NARRATOR (JACK FORTUNE): No creatures have ever dominated the earth like the dinosaurs, but the great mystery at the heart of the dinosaur story is how they came to take over the world. Since the very first dinosaur fossils were found scientists have puzzled over this one big question. Now scientists are exploring a remarkable new fossil site in a remote part of Argentina that may hold the answer.
DR PHIL MANNING (Yorkshire Museum): There is a very, very good possibility that these dinosaurs are telling us the big story in terms of the evolution of the whole of the dinosaurs.
NARRATOR: It may be that scientists can now begin to fill in the last great gap in our knowledge of the dinosaurs, how they rose up from humble beginnings to rule the world. In Argentina a group of scientists prepare for a major expedition. Their journey will take them back in time to a world that is almost completely unknown to science. It is a world that holds the key to the most extraordinary time in the dinosaurs evolution. Their destination is the heart of the Patagonian wilderness. Here they hope to find the clues that will fill in the great black hole at the heart of the dinosaur story.
DR OLIVER RAUHUT (Trelew Palaeontological Museum): What we have here is a locality that spans this timespan that we know very little about. We have the hope to really get now a big piece of the puzzle concerning the black age of the dinosaur era.
NARRATOR: This is the journey back in time that every dinosaur scientist in the world wants to take because it is a journey that may finally unlock the secret of how the dinosaurs came to rule the world. 150 million years ago Earth was the planet of the dinosaurs. It's a period scientists call the late Jurassic.
DR MATT CARRANO (Stony Brook University, New York): In many ways the late Jurassic is really the golden age of dinosaurs. We have a tremendous diversity of different kinds of dinosaurs, meat-eaters and plant-eaters, forms with armour. All around the world we see these dinosaurs.
NARRATOR: But it wasn't just the variety of dinosaurs that made this their golden age. The late Jurassic was also the era of the giants.
MATT CARRANO: We have dinosaurs of tremendous size, we have plant-eaters 40ft tall, we have animals that must have weighed 50 or 60 tons, so these are immense animals and they're everywhere.
NARRATOR: But the dinosaurs didn't start off like this. 50 million years earlier in the early Jurassic they were very different.
MATT CARRANO: We have many fewer different kinds of dinosaurs, they're relatively primitive looking dinosaurs and compared to what we see in the late Jurassic they're much less spectacular animals. We don't see the sizes and shapes and diversities that we see at that time. They're relatively dull looking animals compared to what we see later.
NARRATOR: And there was nothing to suggest that these early dinosaurs would grow and diversify to so completely dominate the planet for so long. Something turned the primitive dinosaurs of the early Jurassic into the amazing creatures of the late Jurassic, but what was it?
PHIL MANNING: Finding the roots of these giants is actually quite a complicated thing to do. In fact, finding their roots has been sort of something which palaeontologists have been trying to do for the last 200 years. It's a big problem.
NARRATOR: All scientists knew was that whatever had transformed the dinosaurs it happened in the 40 million years of the mid-Jurassic, but mid-Jurassic fossils are very rare.
PHIL MANNING: The frustrating thing is with the middle Jurassic the curtains have been drawn so whatever happened there has been almost hidden from view.
NARRATOR: For decades scientists hoped to find a site that would unlock the secrets of this palaeontological black hole. It would have to be something very special.
MATT CARRANO: What would be ideal is for us to find a site in the middle Jurassic that not only had dinosaur fossils, but had a wealth of dinosaur fossils.
PHIL MANNING: Enough of, of an animal to not only reconstruct it but have maybe several of that animal so you know you're right in your interpretation. Single bones will not do.
NARRATOR: This window on the mid-Jurassic would have to contain more than just dinosaurs.
MATT CARRANO: We need fossils of all the other animals that lived with the dinosaurs.
NARRATOR: And not just the animals.
PHIL MANNING: You want to look at the plants so you can actually try and work out what was around, what these animals might have been eating and start building up more of a, how can I say, a piece of that ecosystem.
MATT CARRANO: So not only can we piece together the puzzle of dinosaur evolution in the middle Jurassic, but we can look at all the environmental changes that might have happened as well.
