Original subtitles

Dinosaurs.

Masters of the planet for 160 million years.

The biggest, baddest animals ever to walk the Earth.

They had claws a foot long.

And enormous, bone-crushing jaws

with teeth the size of carving knives.

Weighing up to 80 tons, the ground would literally shake when they moved.

But how do we know so much about them?

For over 40 years, Horizon and the BBC have followed

the world's palaeontologists on their quest to find out

what these elusive creatures were really like.

As a palaeontologist, I love digging up the possibility

of monsters of my childhood,

looking for strange beasts that once roamed where I live now.

Over time, with only bones and tiny fragments of information

to go on, scientists have managed to piece together

the complex jigsaw puzzle that is the life of the dinosaurs.

There have been astonishing new finds,

controversial theories...

And extraordinary revelations about these giant reptiles.

Quick, agile, fast-moving.

15,000lbs of gut-crunching terror.

These tantalising clues and breakthrough new technology have enabled scientists

to reach for the answers to the biggest questions of all.

Do we really know what happened to the dinosaurs?

And is there a chance that some might still be alive today?

When Horizon first began reporting on dinosaurs over 40 years ago,

palaeontology was a science based on a lot of speculation

and not that much evidence.

Scientists really had just bits and pieces to go on.

So it's hardly surprising that the dinosaurs we came to know and love

were really just a mixture of fact and fantasy.

The largest flesh eater the world has ever seen.

I'm not afraid!

All children now learn at an early age,

but are reluctant to believe that tyrannosaurus

and all the other dinosaurs followed a well-trod trail to oblivion.

I see a little hole up in his nose.

They've heard other stories about dinosaurs too,

many of which are myths,

replacing the fairy stories of earlier generations.

For our limited knowledge of these pre-historic monsters

provides numerous questions, but very few answers.

CHILDREN LAUGH

Look at that. Nine feet tall. What a monster.

For years, scientists had grappled with fundamental questions.

They didn't know what dinosaurs ate, how they bred.

Sometimes they weren't even sure how the skeletons fitted together.

They also couldn't work out

whether one of the major groups of dinosaurs,

the sauropods, lived on land or in water.

But one of the first major finds covered by Horizon

revealed evidence of sauropod behaviour frozen in time.

Footprints.

In the bed of the Paluxy river in Texas are tracks made

by dinosaurs 70 million years ago,

when the hard limestone rock was mud.

This is evidence that convinces the most doubting tourist.

Some are tracks of the meat-eating dinosaurs,

Others of the heavy, long-necked, long-tailed dinosaurs.

It is these which pose a problem.

Were the creatures who made these tracks swamp dwellers,

or did they move around on land?

30 years ago, the river was dammed and the tracks photographed

and carefully plotted. The shallowness of the footprints

and the absence of tail marks suggested a herd of the animals

living in water sufficiently deep to keep their tails out of the mud.

It's difficult to believe that such huge creatures

weighing up to 80 tons could support themselves out of water.

But here is evidence for just that, a tail mark.

If these creatures could support themselves out of water

on one occasion, couldn't they be ordinary land dwellers

who occasionally ventured to the swamps?

At the time, a single tail mark was not enough to convince

the palaeontologists that sauropods were anything but aquatic.

But as more skeletons were discovered,

their similarities to animals living on land became clearer.

So then we have long straight limbs and a long neck,

adaptations not for a hippo-like existence,

but for living on land, feeding high on trees.

Scientists looked again at the fossil footprints...

And turned to living animals to try and determine how fast

the sauropods could move.

Dinosaur bones are only one source of information.

Present day animals are another.

Neil Alexander is Professor of Zoology at Leeds University.

His main research interest is analysing how animals move.

Recently he's found a way of applying his work

to answering a seemingly impossible question.

How fast did dinosaurs walk?

On the beach at Southport, some vital evidence was laid out.

These are replicas of some of the biggest footprints ever found.

They were found in Texas, and they're not new footprints.

They're footprints made something like 100 million years ago,

preserved as fossils.

Now, these big fellows, these are the hind feet.

It was a four legged animal,

and these hind feet

are about three feet long. Stride length here of eight feet

from right hind foot down to right hind foot down again.

