Читать книгу Dino Gangs: Dr Philip J Currie’s New Science of Dinosaurs - Josh Young - Страница 6

What exactly is a dinosaur? Dinosaurs were a diverse group of animals that first appeared some 225 million years ago and lived on all seven continents, with more than 1000 species having been identified. Though they had a slight resemblance to lizards, dinosaurs distinguished themselves from other animals on the planet at the time because many of them could walk on two legs. The primary characteristics that scientists use to classify these terrestrial animals were an upper leg bone with a ball and socket joint at the hip, a streamlined lower leg bone and an ankle that functioned as part of the lower leg bone.

Dinosaurs roamed and dominated the planet for more than 160 million years during the Mesozoic era, making them one of the most successful animal types ever.

The Mesozoic era is divided into three periods: the Triassic (225 million to 213 million years ago), the Jurassic (213 million to 144 million years ago) and the Cretaceous (144 million to 65 million years ago). Dinosaurs were at their most diverse and evolved in the Late Cretaceous period, the time when tyrannosaurids ruled the Northern Hemisphere, and it is this period that is the primary focus of many palaeontologists.

At the end of the Cretaceous, dinosaurs had been around for some 160 million years and were at the peak of diversity. The different groups were highly specialized in many ways. They lived everywhere on the planet, all the way from the Arctic to the Antarctic at that time, in environments that were quite diverse. In North America, there were coastal regions and areas that were closer to the mountains that were rising at the time, and there were also dry areas and deserts in some places. Each one of those habitats was home to different groups of dinosaurs because each dinosaur species was adapted for specific climates and environments.

Size is often the first thing people think of when dinosaurs are mentioned. Though the dinosaurs portrayed in popular culture are usually shown to be huge, the fact is that most dinosaurs were the size of humans or even smaller. Because the fossil record is incomplete, scientists sometimes use educated guesswork to estimate the absolute size of the biggest and smallest dinosaurs.

The longest dinosaur is believed to have been Seismosaurus, which lived during the Late Jurassic period. Seismosaurus was a sauropod, or plant-eater, with a long neck and swooping tail that from head to tail measured between 35 and 40 metres (115–135 feet) long. The length is estimated because only part of the best Seismosaurus skeleton was recovered from its northern New Mexico site and the specimen is still being prepared. Despite Seismosaurus’ impressive length, it weighed less than 30 tonnes – heavy but nowhere near the heaviest sauropods (found in Argentina) that are believed to have weighed as much as 100 tonnes. (The largest carnivores were Tyrannosaurus rex and Giganotosaurus carolini, both of which lived in the Cretaceous period, the former in North America and the latter in South America. They each measured about 13.5 metres (45 feet) long and weighed about 6 tonnes.) By comparison, a blue whale can grow to 30 metres (100 feet) in length and weigh a shocking 180 tonnes.

On the opposite end of the scale, a herbivore find that was aptly named Mussaurus, or ‘mouse lizard’, would have fit in the palm of an adult’s hand. Compsognathus, or ‘elegant’ dinosaur, one of the smallest adult carnivores, was the size of a large chicken and weighed approximately 3 kilos (7 pounds). Microraptor was an even smaller carnivore find in China from the Early Cretaceous.

Dinosaurs fall into two basic categories: herbivores (plant-eaters) and carnivores (meat-eaters). The plant-eaters greatly outnumbered the meat-eaters. Some of the best-known herbivores were all quadrupeds, including sauropods, anklyosaurs, stegosaurs, hadrosaurs and horned dinosaurs like Centrosaurus. Sauropods had long necks, tiny heads and massive bodies. Ankylosaurs were very wide-bodied with massive plates of bone covering them like armour. Hadrosaurs are commonly called duckbilled dinosaurs because their mouth looked very much like a modern duck’s bill. Stegosaurus was distinguished by rows of bones along its back that developed into plates. Centrosaurus is one of the most common ceratopsians, or horned dinosaurs, which can be identified by their unique skull features not found elsewhere in the animal kingdom. On the tip of the upper jaw is a rostral bone, which forms what looks like a parrot’s beak.