NARRATOR: But despite years of searching they couldn't find this perfect site and without it the evolutionary master key to the mid-Jurassic remained elusive. Finding a mid-Jurassic site became a great quest of dinosaur palaeontology and then from Argentina came an unusual discovery. A worker in the nuclear industry was prospecting for uranium in the heart of Patagonia. Suddenly his Geiger counter sprang into life, but when he dug down it wasn't a uranium mine he found, but dinosaur bones. The surprise wasn't that the bones had become radioactive - that often happens when fossils form - what was really amazing was that the radioactive dinosaur came from the mysterious mid-Jurassic and all around, as far as the eye could see, were rocks of the same age. News of the find reached palaeontologist Oliver Rauhut. He decided to take a look.
OLIVER RAUHUT: The area up here in Patagonia has some fantastic outcrops of middle Jurassic rocks. That means we have a real chance to get, for one thing, dinosaurs from the middle Jurassic which are very, very rare worldwide. With this we have the hope to really get a big piece of the puzzle in the middle Jurassic.
NARRATOR: Finding one dinosaur wasn't enough to unlock the secrets of the mid-Jurassic and there was no guarantee they would find more and worse, the site was huge, so Oliver quickly pulled together a small team to start searching this vast area for bones. It was a massive task. They would have to comb over a thousand square miles of trackless desert. They might have spent weeks looking and found nothing, but Oliver was lucky. On day one he struck fossil gold. It was a dinosaur leg bone and crucially, it was from the mysterious mid-Jurassic, so the radioactive dinosaur was not a one-off.
OLIVER RAUHUT: The next 10 days we started looking for fossils systematically here and we found bones over here, we found more bones over there. We went further inwards here, further into the valley and found even more material there.
NARRATOR: And that was just the first valley. Within a few more weeks his team had discovered the remains of two big meat-eating dinosaurs and no fewer than six giant plant-eaters. They really had found something truly spectacular - a mid-Jurassic dinosaur graveyard and then it got even better. They found more. Fossils from the mid-Jurassic that they'd never expected to find, everything from strange fish to flying reptiles, even plants.
DR ADRIANA LOPEZ-ARBARELLO (Trelew Palaeontological Museum): It was amazing, it was like to open a Pandora's box with many different animals - flying reptiles, mammals, crocodiles.
NARRATOR: Their fossil treasure trove was so complete it even included exquisitely preserved dinosaur skin. It was a mid-Jurassic time machine, an almost complete ecosystem from this missing period in dinosaur evolution. It was everything they'd been looking for.
PHIL MANNING: It was as if someone had unearthed a Holy Grail in the middle of the Jurassic precisely where people wanted to look in terms of resolving this perplexing question.
NARRATOR: Finally they had a chance to find out what had happened to the dinosaurs in the mid-Jurassic because until Argentina scientists had so few facts all they had to go on were their theories, but all these theories did have one thing in common: they all had something to do with one of the most extraordinary events ever to have happened in the Earth's history, because back at the time of the early dinosaurs the Earth was a very different place to today.
OLIVER RAUHUT: You see at the beginning of the Jurassic what we had was all the land masses, all the continents lumped together in one giant super-continent that was then surrounded by just one giant ocean.
NARRATOR: This super-continent was the world of the early dinosaurs, but forces deep within the Earth were already at work.
OLIVER RAUHUT: During the Jurassic this continent started to break apart.
NARRATOR: This was the birth of the world we know today. It was such a huge event scientists instinctively felt it had to have some effect on the evolution of the dinosaurs.
OLIVER RAUHUT: Well this break-up of the super-continent probably had to do something with the diversification of the dinosaurs, but what it was that is what we're trying to find out.
NARRATOR: Finding out how this event had transformed the dinosaurs had obsessed palaeontologists for years. The site in Argentina gave them the chance to test out all their theories. One theory emerged on the other side of the world from Argentina. The clues came from one of the few other places in the world with some mid-Jurassic fossils: Yorkshire. It's not quite on the same scale as Patagonia, but Paul Wignall likes it. He's been coming to this beach for years to explore life in the Jurassic oceans.
