Читать книгу Dinosaurs - Douglas Palmer - Страница 14

The dinosaurs first appeared around 230 million years ago in Late Triassic times. Over the following 165 million years, through the Jurassic and Cretaceous periods, they increased enormously in both abundance and variety, only to die out abruptly 65 million years ago at the end of the Cretaceous. But in a way the dinosaurs are still with us, only they are now all feathered and we know them as birds.

The earliest dinosaurs included members of both major groups: the saurischians and ornithischians. Many of the oldest dinosaur fossils have been found in South America, which at the time was connected to Africa, India, Antarctica and North America as part of the much larger supercontinent of Pangaea. These fossils have sufficient features in common to suggest that they were derived from an even earlier and as yet unknown common ancestor in Middle Triassic times.

By the end of the Triassic period many different groups of dinosaurs had evolved as well as the flying pterosaur reptiles. As global climates became drier, conifers became more abundant and plant-eating dinosaurs became more common than other reptilian plant eaters. By the Jurassic period the main kinds of dinosaurs had appeared, but alongside them were crocodile-like reptiles, turtles and early mammals. The dinosaurs had also spread across Pangaea from the southern hemisphere into the northern hemisphere and most continents to become a worldwide success.

The Jurassic saw the rise of the first really gigantic plant-eating sauropods and some of the massive predatory theropods such as the allosaurids. The earliest fossils of primitive birds (Archaeopteryx) date from the end of this time period, showing that their ancestry and origin from dromaeosaur (raptor) dinosaurs had occurred earlier. Unfortunately, no fossil record of this important event has been recovered, and much of our information on these birds is from later Cretaceous fossils.

By the end of Cretaceous times, 65 million years ago, dinosaurs were well established across the earth from pole to pole. However, they all suddenly died out in what is called the end-Cretaceous extinction event, along with many other kinds of animals and plants both on land and in the sea. The event coincides with the collision of an 11km-wide (7-mile) asteroid-like rock with the earth. What is surprising about this is that many other kinds of reptiles such as the crocodiles and turtles survived, along with the mammals and those dinosaur descendants, the birds.