The dinosaur footprints are only part of the information needed.

Now something called a Froude number has to be worked out.

It's a mathematical formula relating the size of an animal's legs

to the way its stride increases as it moves faster.

Now we're going along at about five miles an hour,

and the horse is walking.

Each foot is moving in its own time.

There are no two feet going together.

We're going to speed up a bit, and then you'll see the gait change.

If we go up now to about ten miles an hour, there we are...

The diagonally opposite feet are moving together.

Now if we speed up again and go further, there we are...

Going through a canter into a full gallop.

In the gallop, we've got the two forefeet moving about together,

the two hind feet moving, again, about together.

And now we must be going at something like 20 miles an hour.

Professor Alexander has studied dozens of animals,

from tiny gerbils to huge elephants, and worked out their Froude numbers.

From these, he's able to estimate

what the Froude number for any animal of any size will be.

And so we find out what the Froude number is for the dinosaur,

and how fast the dinosaur was going.

And it's awful slow.

Two miles an hour.

Now, two miles an hour, that's a slow walk for a man.

For something with legs three times as long as a man,

it's a very slow walk indeed.

Professor Alexander's work had reinforced the widely held view

that dinosaurs were slow, lumbering reptiles.

But the discovery of a new kind of dinosaur

would change our thinking.

We have right over here one that I discovered myself,

which I think is one of the most interesting dinosaurs

that's ever been found. In fact, I also think

it's one of the most important dinosaurs ever found.

Let me show you some interesting things

about this fellow. First of all, it's a carnivorous dinosaur.

But not a big one like tyrannosaurus,

it's just a little fella,

probably about four or five feet high,

maybe about eight or nine feet in length.

Weighed maybe about 175 pounds, about your weight or mine.

One of the curious things about him is the construction of his foot.

And the peculiar thing about this is the very large, sickle-like claw

on the one toe.

And remember that in addition to this long bony claw,

there was a horny sheath that fit over that

so that the total claw was probably half again as long.

Obviously not designed for walking,

and quite certainly an offensive weapon.

This strange structure

which we had never seen before in any of the carnivorous dinosaurs

is the reason I coined the name for this that I did. Terrible claw. Deinonychus.

But he was a fella

I wouldn't want to meet on a dark street at night, I'll tell ya.

John Ostrum realised Deinonychus was a ground-breaking dinosaur,

one that overturned long-held ideas about how they moved.

So the picture that we get from Deinonychus

seems to be completely different

from the old picture that we had of dinosaurs as sort of sluggish,

sun-basking animals like modern lizards

and turtles. Deinonychus seems to be a very quick, agile,

fast-moving, two-legged predator.

Good balance control means a high neurological development.

This discovery is what sort of pushed me over the brink

into looking at dinosaurs in a whole new light.

Other dinosaurs too were suddenly seen as fast-moving, agile creatures.

And Ostrum's new ideas about them developed like this.

In the animal world, there's a major division. In one group,

there are mammals which are active, hot-blooded creatures.

In the other are reptiles, which are generally less active

and cold-blooded.

Where do dinosaurs fit?

Since like mammals, they were very active,

Ostrum reasoned perhaps they were hot-blooded too.

This idea was revolutionary.

Was it really possible that dinosaurs,

ancient reptiles, could be warm-blooded?

It would be another 30 years before deep bone analysis

revealed that he might be right.

For years, palaeontologists have been looking

at the outsides of dinosaurs.

+On the outsides, we can understand how dinosaurs evolved

and their anatomy changed over time,

but deep inside the bones, we can trace dinosaur life.

By analysing thin cross-sections of fossilised dinosaur bone,

Kristi Curry Rogers is helping to rewrite

what we know about dinosaurs from the inside out.

I think let's go with this one.

And the smaller one.

Those two look good. OK.

One of the things we see

when we crack open dinosaur bones is a story of a very fast growth rate

throughout life history. We see that dinosaurs

were growing very, very quickly on a par with modern mammals and birds,

not like reptiles at all.

This is a great example from a young Apatosaurus,

a young, large sauropod dinosaur.

All of these white spaces we see are places where blood vessels

used to flow through this bone when the animal was still alive.