‘Fossils – objects that have gone through permineralization.’

The best-known bipedal carnivores are Allosaurus, Velociraptor, Albertosaurus and Tyrannosaurus. Allosaurus, or ‘different lizard’, was a large predator in the Jurassic period with extremely sharp teeth and it often measured over 8.5 metres (28 feet) long. Velociraptor and its closest relatives (the ‘Raptors’) were feathered dinosaurs, most of which only grew to the size of large dogs. Each had four claws on each foot, one of which was adapted as a can opener and used to disembowel prey. Albertosaurus is noteworthy for its two-fingered hands and massive head containing dozens of large, sharp teeth. Despite being a top predator in its area and weighing more than 2 tonnes, Albertosaurus didn’t come close to measuring up to the monstrous Tyrannosaurus.

Dinosaurs were given their names by the scientists who described them in scientific papers. Because all languages are different, the names are then translated into Latin or ancient Greek, the common languages of scientists the world over, despite being ‘dead’ tongues. There is no set way to name a dinosaur. Some of the names focus on a characteristic of the dinosaur and others on how it might have lived. For example Tyrannosaurus rex means ‘tyrant lizard king’, while Tarbosaurus translates as ‘terrifying lizard’. Other names refer to the locations where they were discovered; Albertosaurus was first found in Alberta, Canada. Dinosaurs have also been named as tributes to people, such as Othnielia, which was named in honour of palaeontologist Othniel C. Marsh.

What scientists know about dinosaurs has come from fossils – objects that have gone through permineralization, the process by which minerals are deposited in the pores of bones and turn to stone. Fossilization is a very fickle process. The sediments and groundwater must be right for preservation. There must be an accumulation of sediments and no rain and wind washing them away over the millions of years. In the mountains, erosion prevents long-term accumulation of fossils, because if bones were buried, it wouldn’t be for very long because the sediments would be washed out of the mountains into the low lands.

Fossil is derived from the Latin word meaning ‘dug up’, and this is truly the case for dinosaur fossils. Most evidence of dinosaurs comes from original bones infilled with minerals, rather than from imprints of them frozen in time or bone that has been replaced by stone. In some cases, dinosaur bones were encased in ironstone nodules after they were buried, and this protected the bones from water-carrying minerals in solution so that to this day the fossils look just like modern animal bones. Others lines of evidence come from footprints, eggs and even skins that have been fossilized and preserved in stone. Scientists are able to date many dinosaur fossils from the rocks they are found in, and this has enabled them to establish an accurate timeline for dinosaurs despite the fact that they lived millions and millions of years ago.

The dinosaur timeline was established largely through a process called radiometric dating of fossils. This involves comparing radioactive isotopes to the decayed material found in the same rocks or the surrounding layers of rocks. In North America, significant volcanic activity created ash beds that contain radioactive material. In this case, when scientists find a layer of ash, then a layer of fossils, then another layer of ash, they know that the fossils between those two ash beds are bracketed by the two dates, the lower bed providing the older date and the higher bed the younger. They can then compare those fossils to similar ones found in Europe and conclude the Europeans ones are the same age. The radiometric dating techniques used provide dates of plus or minus 10 per cent accuracy, so 90 million years would have left a 9-million-year margin of error. But more sophisticated testing now provides dates to plus or minus a couple of hundred thousand years. In human age, that’s not very close, but in terms of dinosaurs, it is.

Dinosaur footprints. Four trackways of dinosaurs moving to the upper left, and at least one other dinosaur coming from the upper right and one from the lower left.

James Steinberg/Science Photo Library

Fossilized dinosaur footprints have been helpful to palaeontologists in determining what kinds of dinosaurs lived in certain areas. Though they can only rarely identify the species of the animal through footprints, palaeontologists can tell the general type of dinosaur that made the prints. Footprints are particularly revealing in situations where there are consecutive footprints that continue in one direction. These are called trackways, and they have enabled palaeontologists to draw both physical and behavioural conclusions about dinosaurs. Trackways reveal that most dinosaurs walked upright and did not drag their tails. They also show evidence of which dinosaurs were living together. And in certain situations, the stride lengths in the trackways can be measured and used to estimate speed. Without fossilization, this would not be possible.

