My Science School

 

 

                                A group of mammal-like reptiles preceded the appearance of the dinosaurs. However, these early mammals gradually disappeared during the Triassic Period and were replaced by the true mammals. It is difficult to decide exactly which of these extinct animals was a reptile, and which had become a mammal. It is quite likely that the later reptiles had hair and other mammal-like characteristics.

 

 

                            It has been suggested that dinosaur DNA (the substance found in all cells) might have survived in the remains of blood-sucking insects preserved in fossilized tree resin, or amber. The film Jurassic Park was based on this appealing idea. However, we have to be content with the dinosaur descendents that still exist, such as birds, because only tiny and incomplete traces of DNA have been found in this way.

 

 

 

 

 

Was the Earth struck from space?

There is plenty of evidence that the Earth was struck by huge comets and possibly by great stony asteroids. These objects would have devastated huge areas of the land and changed the Earth’s climate for many years. At the time of the dinosaurs’ extinction there is evidence to suggest that an asteroid or comet struck the Yucatan Peninsula in the Gulf of Mexico. It produced tidal waves and global dust clouds that shut out the Sun’s light. Large amounts of the rare element iridium have been found in rocks deposited at this time. Iridium is believed to be characteristic of comets and meteorites.

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                  The extinction of the dinosaurs was not an isolated event. At the same time most marine reptiles and pterosaurs also died out. Also, tiny plankton whose shells form chalk deposits became extinct, as well as ammonites and the remaining species of trilobites. It is very difficult to imagine the causes of this extinction. For example, why did turtles survive, while marine reptiles, well adapted to their environment, such as ichthyosaurs, died out?

 

                        Dinosaurs lived for an enormously long time — some 150 million years — before they died out about 64 million years ago. During their time on the Earth they dominated the land, while other reptile relatives were the dominant forms of life in the sea and in the air. Before the dinosaurs finally disappeared there were two mass extinctions when a large number of species died out. The dinosaurs survived, however, until the end of the Cretaceous Period.

 

Could dinosaurs have continued to evolve?

                     Until they mysteriously disappeared, more and more strange forms of dinosaur were still evolving. Some types disappeared and were replaced by newer forms. This rapid rate of change is in stark contrast to animals such as crocodiles, which hardly altered after the first ones appeared. There is no reason to suppose that dinosaur evolution would not have continued. It might eventually have led to a form of life just as intelligent as human beings.

How did climate change affect the dinosaurs’ extinction?

During the late Cretaceous Period, the world’s continents were drifting into new positions. This constant shifting within the crust led to a huge increase in volcanic activity. Volcanoes spewed out hot lava and gases, which could have built up in the atmosphere to such high levels that they affected dinosaurs and their plant food.

 

 

                       A South American bird called the hoatzin may have some of the characteristics of the very early birds. The young hoatzin has claws on the front of its wings like the Archaeopteryx, which it uses to scramble about in the branches. The adult birds are very clumsy flyers. Unlike all other birds, the hoatzin does not have a specialized gizzard in which food is ground up. Instead it grinds its food inside a large muscular crop. This feature is regarded as being very primitive.

Why is there so much argument about the origin of birds?

The difficulty in explaining the evolution of birds is that there are so few fossils. Bird bones are very fragile and honeycombed, with small air spaces. They crumble away very easily after death, and consequently bird fossils are extremely rare.

The only fossils of ancient birds are those where the animals fell into shallow freshwater lagoons and were covered very quickly with mud, which prevented decay or scattering of the remains by predators or scavengers. These special conditions appeared only occasionally, however. Until more types of bird fossil are found it will be difficult for scientists to fill in the evolutionary gaps and find out more about the origin of birds.

 

                One recent fossil of Archaeopteryx was identified as Compsognathus, a small theropod dinosaur, until it was later re-examined. The problem had arisen because there were no obvious feather impressions, which shows the similarity between dinosaurs and birds.

Which recent discoveries have changed views on the origins of birds?

Many recent discoveries show how birds may have evolved. A fossil found in Madagascar had a sickle-shaped claw on it foot like the predatory theropods Deinonychus and Velociraptor. It also had hollow bones like a bird, and traces of the points where feathers would have been attached. Fossils of a whole range of feathered dinosaurs are now being found in many countries, and especially in China. All these creatures were built along the same lines as birds. Some were undoubtedly dinosaurs and would have used their feathered forelimbs to help them run, rather than to fly.