This is completely different than the bone

we might see of a reptile, like a crocodile, or a turtle.

Instead this is lot more similar to those bones of mammals and birds.

What she's discovered from deep within the dinosaur bones

has reinforced the idea that at least some of them were warm-blooded.

Dinosaurs, just like other modern animals,

probably were fairly well adapted for whatever thermoregulatory strategy.

I think they were perfectly well adapted to deal

with the problems of maintaining a body temperature.

Advances in technology were allowing scientists to break new ground,

proving that dinosaurs weren't just giant lizards

but a truly unique kind of reptile.

But like a detective looking for clues,

finding a whole dinosaur skeleton was the palaeontologists' dream

and, in 1990, an American fossil-hunter hit the jackpot.

For Pete Larson, and his then girlfriend Susan,

the day had started as an ordinary, everyday fossil hunt.

We were out actually digging on a triceratops skull

that my ten-year-old son Matthew had found.

We were just having a grand old time, it was a very nice, small triceratops skull.

And all of a sudden, Susan walks up with a couple of bone fragments.

And I said, "Is there more?"

And she said, "There's lots more."

Nothing could have prepared them for what they'd found.

I looked up the face of the cliff and saw an expanse about eight feet wide

and perhaps two feet deep with bones jutting out everywhere.

And as I crawled up to the top of this exposure,

I saw three articulated vertebra.

I knew they had to come from a T rex because of the size of the curve of those bones,

they were obviously parts of vertebrae

from a meat-eating dinosaur.

And when I saw those three articulated vertebrae,

I knew this was going to be the most important specimen we'd ever dug up.

I just knew it.

Pete Larson marvelled at the size of the partially-exposed killer dinosaur.

And nicknamed it "Sue" after his girlfriend.

It was like clawing our way to the top of Mount Everest,

and as we were uncovering it, we could see the top,

and as we got her out of the ground, we were there.

We had climbed the Mount Everest of palaeontology.

We got the biggest, baddest of all the T rexs that ever was.

And it got even better.

Sue was extremely well preserved and nearly complete,

exactly what Pete Larson had dreamed of finding.

At long last, here was a chance to study the world's ultimate killing machine

in extraordinary detail

and all from just this one specimen.

Deep within Sue's well-preserved skull,

scientists were about to discover something they'd never seen before.

And cutting edge technology would allow them to see it in exquisite detail.

Basically, when it comes down to it, I was told to describe the thing inside and out

I took that literally.

I knew they wouldn't let me break the skull apart

so CT scanning is the answer.

CT scanning is an advanced x-ray imaging technique.

It allowed Chris Brochu to build up

computer images of slices through the head

which he moulded together to produce

a three-dimensional likeness of a T rex skull.

Then, painstakingly, millimetre by millimetre, he followed the contours

on the inside of the skull to reveal the structure of a T rex brain.

The first time I saw the individual slices themselves,

they didn't seem all that exciting.

It wasn't until I built the first animation,

the first flip through a bunch of slices all going through the skull,

that was when it really struck me

that there were a lot of things here to see.

The CT scans revealed something scientists had never before been able to see in such detail.

Protruding from the delicate network of brain tissue,

was the optic nerve.

This nerve was responsible for relaying information

from the eyes to the visual centres in the brain.

And it was big enough to carry a LOT of information.

The scans seemed to confirm T rex did indeed

have a key attribute of a skilled predator.

It would have been able to seek out its prey at a distance

and destroy it with the accuracy of an assassin.

T rex could see its prey, but that didn't automatically make it

an efficient killer.

To get to grips with its enormous jaws,

scientists devised a risky experiment.

Watcha, watcha.

Gators and crocodiles make a great model

for studying the feeding biomechanics

of extinct theropod dinosaurs.

They have very similar musculature, and the basic leverage of their jaws

and things like that are just a good analogy for tyrannosaur feeding.

OK, grab that pole!

Let's go.

Watch your feet, watch your feet. Remember she can run forward.

One, two, three...go, go, go!

Watch your feet, Ray.

This is a female American crocodile, Stevie.

A youngster at 31 years old, she's only half the size she could become.

She may be small, but her strength is obvious.

Stay in line with her. Back up, back up, back up!