There are long-simmering controversies among palaeontologists about the details of how dinosaurs were born, how they lived and how they died. Although there is no exact way to determine how long each species of dinosaur lived, scientists estimate that most species lasted between 2 and 5 million years. Their life spans varied by species and size. Some of the larger dinosaurs, such as Allosaurus, are believed to have lived for around 50 or 60 years, while smaller ones like Compsognathus may have lived for only 5 to 10 years. Velociraptor is estimated to have lived for about 20 years, and Tyrannosaurus and Tarbosaurus about 30.

One of the most hotly debated questions was whether or not dinosaurs were warm-blooded. The advances in that area changed dinosaur science and offered a prelude to more complex theories.

In the late 1960s, palaeontologist John H. Ostrom led the way in arguing that dinosaurs were warm-blooded. Ostrom was a professor at Yale University and in his later years served as Curator Emeritus of Vertebrate Paleontology at the Peabody Museum of Natural History. This was a radical idea at that time. Ostrom’s 1964 discovery and subsequent study of Deinonychus led him to conclude that the animal’s horizontal posture and sleek body, combined with the sickle-shaped claw on each foot, dubbed the ‘terrible claw’, offered convincing evidence that it was an active predator with a high metabolism. He could also see that it looked very much like Archaeopteryx, the first bird. He had also found multiple specimens of Deinonychus in the same quarry in Montana. This caused him to assert that small, meat-eating dinosaurs were behaviourally complex and may have lived in packs. Ostrom’s student, Robert Bakker, further argued these characteristics meant that dinosaurs were, in fact, warm-blooded.

Velociraptor.

Joe Tucciarone/Science Photo Library

These revolutionary theories changed the way dinosaurs were shown and started what Bakker later dubbed the ‘dinosaur renaissance’, a period of study that eventually would double our recorded knowledge of dinosaurs. ‘All of those ideas were coming out of that one find,’ Phil Currie says.

There are now many lines of argument that dinosaurs were warm-blooded. One is bone histology, or microscopic anatomy. ‘The structure of dinosaur bones is very much like the structure of mammal or bird bones, and it’s quite different than what we see in reptiles,’ Currie says. However, he cautions that, by itself, bone histology doesn’t prove warm-bloodedness, because it may just indicate very active growth. ‘Then you argue you don’t have active growth unless you can sustain it, and you don’t sustain it unless you have warm-bloodedness,’ he adds.

The predator–prey ratio is another line of evidence. Dinosaur finds indicate a 5 per cent predator-to-prey ratio. This indicates that the predators were very active and had to maintain a low ratio of predators to prey otherwise they would consume all the available food. The opposite is seen in cold-blooded animals, where the predator-to-prey ratio can be as high as 50 per cent. Cold-blooded animals like snakes can survive a month on one meal, meaning that their environment can sustain far more predators.

Dinosaur-egg finds, coupled with nesting habits, have added another layer of evidence to the argument that dinosaurs were warm-blooded, and they have also helped palaeontologists study parental behaviour.

The very first dinosaur eggs reported scientifically were found in southern France and England back in the 1800s. However, at the time they were believed to be bird eggs, and therefore they were not properly categorized, despite the fact that they were discovered with dinosaur bones. The first identified dinosaur eggs were discovered by explorer Roy Chapman Andrews in Mongolia in 1923. At that point, scientists re-examined the eggs found in France and England and determined they were dinosaur eggs as well.

The early finds of dinosaur eggs were confused with bird eggs because the knowledge base and number of finds were both limited. Now scientists can easily tell a bird egg from a dinosaur egg by viewing a slice of the egg under a microscope and studying its anatomy. ‘If you look at an eggshell closely, you can see that there are pores going through it and in fact there is a crystalline structure to it,’ Currie explains. ‘Every species of animal has a different crystalline structure to its eggshell. Birds have eggs that are very close to dinosaur eggs, but they are a little bit different in their layering. Basically, dinosaur eggs have an extra layer in the crystalline structure – dinosaur eggs have two layers and bird eggs have a third layer on the outside, which generally isn’t found in most dinosaur eggs.’

