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.
It can be surprisingly difficult for the remains of any land-based animal to become fossilized, which is why we have relatively few dinosaur and human-related fossils. When most animals die on land, their corpses are scavenged and are often totally destroyed unless they contain large bones or other indigestible structures such as teeth. Prolonged exposure to weathering and erosion by wind and water further scatters and wears away the remaining bones. The preservation of an entire dinosaur skeleton would have required quick burial below new layers of sediment, a situation that occurs only in certain circumstances.
Caption: Bodies are food for scavengers.
Caption:
Exposed to the elements, the flesh decays.
Fossil hunters are able to spot the remains only after the sediment layers (strata) have been brought back up to the surface and re-exposed through earth movements or deep erosion. Experts have now learned to search out the right kind of strata that were originally laid down in the sort of environments occupied by dinosaurs and where their remains might have been buried. The deposits of rivers, lakes and near-shore deltas have proved good prospects, along with the deposits of more arid environments where footprints can be preserved.
Most dinosaurs are known only from incomplete and fragmentary skeletal remains. Commonly, the skull is missing, along with the hands and feet. Of the 600 or so different kinds of dinosaurs known so far, most are represented by just a single species. We have probably found only the tip of the proverbial iceberg. Even when a fossil has been found, identification is not necessarily certain as it depends on whether the most important characteristics have been preserved. Sometimes specimens of the same kind of dinosaur have been given different names because they have been found in different countries, or because different bits of the same animal have been found in different places.
Caption:
Resurrection in rock.
The first dinosaur fossils were probably found in China many hundreds of years ago, when they were thought to be dragon bones. In modern times it was the early 19th-century discovery of some puzzling fossil bones in the south of England that really started the dinosaur story. In the 1820s, a large 25cm-long (10in) jawbone set with sharp, blade-shaped teeth excavated from Jurassic strata in Oxfordshire was described by William Buckland as Megalosaurus, a new kind of giant and extinct reptile. Around the same time, Gideon Mantell was trying to reconstruct another large fossil reptile that he called Iguanodon from a jumble of bones found in Sussex.
But both Buckland and Mantell were trounced by anatomist Richard Owen, who was the first to distinguish and name the Dinosauria (meaning ‘terrible lizard’) as an extinct group of reptiles in 1842. Owen realized that both Megalosaurus and Iguanodon had features that distinguished them as dinosaurs. For the reopening of Crystal Palace in south London in 1853, Owen and the sculptor Waterhouse Hawkins built the first life-sized dinosaur models, which still exist at the site today. They saw Megalosaurus and Iguanodon as huge, lumbering, four-legged and rather elephantine beasts with massive tails.
By the end of the 19th century the image of the dinosaurs had been transformed by the discovery of much more complete fossil skeletons, especially in North America. It was realized that some dinosaurs had moved around on their large, powerful hind legs, and it was discovered that the dinosaurs could be divided into two main groups: the lizard-hipped saurischians; and the bird-hipped ornithischians. Some of the first giant sauropod skeletons had been found, as well as one of the best known of the giant carnivorous theropods – Allosaurus.
Since then, scientific understanding of the dinosaurs has been revolutionized by new techniques of investigation and new fossil discoveries. From the 1920s, new dinosaur-rich locations have been found in many parts of the world, from Mongolia to Argentina and China. Over the last few decades, Cretaceous localities in China have revealed exceptionally well-preserved and complete skeletons, incredibly some of which even retain indications of the original soft tissues. These finds have caused another revolution in our understanding of dinosaur evolution.
Scientists divide dinosaurs and all other organisms into groups of related forms. Traditionally, the groups have been placed in classes, orders, families and so on, but a new system of classification called cladistics has supplemented these names. Here, branches (clades) are defined by characters that evolved at, or immediately before, their origin. For instance, the bird clade, called Aves, is defined by the possession of wings and primary flight feathers.
Each group has a number of unique designated characteristics that should be found in each of its members. For instance, the ankylosaurs were just one of several groups of four-legged plant-eating dinosaurs but they can be distinguished from the others by rows of bony plates embedded in the skin of their backs, a distinctive bony skull form and small denticulate teeth. The ankylosaurs can be grouped together with the stegosaurs (characterized by their prominent bony back plates) as thyreophorans because of the extensive nature of the armour they had on their backs. The thyreophorans include all dinosaurs that are more closely related to Ankylosaurus than to Triceratops, which belongs in another related group called the marginocephalians, consisting of bone-headed and horned plant-eating dinosaurs.
Both thyreophorans and marginocephalians