Back up. Who's got tape? I have tape.

She's heavy.

Because her jaws are thought to work in a similar way to T rex jaws,

Erickson plans to measure her bite to see what it may reveal

about the power behind a T rex bite.

Yet, as she's small and he's not tested her before,

he has no idea what kind of results he'll get.

All the way with that...

I'm all set.

Erickson needs to get the crocodile to crunch onto

a specially-designed pressure sensor,

which will record the force of the bite.

OK, everybody ready?

The tricky bit is getting the timing right.

The bite needs to be a spontaneous one.

Here we go. Hang on.

819lbs. Good bite.

An 800-lb bite is comparable to what a lion could do or

a spotted hyena, which is the bone crushing champion among mammals.

A very small crocodilian is capable of doing bite forces

equal to what some of these large carnivoran mammals do.

If you matched up an equal-sized crocodile say to a large lion,

the crocodile will bite three times more forcefully.

Watch your legs.

If jaws like these give crocodiles a bite force

well above what their weight implies,

then Erickson believes the same must have been true of T rex jaws.

His work suggested the power of a T rex bite

may have been on a scale beyond anything we have ever seen.

It's not a natural thing

to stick your hand inside the mouth of a crocodile, but...

Probably shouldn't try this at home, kids.

To get an idea of how much more powerful,

Erickson worked on doing more than just scale up the bite.

Snout width is 14.2.

He measured every physical detail of his crocodiles to try to map

the differences in skull shape and body weight

compared to an animal the size and shape of a T rex.

50.2 head length...

Erickson's preliminary maximum estimate of a T rex bite

could be as much as 40,000lbs of force.

That's about 50 times more powerful than our crocodile.

T rex would have had easily the most powerful bite

of any animal that has ever lived.

The combination of new finds and advanced technology

has enabled palaeontologists to interpret fossils

with greater certainty.

We now know more than ever before

about what dinosaurs looked like,

how fast they grew, their skill as predators,

and how they moved.

All building a convincing picture

of how the dinosaurs came to dominate the Earth

for over 160 million years.

160 million years is a pretty long time

and makes dinosaurs some of the most successful animals

ever to have walked the Earth.

After all, modern humans have only been around for a couple of hundred thousand years.

Evidence of dinosaur life fills the geological record

but then suddenly, 65 million years ago,

it all disappeared.

The dinosaurs vanished.

Scientists spent years scrutinising dinosaur bones, looking for answers.

You got something? Yeah, this is a vertebrae...

They struggled to come up with ideas to explain the mass extinction.

Perhaps the climate deteriorated, becoming too hot...

..or too cold.

Or suddenly too wet...

..or too dry.

There were problems maybe of reproduction

or maybe their eggs were eaten by the tiny furry mammals.

IT BURPS

Maybe it was God's will or lack of standing room in the ark.

But it was only when they turned their attention to rocks,

rather than bones, that scientists had a breakthrough.

Geologists searching for clues to the extinction discovered

an unusual layer of clay in the geological record that marked

the boundary between the time of the dinosaurs and the time of mammals.

Nobel prize winning physicist Luiz Alvarez and his team

took up the challenge.

You see this clay layer here, about a half-inch thick.

That's when the dinosaurs went out.

We really don't know how long it took, why it's there.

So I said, "Maybe some of the tricks I know as a physicist

"might help unravel that story."

And we talked about it for the next couple of weeks

and finally decided to look for iridium

as a measure of the deposition rate.

A small quantity of the metal iridium

constantly falls to Earth from space,

and the team expected to find only trace amounts.

But their tests showed something astonishing.

There was so much iridium in the clay layer

there could only be one source.

Alvarez's radical idea was that it had been brought to Earth

by a meteorite.

The vast majority of iridium-bearing meteorites started life as asteroids.

Most of them, in an orbit between Mars and Jupiter,

never come anywhere near the Earth.

But the theory goes that a few are occasionally

swung out of line by the enormous gravitational pull of Jupiter.

A very few of these finish up in an orbit which crosses the Earth's.

Most of time they pass harmlessly by, but every now and then, they collide.

Alvarez's theory is that 65 million years ago a huge asteroid,

six miles wide, smashed into the Earth with devastating effects.

