Dinosaurs. Douglas Palmer
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theropods active hunters that chased down their prey, or were they scavengers who used their size to chase other meat eaters away from their kills? And did the giant plant-eating sauropods continue growing over a long period or did they grow faster than many of the large animals that are alive today? There is still much to be discovered, and who knows, maybe one day you will be the one to answer some of these questions.
Since the early decades of the 19th century, fossil hunters have dug up hundreds of different kinds of ancient reptiles that we now know as dinosaurs. From tiny bird-sized creatures to the largest animals ever to have lived on land, the dinosaurs were an extraordinary group of animals. Ancient peoples populated their myths and legends with all sorts of strange monsters such as dragons. Today, the role of these creatures has been filled by the dinosaurs – real animals, many of which were just as strange as any fictional beast.
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Therizinosaurus, fantastic fiction or actual dinosaur?
The dinosaurs first appeared around 230 million years ago and altogether form a special group of reptiles with certain distinct characters that were first recognized in the mid-19th century by the English anatomist Richard Owen. These features are preserved in the structure of their skeletons, especially their hip bones, and they separate dinosaurs from other reptiles both living and extinct. A number of extinct and distant reptile relatives of the dinosaurs, such as the flying pterosaurs and the sea-dwelling ichthyosaurs and plesiosaurs, are often described as if they were dinosaurs, but this is not the case.
Caption:
Iguanodon, first described by Gideon Mantell.
Caption:
Ornithomimus, a small and agile Ostrich-like dinosaur.
Caption: A crocodile, a living relative of the dinosaurs.
The closest living relatives to the dinosaurs are the birds and crocodiles, with which they share certain features. All three groups share the presence of extra openings in the skull and jaw. With the birds they have in common lightly built bones, the bony structure of their legs, a hinged ankle, and aspects of the skull and jaws. And while the dinosaurs are also similar to crocodiles, they differ from them and other reptiles in the bone structure of their legs, feet and hips. Crocodiles, like most other living reptiles, sprawl with their limbs held out to the side of the body. However, the dinosaurs have their limbs tucked in under the body, a structure that allows them to walk and run more efficiently.
The image and idea of dinosaurs as once living animals has changed enormously since their fossil remains were first discovered and recognized as belonging to a distinct group of extinct reptiles. Even the iconic status of the giant meat eaters like Tyrannosaurus rex has been partly displaced by that of small, fast-moving and highly active predators, some of which are known to have been covered with hairlike down and feathers.
Caption:
Archaeopteryx, a feathered dinosaur descendant.
Nobody has ever seen a live dinosaur, so our ideas about what exactly they looked like and how they behaved mostly have to come from our interpretation of their fossil record. Using living reptiles as a model for how dinosaurs might have looked led early investigators astray, because in many ways the dinosaurs were very different from crocodiles and lizards.
Once it was realized that dinosaur skeletons were put together in a rather different way from those of any living animals, scientists had to go back to the basics of anatomy to try to reconstruct what the creatures really looked like. As more and better preserved skeletons were found, many surprise discoveries were made. Some dinosaurs – for example, sauropods such as Diplodocus – were immense, with massive bodies supported on four pillar-like legs; if anything, they resembled large plant-eating mammals. Other dinosaurs were more like birds in that they supported their body weight on their two hind legs. These dinosaurs could be pretty immense, too, especially some of the meat-eating theropods such as Allosaurus.
Accurate reconstructions of the skeletons found have allowed scientists to gain an idea of the size and shape of the 600 or so different kinds of dinosaurs. They can also show how the dinosaurs moved about and, along with the shape of their teeth, indicate how they fed. However, very rarely do fossils tell us anything about other aspects of dinosaurs’ behaviour, such as their breeding or social habits, or the fine detail of their appearance, including the texture and colour of their skin. Surprisingly, however, new discoveries are revealing some of these details to us, as we shall see.
For more than 200 years anatomists have realized that the careful study of the way the bones of a skeleton fit and work together – called comparative anatomy – allows the reconstruction of extinct beasts whose body shape may be quite unlike that of any living animal. The reason for this is that all animals with bony skeletons, from fish to humans (known collectively as the vertebrates), share a common structure. This is still true even when parts of the body perform quite different functions. For instance, the human arm and the wings of birds and bats have a basic structure similar to the front leg of a Diplodocus, the arm of Tyrannosaurus and the paddle of a whale. By assessing the different proportions and structures of fossil bones with equivalent bones in living animals, their functions in the extinct animal can be predicted.
Caption:. Bodybuilding: Velociraptor skeleton and musculature.
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Stegosaurus, with its distinctive bony back plates.
Detailed study of fossil bones can also indicate the presence of other features such as where muscles attached and how big they were, and where blood vessels and nerves ran. The thickness of a bone’s wall and its interior structure indicate how strong it was and whether it could carry a heavy body or was instead adapted for swimming or flight.
Sometimes, fossil skeletons are found that are nearly complete and have their various bones still articulated in their true anatomical positions. Such discoveries have helped confirm what anatomists have predicted, but they have also sometimes led to surprises. Some dinosaur structures are so original and unusual that they cannot easily be predicted by comparative anatomy. For instance, what exactly the back plates of a Stegosaurus and the hand claws of a Therizinosaurus were used for is still something of a puzzle.
Very occasionally, imprints or mineralized soft tissues are preserved that show that a particular dinosaur had a scaly skin or a feather-like covering. Even internal organs such as the gut and stomach contents have been preserved in some instances, giving important clues as to how the animals lived.
Caption:
The dome on the skull of Pachycephalosaurus could be up to 25cm (10in) thick.