Dinosaurs. Douglas Palmer
fight one another. Disputes over food, territory and mates commonly lead to conflict, and dinosaurs were no exception to the rule. However, our understanding of dinosaur disputes is biased by what information we can recover from the fossil record and what can be inferred from general principles of animal behaviour.
Obviously, the main arena of conflict results from the predator–prey relationship between meat eaters and plant eaters. Predators have to be appropriately armed to catch and kill their prey and, as we have seen, this is primarily a matter of being able to detect the prey, surprise it and then catch it with claws or teeth. Inevitably, eggs, babies and juvenile dinosaurs would have been most vulnerable to predation. From the prey’s point of view, the matter is largely one of flight or fight – in other words, being able to run away or defend itself.
Some of the small plant-eating dinosaurs opted for the flight mechanism and were fleet of foot. The two-legged ornithomimids, for example, may have been able to run at speeds of up to 40kph (25mph). At the other end of the scale, giant sauropods, once grown up, were so big they would have been invulnerable. Medium-sized but heavy, slow plant eaters such as the ceratopsians, stegosaurs and ankylosaurs evolved a variety of types of armour and defensive weaponry. The ceratopsians are characterized by their helmet-like skulls with prominent rhino-like horns. The stegosaurs had plates and spikes along their back and tail, while the ankylosaurs had tough bony plates embedded in their skin as well as bone-crunching tail clubs. The same weaponry was probably also used by males when fighting one another in territorial disputes or over females. Some of the defensive weaponry that is not as structurally robust as it looks, such as the neck flanges and frills of the ceratopsians, may also have had other functions, such as for male display, signalling and species recognition.
Caption: A Repenomamus with a baby Psittacosaurus.
Medium-sized plant eaters that were neither fast movers nor well armoured developed other means of defending themselves against predators. The hadrosaurs, for example, were equipped with a variety of strange bony structures on their skulls, some of which may have been used to amplify calls to other members of the herd to warm against a nearby predator.
And, finally, we now know that dinosaurs did not have it all their own way. Recently, the fossil of a badger-sized mammal called Repenomamus has been found with the remains of a baby psittacosaur in its stomach cavity.
Caption:
Brachiosaurus, quite possibly too large for most predators.
When dinosaur footprints were first seriously studied they presented a considerable puzzle. Footprints and tracks found in the early 19th century in Massachusetts, USA, were initially seen as evidence for the Old Testament Flood. But by the 1830s the Reverend Professor Hitchcock of Amherst College had developed the new science of palaeoichnology, or the study of fossil footprints. He thought that the abundant three-toed footprints that occurred in strata dating from the Late Triassic (210 million years ago) and Jurassic (200–145 million years ago) must have been made by birds, some of which had to be much larger than any species living today. We now know, however, that these particular prints were made by two-legged (bipedal) dinosaurs.
Sets of trackways made by the same kind of animal have been found, showing that certain dinosaurs moved around in herd-like groups, especially plant eaters such as the iguanodontids and hadrosaurs. Presumably this behaviour was for protection. In contrast, many theropod trackways tend to be solitary, showing that they were lone hunters, although not all behaved in this way – some theropods are also known to have grouped together.
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.
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