DR PAUL WIGNALL (University of Leeds): What we see here is a lovely record of life on the Jurassic seafloor a good sort of 180 million years ago. There's a whole range of sort of typical things that lived on and above the Jurassic seafloor. There's various types of shellfish such as this that lived in the sediment. These as well, they're nice typical sort of Jurassic shellfish called ammonites living and happily swimming above the seafloor. There's a whole range of different fossils, a really big diversity of things.
NARRATOR: The rocks here are littered with this abundance of Jurassic sealife, but it doesn't last. Further up the cliff there's a black layer and here the fossils just disappear.
PAUL WIGNALL: As you can see, as you work your way up through here and we get quite a dramatic change at this point here and all that seafloor life is gone, it's just disappeared completely.
NARRATOR: 80% of life in the oceans had vanished. Everything seemed to have died out and all over the world the same rocks recorded the same catastrophe.
PAUL WIGNALL: This layer which we see here on the Yorkshire coast, we can trace that throughout the world. If we go to places in North America or the Himalayas or something like that, then we can see that rocks the same age have, have the same event in them. It's this very sharp event and it just records this mass death in the Jurassic.
NARRATOR: Just before the crucial mid-Jurassic period there had been a mass extinction and it seemed that a massive event such as the splitting apart of the super-continent had to have been responsible. It was one of the most violent periods in the Earth's history.
PAUL WIGNALL: It's very hard to imagine just the scale of the volcanism which starts off at this time. It's vastly greater, several orders of magnitude greater, than anything we see on earth today. Thousands of square kilometres of, of red hot lava just pouring out of these fissures in the ground covering a vast area.
NARRATOR: And it wasn't just the lava. These eruptions had pumped out millions of tons of poisonous gases. The theory was that these gases had effectively suffocated life in the oceans. Paradoxically, all this death could have been the dinosaurs big evolutionary break because it's a well known evolutionary theory that mass extinctions can have a dramatic effect on evolution.
PAUL WIGNALL: Extinction events are obviously bad news for the things that are around at the time, but in a way evolution's able to experiment quite a lot at this time because there's less sort of competition going on at these moments of time and you often get the sort of major innovations and new, new groups start appearing in the aftermath of extinction events.
NARRATOR: So could a mass extinction have been responsible for the transformation of the dinosaurs in the mid-Jurassic? It would have created a world free of competition allowing the survivors to evolve into all the new types of dinosaur. It seemed a perfectly good theory, but the big test was: was there any evidence that it actually had affected the dinosaurs? Argentina was the perfect place to look. At first there were some tantalising similarities between the rocks in Yorkshire and in Patagonia. They had found fossils from the bottom of a mid-Jurassic lake and in these rocks they found one particular layer which was very strange.
ADRIANA LOPEZ-ARBARELLO: One fits here, another here, another one here and it's here there is another one, so many pieces all together in the same layer.
NARRATOR: This layer was packed with hundreds of fish and they'd all died the same way: all the fish were twisted, contorted. They had clearly died a terrible death. Was this then evidence of a mass extinction? When they looked further there were clear signs that the fish had been killed by a volcanic eruption, but then the evidence became more confusing. When Oliver's team looked on land they found nothing, no sign of an extinction event affecting the dinosaurs. Whatever had killed the fish and caused the mass extinction in the oceans, it hadn't affected life on land. That meant dinosaur evolution couldn't have been kick-started by a mass extinction. That theory was wrong. It meant scientists would have to find something else, something equally connected to the break-up of the super-continent and powerful enough to have transformed the evolution of the dinosaurs, so they turned to another theory: climate change. Judy Parish has devoted her career to studying the climates of the past.
PROF JUDY PARRISH (University of Arizona): It's sort of like mystery novels, so this is a little bit like a detective story, putting together bits and pieces of information to figure out what the world was really like in the past. It's really fascinating the idea of this super-continent and just the fact that it would have to have, have to have a major effect on climate, so I got really interested in how this might develop.
NARRATOR: These spectacular cliffs by the Colorado River in the United States are from just before the super-continent split apart. This makes them just the right age to unravel this climatological mystery.
JUDY PARRISH: Geologists have known for a long time that there was something really odd about the climate of this time period. It's in these rocks that we can really figure out what the climate was like.