It was this collision, he believes, that covered the Earth with iridium

and wiped out the dinosaurs.

It's not all that far-fetched.

Only 25,000 years ago,

a much smaller collision caused Meteor Crater in Arizona.

There are larger impact craters on the Earth's surface.

Many have been eroded away over time,

and are rather difficult to recognise.

Nevertheless, so far, over the whole world, more than 200

have been identified,

but none of these is both the right age and size for Alvarez's theory.

However, there's an alternative.

The asteroid may have landed in the sea.

Dr Cesare Emiliani.

We have no evidence at all

of a crater of the size that this asteroid this should have made

either on land or on the ocean floor.

This is a map that shows the structure of the ocean floor.

On the other hand,

we have evidence indicating that plant life on the continents,

in a broad area ranging from the Urals to the Rockies,

suffered somewhat, at the end of the Cretaceous.

While plant life west of the Urals, from the Urals to the Rockies,

around the North Atlantic,

suffered very little or nothing at all.

That would seem to indicate that the point of impact of the asteroid

was somewhere between the Urals and the Rockies.

We have no crater on land, we have no crater on the visible ocean floor

but a portion of the ocean floor since then has disappeared under the continent.

Because the oceanic crust moves towards the continents

and then dives under the continents.

There is a substantial chance that the asteroid

might have hit an area of the ocean floor

that has since disappeared.

If one were to make a wild guess as to where the asteroid may have hit,

one would say somewhere in the North Pacific, round here.

Without finding a crater, it was hard to prove that it was

an asteroid that had killed off the dinosaurs.

But, by 1997, scientists realised

they'd been looking in the wrong place.

A number of circular structures had been found in the Caribbean.

The shape of islands, circular structures on the sea floor,

circular geophysical anomalies.

When you're looking for an impact crater, usually the obvious thing,

because most craters are round, is looking for something big and round.

One of Hildebrand's suspects was on the Yucatan peninsula of Mexico.

There the state oil company, Petrolinas Mexicana, had detected

a strange circular anomaly in the Earth's gravity field.

Chixulub, the dead centre of the big round hole,

but at the surface there's no sign of a catastrophe.

The 200km-wide crater is hidden.

It's buried hundreds of metres beneath the Earth's surface,

so Hildebrand had to investigate it in some other way.

We've taken another 1,400 measurements and combined them with the data that

Petroleos Mexicanos already had to make this map of the gravity field.

Here you can see all this concentric circular structure

that represents the crater.

From here to here is about 180km.

Petroleos Mexicanos had known about this big buried structure for decades.

They'd drilled several wells into it for oil exploration, beginning in 1952.

When they did so, they found what they thought was volcanic rock.

But this contains shock quartz and impact glass and so on.

These are the classic signs,

the deposits you'd expect in a big impact crater.

The rock proved to be precisely 65 million years old -

the age of the mass extinction.

Here at last was the first confirmation that Chicxulub was ground zero.

Hildebrand confirmed the theory proposed 17 years earlier

that a devastating asteroid had hit Earth 65 million years ago.

By 2004, scientists believed they had proof that the impact

had caused a massive explosion...

..quickly followed by an enormous shock wave that had destroyed life for hundreds of miles around

And there was more. Investigations of the layer of rock

that marks the time when the dinosaurs disappeared -

known as the KT boundary -

revealed further evidence of what had happened in the aftermath.

These are called spherals.

They're actually made of round rock globules, so we know they're condensed

from a very hot vapour cloud. And some of the mineralogy in there

tells us these globules originated at very high temperatures.

That's exciting.

You know something hot happened and hot is associated with an impact.

The spherals were evidence that the fireball had vaporised

billions of tons of rock.

In outer space, the vapour condensed into tiny droplets which fell back

all over the Earth as white hot spherals.

From America to New Zealand there seemed to be

evidence of massive burning at time of impact.

It looked as if the world's forests had spontaneously ignited,

as the spherals heated the atmosphere by up to 1,000 degrees centigrade.

If we're looking at 600, 1,000 degrees, then this would instantly

have ignited all the plant matter across the world

and it just would have been sent up in flames.

The impact was also thought to have created vicious acid rain.