Preparing a nest of fossilized dinosaur eggs, originally discovered in Mongolia.

Ria Novosti/Science Photo Library

Scientists have looked to dinosaur eggs for clues not only about how they reproduced but how they lived. Noted palaeontologist Jack Horner, who has published numerous scientific papers on dinosaurs and served as technical advisor to the movie Jurassic Park, and his colleague Bob Makela found some of the first evidence that dinosaurs cared for their young. Horner and Makela studied nests of dinosaur eggs discovered in the 1970s in Montana and dated to the Late Cretaceous. The large quantity of nests and the spacing between the nests led them to conclude that these dinosaurs nurtured their babies by protecting the nests and bringing them food. Horner and Makela named the new species of dinosaur Maiasaura, which means ‘caring mother lizards’. In 1983 in the same area, which was nicknamed ‘Egg Mountain’, they found similar egg sites for the carnivorous dinosaur Troodon.

‘What we learned from Egg Mountain, since we had Maiasaura, which are duckbill dinosaurs, and Troodon, which are carnivorous, was that these dinosaurs were doing basically the same thing,’ Horner says. ‘While there was no evidence that Troodon kept babies in their nests, they certainly kept them on their nesting horizon. These nesting grounds suggest that both groups of dinosaurs were social. We haven’t really had evidence for any dinosaurs of any kind since that would suggest they weren’t social. Basically, every group of dinosaur has been found in accumulations together, suggesting that they lived in some kind of aggregation.’

Another egg breakthrough occurred in 1987 when Currie and his team found the first hadrosaur eggs containing embryos in southern Alberta. ‘Identifying dinosaur eggs has only [come about] in the last 30 years because before that there was no association between eggs, embryos and adults,’ Currie explains.

The examination of eggs has also heightened the public’s interest in dinosaurs by making them seem more accessible. Of course, part of the fascination with dinosaurs is that they are big so many people are surprised that dinosaur eggs are relatively small. Still, the realization that dinosaurs had babies is compelling.

‘For a long time, people had just thought of dinosaurs as being some kind of rocks, not so much as being animals that lived or breathed,’ Currie says. ‘When you know they have babies and are very much like modern animals in so many ways. Even within the eggs – we could see that the babies had been moving their jaws so they could grind down their teeth – it says something about the biology of the animals, which brings it home big time.’

The study of the eggs has revealed how some dinosaurs nested and protected their eggs. Oviraptorids, which were small, roughly human-sized dinosaurs, were originally known as ‘egg snatchers’. In the 1920s, the plant-eating Protoceratops was the most common kind of dinosaur at the site where the eggs were found, so the conclusion was that the nest of eggs found with the oviraptorid skeletons belonged to Protoceratops. But as palaeontologists made more oviraptorid egg finds, they realized that the dinosaurs were, in fact, protecting their own eggs, not stealing those of other dinosaurs.

‘We now know that in the case of oviraptorids that they stayed in one spot when laying eggs. First, they laid a pair of eggs, then they turned a little and laid another pair next to the first pair. And then they turned to lay another pair, and so on. So basically they stood on one spot and laid a circle of eggs around their feet,’ Currie explains.

‘Egg finds offer compelling evidence that dinosaurs were warm-blooded.’

He stands in the middle of his office, expands his arms like wings, and excitedly acts out the scene. ‘At the same time they are doing that, they took their hands and scooped sand onto the eggs. Once the first layer was down, they lay a second layer and then sometimes a third layer, all of which were buried in sand. This process created a trench around the nest,’ he continues, mimicking how the ancient animal might have done it. ‘In some cases, we know precisely how they were laying their eggs. In the case of oviraptorids, it was in 1990 when we found our first nest with a mother on top; since then at least four more nests have been found with the mothers sitting on top.’

Currie believes that in addition to egg finds showing how the mother protected her eggs, they offer compelling evidence that dinosaurs were warm-blooded. ‘The study of eggs can even tell you something about physiology. If the mother is lying on the nest – birds lie on their nests to keep the eggs at a constant temperature – it may well tell us that these dinosaurs were warm-blooded and in fact were brooding their eggs and keeping their eggs warm.’