NARRATOR: Judy's spent years trying to decipher the clues locked inside these 200 million year old rocks, but what she found was something very puzzling. In the rocks there were brown layers which came from old riverbeds. Above this were grey layers made up of lake muds. These two layers seemed to be evidence for a very wet climate, but nearby in the same formation there were very different rocks, fossilised sand-dunes. This was evidence for a very dry climate. It didn't seem to make sense.
JUDY PARRISH: You might see one report that said it was desert and then you might see another report that said it was tropical rainforest in the same place, but clearly both of those can't be right. Well you wouldn't think they could both be right and so it was just sort of trying to figure out what that all meant, how you could, how these two radically different signals in the same piece of geology that, that really was one of the things that made this so interesting and surprising.
NARRATOR: But as she studied these contradictory signals, Judy realised there was one answer that would solve the riddle. The climate had to be two things at once - a hot, dry season and a short but very intense wet season. This would explain how rivers and deserts could appear at the same time. There is a word for this type of climate: monsoon, but Judy's theory is that this was a monsoon like no other, a mega-monsoon.
JUDY PARRISH: The word monsoon comes from an Arabic word that means season, so a mega-monsoon is where that really goes to extremes.
NARRATOR: This climate of extremes, veering from very wet to very hot and dry, was unique to the super-continent.
JUDY PARRISH: One of the things that would have been characteristic of this very strong monsoonal system is that you would have had a similar climate over a very large part of the world, so this would have been a world in which the environments were not particularly diverse.
NARRATOR: According to the theory, a uniform world led to uniform animals. This was why the early dinosaurs lacked variety and as long as the super-continent existed nothing much was going to change, but then of course something did change. As the super-continent broke up areas that had been landlocked for millions of years would have been surrounded by water. Judy's theory was that this would have transformed the world's climate. Different climate zones would have created new ecosystems and new ecosystems meant new life. If Judy was right, this could have been the key to the dinosaurs transformation in the mid-Jurassic.
JUDY PARRISH: Seems likely that some kind of major climate change, or some major climate system has to have some sort of effect on the evolution of life including that of the dinosaurs, so it's really interesting to try to figure out whether those animals evolved in response to any kind of climate change.
NARRATOR: So was climate change the evolutionary master key that transformed the dinosaurs? Until Argentina, there was no way of knowing. This is where the richness of the Argentinean site came into its own.
OLIVER RAUHUT: Look at that, that's really extraordinary. We have like there a rib, a rib there, a rib there, a rib there, a rib there...
NARRATOR: Because they had found such a complete ecosystem they could search for climatic clues amongst the fossils and since the site covered millions of years they'd be able to see how the climate had changed over time and the first clues came not from the dinosaurs, but from the lake where the fossil fishes had been found. Here there were strange fossils called stramatolites.
DR RUBEN CUNEO (Trelew Palaeontological Museum): At the beginning of the mid-Jurassic we have these kind of structures here that represent stramatolites. Stramatolites, like this, were form in this particular case in lake that occupied most of the central part of Patagonia during the early mid-Jurassic.
NARRATOR: Stramatolites are formed by a kind of algae. They thrive in extreme environments which are both wet and very hot, like the mega-monsoon. It was exactly what would have been expected because this was before the super-continent had split up.
RUBEN CUNEO: I think that the initial conditions of the mid-Jurassic are in accordance with this idea of dominant mega-monsoonal climatic conditions dominating, you know, the entire super-continent.
NARRATOR: And then suddenly everything seemed to change. These are fossilised seed cones.
RUBEN CUNEO: You can see, for instance, this wonderful and beautiful seed cones preserved in a petrified way where you can see inside even the, the seeds, the small seeds in there, you know, that is suggesting that these kind of plants cover most of the Patagonian region during the Jurassic period.
NARRATOR: The seed cones could only come from giant conifer forests. These are trees that thrive in warm, wet conditions, completely different from the mega-monsoon and there was more.
RUBEN CUNEO: This is a small piece of a fern plant that probably grew in some small shrubs, you know, the low ones that were part also of the same forest that was covered, or dominated by the, by the conifers.
