Dinosaurs

Caption:

Brachiosaurus, quite possibly too large for most predators.

TRACKS AND TRAILS.

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.

The modern study of dinosaur prints can tell us a great deal about the animals’ behaviour. For instance, from print size, the distance between prints and estimates of the leg length of the animals that made them, it is possible to calculate the speed at which the animal was moving. Some small, lightly built bipedal dinosaurs seem to have been capable of running at speeds of up to 40kph (25mph).

The largest quadrupedal sauropods were so massive that they were capable of only a fast saunter at around 20kph (12mph).

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 problem with many footprints and tracks is that it can be difficult or impossible to link the prints with a particular dinosaur, as very few animals literally dropped dead in their tracks. Instead, interpretation has to come from our understanding of the structure of the limbs, ankles and number of toes possessed by different dinosaurs. Tracks have helped show that some dinosaurs such as the iguanodontids, which were originally thought to have been bipedal, normally moved on all fours even though their fore limbs were smaller than their hind limbs.

THE DINOSAUR ERA.

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.

FOSSIL FORMATION.

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 FINDS.

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.

DINOSAUR NAMES.

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 had distinctive bird-like hip bones that characterize them as belonging to the ornithischians, one of the two main dinosaur groupings. The animals in this group were all plant eaters and most moved around on all four legs. They include such well-known groups as the ankylosaurs, stegosaurs, ceratopsians, and ornithopods (with birdlike feet).

Caption:

A tree diagram showing ornithischian relationships. From right to left:

 Genasaurs and the Lesothosaurus belong to the ornithischian group. .

 From the genasaurs follow two distinct groups: cerapodans and thyreophorans..

Following on from cerapodans:

 There are two groups included in marginocephalians: ceratopsians and pachycephalosaurians..

 Ceratopsians lead to ceratopsids and psittacosaurids..

 From ornithopods follow iguanodontians, leading to hadrosaurids and igaunodontids..

 From ornithopods follow hypsilophodontids and heterodontosaurids..

 From thyreophorans follow ankylosaurians, stegosaurians and Scutellosaurus..

The other main group of dinosaurs is the saurischians, which had a hip structure more like that of lizards. A major subdivision within this group separates the plant-eating, four-legged sauropods and the meat-eating, two-legged theropods. In recent decades the classification of the theropods has been transformed by new evidence indicating that the birds originated within a theropod group known as the maniraptorans.

The theropods group contains the most subdivisions of any dinosaur group, which reflects the considerable range in form and size of its members. These include some of the largest land-dwelling animals, the allosaurs and tyrannosaurs, which reached lengths of 14m (45ft) and weights of up to 6 tonnes (5.9 tons). At the other end of the scale the theropods include tiny dinosaurs such as Microraptor, which had a body length of around 47cm (1ft 7in). Aves also belongs to the theropods and numbers Archaeopteryx among its dinosaur members; it also includes all of the modern-day birds.

Caption:

A tree diagram showing saurischian relationships. From right to left:

 Eoraptor, herrerasaurids, and the two groups of sauropodomorphs and theropods belong to the saurischian group..

 From the sauropodomorphs follow two distinct groups: prosauropods (leading to anchisaurids and plateosaurids) and sauropods..

 From sauropods follow Vulcanodon and eusauropods..

 From eusauropods follow Mamenchisaurus, cetiosaurids and neosauropods..

 From neosauropods follow two distinct groups: diplodocoids (leading to discraeosaurids and diplodocids) and macronarians..

 From macronarians follow camarasaurids, brachiosaurids, Euhelopus, saltasaurids, and titanosaurians..

A final group of small to medium-sized meat-eating bipedal saurischians is still difficult to categorize. These include Herrerasaurus, Eoraptor and Staurikosaurus, which have been found in Late Triassic rocks, especially in South America. Although it is still not clear what their relationships are to the main groups of saurischians, these dinosaurs were certainly more like the theropods than the sauropodomorphs.

Caption:

A tree diagram showing theropod relationships. From right to left:

 Tetanurans, ceratosaurians and coelophysoids belong to the theropods group..

 Ceratosaurians include neoceratosaurians and Ceratosaurus..

Following on from tetanurans:

 From tetanurans follow avetheropodans and spinosauroids (leading to spinosaurids and megalosaurids)..

 From avetheropodans follow coelurosaurians and carnosaurians (leading to carcharodontosaurids and allosaurids)..

 From coelurosaurians follow maniraptorans, compsognathids, ornithomimosaurians and tyrannosauroids..

 From maniraptorans follow the group known as eumaniraptorans, as well as oviraptorosaurians and therizinosauroids..

 From eumaniraptorans follow the group known as avialans (leading to modern birds and Archaeopteryx), as well as dromaeosaurids and troodontids..

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