The evidence that supports this theory comes from the layout of the eggs in the nest. The eggs formed a circle around the outside of the nest like a doughnut. The mother was standing in the middle, which allowed her chest to cover the eggs in the front and her tail to cover those in the rear. With her arms outstretched around the eggs to the sides, she could also protect those eggs.

‘If you look at the feathered dinosaurs where the feathers are behind the arms, the feathers would cover those eggs on top,’ Currie says. ‘Large dinosaurs couldn’t do that – they didn’t have large enough wingspans. In the case of these dinosaurs that have been found brooding, it has been suggested that there were several reasons that these long feathers developed, including as a mechanism for shading the nest, for protecting the eggs and/or for keeping them warm. Brooding doesn’t work if you are cold-blooded.’

What have not been found are Tarbosaurus eggs – yet. ‘It’s probably because of the fact that they would have been in an environment with acidic groundwater so the eggs were destroyed,’ Currie says. ‘Of course, there are other possible explanations, one being that they would have nested farther inland in a place where things weren’t getting fossilized. Still, we may eventually find a Tarbosaurus egg in the right place, where it did have a chance to fossilize. If we are lucky, there will be an embryo inside and we’ll be able to put the whole story together.’

Because of their nesting habits and feathers, it has been established that oviraptorids and dromaesaurids like Velociraptor were probably warm-blooded. That has led Currie and several other palaeontologists to argue that it therefore makes sense that the closely related Tyrannosaurus would have been warm-blooded, though this is still considered somewhat forward thinking.

Currie lays out the argument. ‘Velociraptor and oviraptorids were warm-blooded in all likelihood,’ he says. ‘They are feathered dinosaurs; it doesn’t make any sense to have feathers on your body as insulation unless you are warm-blooded. The advantage [cold-blooded] lizards have is that when they get cold, they just move into the sun and they warm up pretty quick. But if you put feathers on them, it would be like taking an ice cube and wrapping a blanket around it and sticking it outside; it doesn’t work. So there are a lot of reasons to think that the little guys are warm-blooded; notably these include the fact that they are so close to the ancestry of birds, and that they have bone histology like modern mammals and birds.’

Tyrannosaurus, like Tarbosaurus and Albertosaurus, was relatively closely related to Velociraptor and oviraptorids, so Currie concludes that because of the fact that they are closely related to these warm-blooded creatures, they were almost certainly warm-blooded, too. ‘It doesn’t make sense that you have guys in your ancestry who are warm blooded, that all your close relatives are warm-blooded and that your descendants are warm-blooded, but you are not,’ he says. ‘So it makes sense that tyrannosaurs were warm-blooded.’

As radical as it was to accept that dinosaurs were warm-blooded and laid eggs, it was even more revolutionary to see them as big, non-flying birds rather than just scary, oversized lizards. In 1974, John Ostrom, the palaeontologist who found Deinonychus, revised his description after the discovery of a more complete specimen and championed the idea that birds were descended from dinosaurs. Robert Bakker backed it up with additional research, and their work provided the dinosaur renaissance with another major development.

Ostrom’s scientific paper reiterated the idea that had been presented a hundred years earlier by British scientist Thomas Henry Huxley, a defender of Darwin’s theory of evolution, who had proposed that birds were descended from dinosaurs. Huxley came to this conclusion after studying Archaeopteryx, the oldest known fossilized bird. Archaeopteryx lived during the Late Jurassic period, and its features suggested that it was a transitional fossil between dinosaurs and birds. Since Archaeopteryx was discovered in 1861, only 10 specimens have been found, despite Herculean efforts to find more.

Additional evidence for this theory came in 1986 when Jacques Gauthier, a scientist who is now based at Yale University, published a list of more than 125 characteristics shared uniquely by birds and dinosaurs. Currie calls this very, very powerful evidence under any kind of modern scientific analysis. ‘We don’t even have such strong evidence for other transitions such as reptiles into mammals,’ he points out. ‘A lot of palaeontologists by the mid-1980s already believed that birds came from dinosaurs. I got into researching it because of the fact that our Late Cretaceous dinosaurs from Alberta were very bird-like in a lot of ways, and it started me thinking about it and publishing on the subject.’