NARRATOR: The ferns and the pines could only mean one thing.
RUBEN CUNEO: And this is suggesting that the dry conditions we had at the beginning of the mid-Jurassic are changing to, you know, more humid conditions by the end of the mid-Jurassic.
NARRATOR: As the super-continent split up the hot, dry world of the early dinosaurs changed into a lush, forested one. The world of the early primitive dinosaurs had been transformed. Then, as the climate changed, a chain reaction could have started. Lush forests meant more food for plant-eaters.
PHIL MANNING: If you have a environment from the plants are thriving, likewise the dinosaurs are having to adapt at exactly the same sort of rate to keep up with these changes with the plants.
NARRATOR: And it wouldn't just have effected the plant-eaters.
PHIL MANNING: As these animals are getting larger and larger that are feeding upon these plants there is a good chance that the predators, if they're going to keep up with the plant-eaters, they have to get big as well.
MATT CARRANO: In a sense we would have then had an evolutionary arms race progressing through the middle Jurassic.
NARRATOR: There seemed to be a direct link between climate change and the transformation of the dinosaurs. It really did look like a new climate could have led to an explosion in dinosaur size and diversity. But some scientists had spotted a flaw in the climate change theory. Even if the climate had changed, it didn't follow that all the dinosaurs had to change as well.
PHIL MANNING: Climate is an important factor and it's one which obviously affected the dinosaurs in the middle Jurassic, but it is not a single factor that could have done it alone, for the simple reason dinosaurs, if the climate changed became terrible, they would up sticks and move.
NARRATOR: If the dinosaurs could move they could simply go wherever they wanted to find the climate and vegetation they liked best. On its own climate change wasn't enough to explain the transformation of the dinosaurs in the mid-Jurassic. For climate change to have become a major evolutionary force it needed something else, a physical barrier. If the dinosaurs were forced to stay put they would have to evolve to suit the changed conditions, or die.
PHIL MANNING: Movement is the key 'cos if you actually do break the movement across the continent you bring up a barrier, whether it be waters or mountains, and you actually restrict that group and the climate does continue to change. That's when you will start affecting these populations of animals.
NARRATOR: And scientists realised there was a barrier that was beginning to form at exactly the right time, something else that was created as the super-continent split apart: the Atlantic Ocean. In the mid-Jurassic the new ocean flooded in, cutting present day North America off from South America. Because of the oceans, animals would have been isolated on the newly formed continents allowing them to evolve separately and creating many new types of dinosaur. It's a process scientists call vicariance, but vicariance is very difficult to prove.
MATT CARRANO: Up until now it's been very difficult to demonstrate the connection between vicariance and dinosaur diversity in the middle Jurassic. We have lacked any site that has sufficient high resolution to show us any connection between these processes.
NARRATOR: But that was before Argentina. The first clues about vicariance came from something surprising, a discovery that seemed to have nothing to do with dinosaurs.
DR GUILLERMO ROUGIER (American Museum of Natural History): We were actually looking for some small pieces of bone. We have seen that there were bones in those rocks so we were just essentially chipping little pieces of rock and seeing if there were any bones or fragments in there.
NARRATOR: Dr Rougier doesn't do dinosaurs. He's not even looking for plants. He's after something even more precious and harder to find.
GUILLERMO ROUGIER: People working on dinosaurs think that they have a lot of heavy work and so on, but we looking for tiny little mammals you know little creatures like this and like that, we have an even harder time because they can trip over a skeleton of a dinosaur, but for us to identify a little mammal jaw like this in the field for which only a little portion is showing up it's very tough.
NARRATOR: Mammals were the Cinderellas of the Jurassic. They were tiny, like present day mice, and probably nocturnal. Their fossils are so rare that before the site was discovered not a single Jurassic mammal had ever been found in South America before.
GUILLERMO ROUGIER: We were kind of ready to head back home and kind of a little bit dispirited because after four or five days we haven't really gotten what we were looking for, I saw a little thing that was very suggestive of a jaw of a mammal, but there wasn't enough showing to be sure that that was the case.
NARRATOR: But when he looked closer Dr Rougier saw teeth. He had found a single jawbone and it was from a mid-Jurassic mammal.