The fireball had release chemicals, which turned the water deadly.

It was suggested that the acid rain had a pH

so low it was like battery acid.

If you had something that low,

it would literally burn everything on the land

from plants, to dinosaurs to everything else.

Then there was the final clue from the KT boundary -

a high concentration of fern spores.

Ferns flourish whenever all other plants have been killed off by some environmental devastation.

So the predominance of fern spores - known as a fern spike -

suggested something had wiped out every plant on the planet.

Fern spikes were found all over the world, such as in New Zealand.

This, I think, became stronger and stronger evidence

that there was something LIKE global darkness caused by an impact.

So the theory grew up that vast amounts of dust created by the impact

must have blocked out the sun.

This could have plunged the world into freezing darkness for months or years.

Any dinosaurs that escaped burning either froze or starved to death.

The mystery surrounding the death of the dinosaurs

finally appeared to have been solved.

A number of factors may have influenced the extinction,

but research had shown that the impact at Chicxulub WAS the crucial factor.

We should probably be thankful for that mighty asteroid -

if the dinosaurs hadn't been wiped out,

mammals may never have flourished and we might not exist.

So what would have happened if an asteroid hadn't hit the Earth

and the dinosaurs had survived?

It's a thought that's given rise to some novel ideas.

I think that some dinosaurs, like some mammals would have become

increasingly intelligent at a geometric rate,

as did our own ancestors,

and I think, possibly, by this time the dinosaurs themselves

would have approached our own level of brain development.

A sculptor at our museum

and myself have collaborated over the last several months in trying

to estimate what the appearance of one of these highly encephalised

or intelligent dinosaurs might have been, a dinosaur for the 1980s,

and here is an example of what we think it may have looked like.

Their model of a 20th-century dinosaur incorporates

many features of the original reptiles - the binocular vision,

the absence of an external ear, a deep chest cavity with ribs

all the way down the abdomen,

opposable fingers and no external genitalia.

But it looks closer to a human being than a brontosaurus.

In building it, have they perhaps unwittingly favoured our own kind?

I don't think so.

I think just as the birds, bats and flying reptiles all have

a crudely avian form, so too there is a meaning to the human form.

So that we are, in effect, adapted to interact with

an environment as highly encephalised bipeds

or walking animals with a very large brain.

So perhaps were it not for a chance collision with an asteroid,

creatures like this could be ruling the world today

just as they did all those millions of years ago.

Let's imagine that the dinosaurs really did become some sort

of dinosauroid, the great rock doesn't fall out of the sky,

there's a bright light in the sky, the dinosaur says, "What's that?

"No idea". The mass extinction is postponed. In fact, cancelled.

So what's happening then?

We've got the Apes rapidly evolving and they're beginning to

look over their shoulders because just conceivably there are also

these dinosauroids doing rather similar things.

What would have happened?

Would it have been an evolutionary race? Maybe there would have been a winner?

Or maybe, unbelievably, madly, there could have been a co-operation.

The Utopian notion of dinosaurs and humans sharing the planet may appeal,

even be plausible to some,

but most palaeontologists see the dinosauroid as an insult to dinosaurs.

There's probably some good ideas there.

The brain was getting bigger, and they probably

would have continued to outcompete other animals.

But for them to become fully erect like humans is a little bit fanciful.

Dinosaurs would have continued to develop, to specialise.

They would have adapted, but they would have adapted

and specialised as dinosaurs, they wouldn't have become primate-like.

The idea that a dinosauroid could exist as a scientific question...

is bogus. It's about as bogus as it gets.

It is fairly arrogant to think the endpoint of evolution

should emulate human beings.

If the asteroid had never hit,

life on Earth could have been very different.

But that's all just crazy speculation.

Everyone now knows that dinosaurs were wiped out 65 million years ago

and none of them survived that catastrophic asteroid impact.

Or did they?

The idea that dinosaurs may have evolved into something else

was one that had been doing the rounds for many years.

But it began to gather momentum when some palaeontologists

began to increasingly suspect that dinosaurs might still be alive.

The Natural History Museum, London. Within these hallowed halls

lies the fossil that first hinted at the origin of birds.

Discovered in Germany 100 ago,

superstitious quarry workers thought it was a fallen angel.