The theory was slow to make its way into the mainstream. Currie believes this is because most ornithologists don’t work on fossils, and only work on modern birds. ‘Maybe 50 per cent of the scientists working in palaeontology believed birds came from dinosaurs, maybe 5 per cent of ornithologists believed it, and very few people in the public believed it because they had never heard of it.’

In 1996, Currie himself came face-to-face with the evidence from one of the very first specimens showing the transition in what turned out to be a complicated situation where cultures, science and publishing collided.

The story began in 1994 when a farmer in north-eastern China found a fossil of what was thought to be a species related to Archaeopteryx and sold it to a local museum. The farmer made some money on the transaction, so he went out and dug some more and found another fossil, which he sold to a different Chinese museum. Scientists were aware of those finds and figured there were more where those had come from. In 1996, at the Tucson Rock and Mineral Show, there were hundreds of specimens resembling this bird, complete with feathers. Excitement began to grow because this was a bird that had never been scientifically described, and suddenly there were already far more specimens of fossil birds than had showed up in more than 100 years. But were they genuine?

For a long time palaeontologists had talked about the possibility of feathered dinosaurs. This discussion stemmed from the theory that if any dinosaurs were warm-blooded, then the most likely would be small dinosaurs; and if they had any kind of insulation on their bodies, it would probably be feathers. By the late 1970s, many scientists had concluded that dinosaurs gave rise to birds. Though it was mostly speculation and there was scant evidence to prove it at the time, dinosaurs were being illustrated with feathers on their bodies in books everywhere.

Currie first saw a picture of the Chinese’s farmer’s find in a Beijing newspaper in 1996 after a trip to Mongolia. The article reported that a feathered dinosaur had been found, and the story was accompanied by a picture. ‘It was a little picture that didn’t look like anything,’ Currie recalls. ‘The idea of feathered dinosaurs had been with me for a long time, but still, what are your chances?’

Through a contact, Currie made arrangements to see the ‘feathered dinosaur’ specimen. He was slightly suspicious because he had been given an exact time for his viewing. When he arrived 10 minutes early, his contact walked him up and down the street so they wouldn’t be early. At the appointed time, the contact ushered Currie into a room full of reporters. ‘I realized that I had walked into a press conference and that they were going to show me this specimen in front of the Chinese press to see what my reactions were.’

Several different specimens were bought out one at a time. The boxes were then opened in front of Currie. ‘I would see a beautiful insect with spectacular preservation. Then they would bring out another box, and I would open it up and see a spectacular lizard fossil and so on.’

This dragged on for so long that Currie began thinking it was a diversion and that they weren’t even going to show him the feathered dinosaur. ‘I went to see it not because I thought it was feathered, but because I could see from the photograph that it was a complete specimen of a small dinosaur,’ he says. ‘Small dinosaurs are rare; carnivorous dinosaurs are rare. This was obviously a very, very important specimen’ – provided it actually existed.

Finally, the box with the feathered dinosaur arrived unannounced. ‘When they opened the lid of the box, my eyes probably expanded 20 times like a cartoon character,’ Currie remembers. ‘First of all, the specimen was beautifully preserved, but secondly, my eyes were drawn to these things that were around the outside of the body that were supposed to be feathers. In my mind, I had rationalized that it was probably dendrites or some kind of fungal growth. I just didn’t think the chances of finding a feathered dinosaur were all that good. Sure enough, within milliseconds, I knew that what I was looking at was real, and in fact, we did have the first feathered dinosaur.’

‘My eyes probably expanded 20 times like a cartoon character.’

Fossilized tarbosaur skin.