GUILLERMO ROUGIER: It's just what you dream for. We were very happy in the field, but of course once we got back to lab and we, after a month of very careful and painful preparation of the specimen we realised how unique this was, that was something beyond our dreams. It was so good.
NARRATOR: What he could do now was compare his mid-Jurassic mammal with ones from the Northern Hemisphere. If he could find any significant differences between the fossils it could be a sign of vicariance. When he compared them it was clear that the South American mammal was a very similar type of creature to those in the north, but when Dr Rougier looked at the jawbones more closely he spotted something.
GUILLERMO ROUGIER: This is the southern jaw and we can see that there is a groove right here on the back of the jaw.
NARRATOR: The South American fossil had a tiny groove. It wasn't much, but to Dr Rougier it was vital because fossils from the Northern Hemisphere don't have that groove.
GUILLERMO ROUGIER: This animal provides physical evidence that the jaws of the animals from the north and the animals from the south were different at this time and it provides probably the only clue that we have for this in South America. This is a unique specimen. It's fantastic.
NARRATOR: As the super-continent split the jaw showed that mammals in the southern continent were evolving separately from those in the Northern Hemisphere. This was evidence that vicariance was beginning in the mid-Jurassic, at least for mammals, but was there proof of vicariance in the dinosaurs? To find out they needed fossils from different times in the mid-Jurassic.
OLIVER RAUHUT: We need dinosaurs from just before the break up of the continent and just after the break up of the continent and that is what we have here.
NARRATOR: So they carefully dug out the fossils and encased them in plaster to protect them on the long journey back to the lab. Only then could the analysis begin. Back at base, Oliver studied the most complete fossil he had from before the super-continent split up. If the vicariance theory was right then it should look very similar to its relatives in the Northern Hemisphere.
OLIVER RAUHUT: It is a medium sized meat-eating dinosaur which, which still means that of course for our modern standards this would be a pretty enormous beast. The total length would probably be something like around five metres which is two times the length of the modern tiger which is the largest land predator that we have. We know that animals of more or less this size move at something like 40km/h. It probably looked very much like the familiar allosaurus of North America.
NARRATOR: The Argentinean dinosaur was an early ancestor of the predators that went on to dominant the late Jurassic, but the crucial thing was that it was virtually identical to dinosaurs in the Northern Hemisphere and this could only be the case if the dinosaurs were able to migrate freely between north and south.
OLIVER RAUHUT: According to my preliminary analysis, it seems that these animals do not seem to show any differentiation between the dinosaurs from the Northern and Southern Hemisphere and this is the time more or less directly before the final split of the Northern and Southern Hemisphere, the final separation of North and South America.
NARRATOR: One continent, one type of meat-eater. It was just as the vicariance theory predicted, but then they had to compare dinosaurs for signs of vicariance from after the super-continent split up. If the theory was right, there should be some differences between dinosaurs from the north and the south, but because the split had only just happened these differences would only be slight and very hard to spot and at first he couldn't find anything significant. In the end he looked again at the radioactive dinosaur. Of all the fossils this was the youngest, from well after the super-continent split up.
OLIVER RAUHUT: Now the interesting thing about this fossil is that this comes from a part of the rock sequence that we actually now believe to be younger than the other fossils that we've seen and with this kind of fossils hopefully then we can answer the question whether the dinosaurs took different evolutionary pathways in the Northern and Southern Hemisphere.
NARRATOR: And so Oliver went to work. He systematically examined the fossil looking for even the tiniest differences to dinosaurs in the north. He made many measurements of the bones. Then he used a computer programme to compare them to other fossils from the Northern Hemisphere and at last he found something. It was to do with the upper leg bone, the femur. Oliver saw the radioactive dinosaur had a distinctive bulge near the top of the femur, but when he examined femurs from the Northern Hemisphere there was no bulge. It was exactly the kind of tiny anatomical difference he'd been looking for.
OLIVER RAUHUT: So what the analysis of this animal tells us is that obviously the continental break up really had a profound impact on the diversification of the dinosaurs.
