned ceratopsians, thick-skulled pachycephalosaurians, stegosaurs and ankylosaurs.
The dinosaurs and a large number of other species became extinct quite abruptly at the close of the Cretaceous, 66 million years ago. The extinction is thought by many scientists to have been triggered by a massive asteroid (a large rocky object in space) crashing into the Earth. The resulting explosion may have filled the atmosphere with dust, blotting out the sun and lowering temperatures for years on end.
Plants withered away and, with them, the plant-eating dinosaurs, perishing from cold and hunger. Having no prey, the flesh-eaters soon followed. The Age of Dinosaurs was over.
Consultant: Chris Jarvis
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