Dr Philip J Currie

Whether or not it was a legally collected, genuine specimen would take years to resolve. A week after Currie returned to Canada, he began fielding calls first from Japanese reporters and then from British reporters. Days later, on 19 October 1996, the story hit the front page of the New York Times under the headline ‘FEATHERY FOSSIL HINTS DINOSAUR– BIRD LINK‘. The story was accompanied by a drawing done by Michael Skrepnick, the artist who was travelling with Currie in China, and it reported Currie’s assessment that this was, in fact, a feathered dinosaur. ‘The whole world went a little crazy for a while,’ Currie says.

For years, dinosaur feathers continued to provide something of a mystery for scientists. Currie believes there is a possibility all the meat-eating dinosaurs known as coelurosaurs (‘hollow-tailed lizards’) had feathers as babies to provide insulation. The big species then shed those feathers as they grew into adulthood and no longer needed the feathers. The larger a dinosaur became the less its surface area was in relation to its mass or volume. Big animals have a problem ridding themselves of excess body heat. However, a small animal would lose heat really fast. So if small animals are warm-blooded, they have to be insulated in some way, such as with feathers or fur. However, a whale or an elephant is so large that it doesn’t need the insulation. Could it have been the same for Tarbosaurus?

‘It may well be that Tarbosaurus is free of feathers only because it’s big,’ Currie says. ‘When Tarbosaurus were born, they were probably only a half metre or 18 inches long. At that stage, they may have needed feathers. So there was a prediction, which is kind of cool, that if we ever find a small Tyrannosaurus then it should have feathers because it is closely related to these feathered dinosaurs.’

The cool part, Currie says, is that in 2004 a small Tyrannosaurus was found in north-eastern China that was about the size of a German shepherd and it had feathers. Currie explains that the big problem is that there are very few places in the world where conditions are such that feathers would be preserved, though skin impressions are often found in Mongolia and Alberta. Feathers rot away pretty quickly, so typically they decompose before they have a chance to fossilize.

The environment controls what is preserved. In most dinosaur-fossil sites there are no eggs or feathers found. However, in an environment like north-eastern China, where there was a lot of volcanic action, things preserve far better. Volcanic ash would rain down on the lakes. Sometimes the ash would kill a bird or a dinosaur running along the shore, and they would fall into the lake. Because the ash is very fine grained, mixed with the mud in the lake it preserves details very well. More importantly, it alters the chemical environment and kills the bacteria that would otherwise decompose the keratin – the horny material that forms fingernails, beaks and feathers – and leaves these structures preserved for science. ‘Suddenly, you have this amazing situation where you not only get fingernails and beaks but also feathers preserved,’ Currie says. That kind of preservation is critical for scientists to formulate theories about dinosaur feathers that connect them to birds.

Fossilized feathers have also provided scientists of the first evidence of dinosaur colours. Melanosomes, the biological structures that give feathers colour, were recently found to have been preserved in the small feathered theropod dinosaur seen by Currie in 1996 and subsequently named Sinosauropteryx, which lived 125 million years ago. The melanosomes allowed scientists to determine that the dinosaur had a red Mohican with a red and white striped tail. However, scientists have not been able to determine the colour for most other dinosaurs, even those whose skin has been preserved. Early artistic renderings of dinosaurs were in browns and greys and were based on the colour of the larger modern animals such as elephants and Komodo dragons, but scientists still have little to no evidence that dinosaurs were similar in colour to these.

‘We are absolutely nowhere with the colour of Tarbosaurus,’ Currie admits. ‘So far all we have is tarbosaur skin, but we don’t have any evidence of colour banding to show us that there might be melanosomes preserved. Of course, that doesn’t mean that they are not, because what might happen eventually is that somebody might take a look at the skin impression and find out that, yes, there are melanosomes there and we can actually figure out the colour on these guys.’

A close up of Komodo dragon skin – one of the largest reptiles alive on Earth at present – shows what dinosaur skin may have looked like.

Steve Gschmeissner/Science Photo Library

The fact that birds are the direct descendants of dinosaurs means that dinosaurs are not extinct. There are some 10,000 species still around. In fact, dinosaurs are actually divided into two groups, avian (those that fly, which we call birds) and non-avian (the land-dwellers that we normally think of as dinosaurs). For the sake of simplicity, when everyone from the layperson to the veteran palaeontologist uses the generic term ‘dinosaurs’ in conversation, they are generally referring to non-avian dinosaurs.