NARRATOR: It was a clear sign. Dinosaurs in the south were beginning to evolve separately to those in the north. It was vicariance in action. It's still early days in Argentina and Oliver's work may yet yield many new and different avenues of research, but already it is giving us a glimpse of what might have been happening in this mysterious, dark age of the dinosaurs. The transformation of the dinosaurs began when the world was ripped apart in the mid-Jurassic. As the sea flooded in dinosaurs became cut off from each other allowing them to evolve separately. The newly formed continents began to develop new climates. This more varied world offered the dinosaurs a host of new opportunities. There were habitats to suit dinosaurs of every shape and size and it was a world they would go on to rule for almost 100 million years.
Why did dinosaurs like brachiosaurus suddenly become so large? For years scientists have been trying to find the mysterious evolutionary master key responsible for transforming the dinosaurs into world-beaters. In the early Jurassic, 200 million years ago, they were a relatively small group of primitive creatures. By the late Jurassic, 50 million years later, they had become the magnificent array of carnivores and giant plant eaters that would dominate the planet for millions of years. In between lies the mysterious period of the middle Jurassic in which all these changes must have happened. But what were they? What was it that transformed the dinosaurs?
Was there some terrible mass extinction? Had there been an amazing change in the environment? All this was speculation and theory. How and where would evidence come to light? Fossils from the middle Jurassic are incredibly rare. All anyone had to go on were a few small outcrops of rock dotted around the world.
Then a treasure trove of fossils emerged from the midst of an Argentinian wilderness in the 1990s; thousands of square miles of mid-Jurassic rocks. On their first season in the field, palaeontologist Oliver Rauhut and his team unearthed two giant meat-eating dinosaurs and six huge long-necked dinosaurs. And there was much more: early mammals, crocodiles, fish and even plant life. They had uncovered a complete mid-Jurassic eco-system, a wonderful snapshot of life from this dark age of dinosaurs.
"It's as if someone has unearthed a holy grail of dinosaur palaeontology," says British geologist, Dr Phil Manning. Oliver Rauhut describes the site as, "an extraordinary window on the mid-Jurassic." Above all, the hope is that this site may contain all the information they need to find the mysterious evolutionary forces that have eluded palaeontologists for so long.
No mass extinction?
Already they've been able to test out many of their theories and draw some exciting conclusions. For instance, one theory about what might have happened in the mid-Jurassic clearly does not seem to be supported by the finds in Argentina: the mass extinction theory.
The laws of evolution say that a major extinction event could have caused an explosion in dinosaur diversity like the one in the mid-Jurassic. Death on such a vast scale clears away the competition, allowing the survivors to evolve rapidly into new ecological niches. But there's no evidence in Argentina for an extinction event affecting the dinosaurs.
A second theory was that a major climate change could have transformed the dinosaurs' environment, leading to the evolution of many new types of dinosaur. In Argentina there is indeed evidence for a dramatic change in the climate. At the time of the early, primitive dinosaurs all the continents were gathered together in one giant super-continent (Pangea). The climate of the super-continent was dominated by extremely hot and dry conditions - with rainfall concentrated in a short bursts. Scientists call this the time of the mega-monsoon. Then in the middle Jurassic Pangea began to split apart. The Argentinian site offers evidence that as the super-continent split up, the climate changed to a more moderate, less extreme climate.
Barriers to migration
Many scientists believe that on its own climate change isn't enough to explain what happened to the dinosaurs in the mid-Jurassic. As Phil Manning points out, the dinosaurs could in theory simply move to find the climates they were most adapted to - unless something stopped them from moving, some major physical barrier that meant they couldn't follow the climate zones. When scientists looked into this, it became clear that as the super-continent split up, such a barrier was being formed. Today it's called the Atlantic Ocean. This major barrier would allow an evolutionary process called vicariance to operate - animals on different sides of the barrier are able to evolve separately. The problem was there was no proof of vicariance in the mid-Jurassic. Until Argentina.
Fortunately the site has fossils from just before and just after the super-continent split in two, so it's ideally placed to judge whether vicariance was beginning to take effect. And the early results are lending support that this may have been a key factor in explaining what happened to the dinosaurs in the mid-Jurassic.