How might flight have developed?

The evolution of birds has met with more argument than almost any other part of palaeontology. The reason why flight developed will probably never be proved. Some scientists suggest that small feathered dinosaurs ran along the ground flapping their arms to help them catch their prey. Others suggest that flight started when the ancestors of modern birds climbed and leaped about in trees, using their feathers to extend their leaps and eventually to glide.

The simple structure of the wings of Archaeopteryx means that it would have been an extremely clumsy flyer. It probably needed to use the fingers that were attached to its wings to help it scramble about in the trees.

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                          A single fossilized feather was discovered in a German quarry in 1860. It was identical to the feather of a modern bird, and a year later Archaeopteryx was discovered, also in Germany. This find caused a sensation, because Darwin’s theory of evolution had just been published and was causing enormous interest. Archaeopteryx looked like a small dinosaur, but there were clear impressions of feathered wings and a tail. It had teeth in its jaws, a long tail and fingers extending from the front of the wing. It appeared to be a true ‘missing link’.

                      Modern birds never have teeth, and their tail is reduced to a small stump; the ‘parson's nose’. Archaeopteryx had clearly evolved from a reptile, but the type of reptile is still a matter for dispute.

Why was Archaeopteryx thought to be a fake?

Ever since its discovery, scientists have argued about Archaeopteryx. In 1985 some scientists expressed the views that the fossils (several had been discovered) were nothing but an elaborate hoax. They claimed that the feather impressions had been printed over the remains of a small dinosaur. Detailed re-examination proved this claim to be false, and there is no doubt that Archaeopteryx did have true feathers. Whether it was a dinosaur or a bird is open to argument, however.

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Pterodactylus had a wing span 40 cm across. This means that it would have been a fast and agile flyer.

Did pterosaurs have feathers?

Pterosaurs did not have feather, because their large wings were more like those of a bat than a bird. However, pterosaurs did have fur! Fossils have shown that the body of many types was densely covered with hair, and even the wings were lightly furred. This seems to suggest that pterosaurs were warm-blooded, as there would be no point in the body of a cold-blooded creature, such as a modern reptile, being insulated.

Did pterosaurs evolve into birds?

Despite their apparent similarities, pterosaurs were quite different in structure to birds, and they never developed the powerful breast muscles needed to beat their wings in the same way as birds. Similarly, they could not fold their wings away like modern birds, so would always have been clumsy when moving on the ground.

How did pterosaurs fly?

Scientists used to think that pterosaurs were unable to flap their wings and fly like a bird. Instead they probably launched themselves off cliffs and glided on upward currents of air. More recently it has been suggested that pterosaurs were actually very efficient flyers. Some of the smallest types would not have been effective gliders and must have fluttered their wings like modern birds. This would not have been possible for the giants, which must have been pure gliders.

Picture credit: google

 

 

 

As with most other extinct animals, new discoveries about pterosaurs are constantly being made. The size of pterosaurs that have been discovered has increased enormously in recent years. One of these, Quetzalcoatlus, had a wing span greater than 15 m, which is larger than that of many light planes. Its body was about the size of a human’s, but much lighter.

Palaeontologists have difficulty in understanding the diet of Quetzalcoatlus. This pterosaur has been found in inland fossil deposits, so it would not have been a fish-eater. Some people think that it may have been a scavenger, like a vulture. However, it is difficult to imagine such an enormous and awkward creature scrabbling around on a carcass on the ground.

What did pterosaurs feed on?

All pterosaurs were carnivorous, and most are thought to have fed on fish because of their long pointed teeth. As the pterosaur swooped low over the water it would have snatched fish from the water’s surface. Some toothless pterosaurs may have had a pouch in which small fish could be trapped, like that of the modern pelican.

One extraordinary form called Pterodaustro had a long upturned curved beak filled with hundreds of fine bristles. These were probably used to sieve tiny shrimps from the water, like the beak of the modern flamingo.

How did pterosaurs get around on the ground?

The experts are unable to agree about how pterosaurs moved about on the ground. We know for certain that their huge wings could not be folded away as neatly as those of a modern bird, and they would have been very ungainly on the ground. Most palaeontologists think that pterosaurs scuttled around on their hands and on the feet of their hind legs, with their wings folded and trailing behind them. A different view is that some pterosaurs may have scuttled about upright, running on their hind legs.