Dinosaurs were divided into two orders by the British palaeontologist Harry Seeley in 1888: the saurischians and the ornithischians. Seeley characterized these orders (or lineages) by the arrangement of the bones in the hip. The saurischians, which have a pubic bone that slopes down and forward, were named ‘lizard-hipped’ because their hip structure resembled that of a lizard. The ornithischians, which have a pubic bone that slopes down and backwards, were named ‘bird-hipped’. Despite the fact that ornithischians were named bird-hipped because their hip structure was similar to birds, Seeley did not identify any similarity to birds. In fact, further study determined that modern birds actually evolved from the lizard-hipped saurischian dinosaurs, not the bird-hipped ornithischians.

The saurischians include two major dinosaur groups, the sauropods (large herbivores such as Apatosaurus and Diplodocus) and the theropods (meat-eaters such as Velociraptor and Tyrannosaurus). The ornithischians include armoured dinosaurs (such as Ankylosaurus), horned dinosaurs (ceratopsia), and duckbilled dinosaurs (hadrosaurs). Though scientists have concluded that the oldest dinosaurs were 225 million years old, they do not know how much earlier in time the common ancestor of the lizard-hipped and bird-hipped dinosaurs lived.

As with most areas of the study of dinosaur science, there is also a major controversy about the extinction of what we commonly call dinosaurs. Did dinosaurs die out catastrophically as a result the Cretaceous–Tertiary extinction event that occurred 65 million years ago, or did they die out gradually over a long period of time due to climate changes or environmental forces? There is quite a bit of evidence to suggest that an asteroid hit the earth 65 million years ago and that not only wiped out dinosaurs but a great numbers of other animals and plants as well. Except for a few explainable aberrations, there are no non-avian dinosaur fossils above the Cretaceous–Tertiary boundary in rocks younger than 65 million years old.

Like many palaeontologists, Currie believes that when the asteroid hit natural selection was already at work. By the end of the Late Cretaceous period, temperature shifts on the planet were becoming extreme. He points to the rocks along Alberta’s Red Deer River that stretch into Dinosaur Provincial Park as evidence that factors such as climate change were already at work phasing out certain species.

‘If you look at 10 to 15 million years before the asteroid hit, you have more than 40 species of dinosaurs in this region,’ he explains. ‘By 5 million years before dinosaurs became extinct, you have about 25 species of dinosaurs. The rocks that were laid down a million or so years before the end of the Cretaceous along the Red Deer River have fewer than a dozen species of dinosaurs. That is telling me loud and clear that there was something else going on to reduce their diversity, and I suspect it was climatic.’

‘There is no black or white in palaeontology, only differing shades of grey.’

Further, Currie argues, if an asteroid hit and simultaneously wiped out all the dinosaurs, then there should be an abundance of fossils present on the Cretaceous–Tertiary boundary. However, the preserved evidence does not show this. In North America, the fossil record shows that only a few dinosaurs, including Ankylosaurus, Triceratops and Tyrannosaurus, lived until the end of the Cretaceous period. Studying the succession of faunas over a 10-or 15-million-year period shows that species’ diversity was dropping off.

‘Prior to the asteroid hitting, something else was going on,’ Currie reasons. ‘It’s like everything: as humans what we do is try to come up with a simple answer, but in nature there is not necessarily a simple answer.’ He acknowledges that this is just one of many theories on extinction. ‘It’s very easy to come up with a new theory for dinosaur extinction; it’s not so easy to go out and get the evidence. The problem is that it takes years and years of collecting evidence.’

The same can be said about almost all dinosaur theories. There is no black and white in palaeontology, only differing shades of grey. Short of having a time machine to travel back to the Jurassic or Cretaceous periods, palaeontologists must take to the field and dig for evidence, and then hold new fossil finds up against existing ones. Eureka moments are very rare. Discoveries take years to be prepared, studied and then scientifically described before they are presented to the public for further debate. Even under the best of circumstances, they are met with doubting eyes and contradictory theories rather than front-page headlines.