Archaeopteryx turned out to be something almost as remarkable.

The size of a pigeon, it possessed teeth, a long, bony tail,

and claws on its arms. All features of reptiles.

At the same time, it was very much like a bird.

It actually has impressions of the wing feathers, both wings

and long tail feathers, but the tail has a long set of bones

running down it as well, which modern birds don't have at all.

So Archaeopteryx really does seem to be a primitive bird.

These fossils, I find it exciting to look at them

because they have so much scientific information in them,

but they're very beautiful objects to look at in their own right.

They really are an exceptional snapshot record of evolution.

That fossil was to be the key to something that John Ostrom

had been thinking about for decades.

When he'd first described Deinonychus in the 1960s,

he'd noticed that its skeleton was strangely similar to that of a bird.

Archaeopteryx helped him refine his ideas.

Neat animal, isn't it?

I think so.

And what made me even more excited

was when I saw structures in that animal

that I subsequently recognised

in Archaeopteryx.

This is the Solnhofen Archaeopteryx. The Solnhofen specimen, yeah.

John Ostrom's crucial realisation was that

his beloved Deinonychus shared many anatomical features with Archaeopteryx.

He compared in detail the skeletons of predatory dinosaurs,

Archaeopteryx and modern birds.

He found a whole set of similarities - most notably in the skull,

the hind limbs and the forearms.

For a start, they all have the same number of fingers.

This is the skeleton of a modern pigeon.

Three fingers in the hand of a modern bird,

three fingers preserved in the hand of Deinonychus, and that particular

kind of hand morphology is also supplemented by

the strange wrist bone that allows for the flexibility

that produces the flapping strokes.

The similarities between birds

and predaceous dinosaurs is amazing to me.

But, as with all groundbreaking new theories,

Ostrom's idea had its detractors.

The dinosaur-bird theory has tremendous popular appeal,

one can vicariously study dinosaurs at the back yard bird feeder,

and one can buy a piece of dinosaur leg at the local Kentucky Fried Chicken.

So it has tremendous appeal to the public.

Unfortunately, it seems to be wrong.

Alan Feduccia argued that birds evolved long before dinosaurs came along.

They descended from much more primitive reptiles,

and any similarity between birds and predatory dinosaurs was superficial.

They resembled each other because they both walked on their hind legs,

not because they were closely related.

Such vocal sceptics were going to need better proof

if they were to be convinced of the dinosaur-to-bird theory.

And in 1999 in Tucson, Arizona, fossil collectors thought

they might have come across a specimen that fitted the bill.

I carried it out to the light

of the sunlight so that I could see it cross lit.

And there were a number of beautiful teeth in this skull.

And that was very exciting.

And then we studied also the tail that was a dinosaur...

a very dinosaur-like tail.

I got this incredible high feeling,

the feeling of discovery - that wonderful time

when everything clicks into position.

The two fossil dealers thought they could be looking at

one of the most important fossils ever found.

A specimen that would prove beyond doubt

one of the most controversial theories in all of evolution.

This fossil, this clearly cross between a bird

and a dinosaur was what everybody had been looking for.

And here it was, right there, right in front of my eyes,

and I was one of the first people to see it.

I looked it over very carefully.

Literally under a magnifying glass.

And I was looking for any tell-tale features, particularly on the tail.

I wanted to look at that tail very carefully

because it was very clearly a dinosaur tail.

The world of palaeontology was gripped

and a team of experts was assembled to investigate.

After several months, they confirmed that it was the missing link.

It had a bird-like front and legs, and a dinosaur-like tail.

They called it Archaeoraptor,

ancient hunter,

and proudly presented it to the world.

Scientists could now say that dinosaurs evolved into birds.

One of the most important theories in evolution was finally proved.

But not everyone was convinced.

At the university of Texas, Tim Rowe had used a CAT scan

to study the fossil.

The results threw up some serious questions about how it fitted together.

I'm going to show you two slices.

The first is this slice here through the skull

and these other elements here,

and the second slice will be back through the ankle and tail,

this critical region here through one of the legs.

When we go to these slices, here's what we see.

Here's the skull.

We can see the skull is part of this upper layer of shale.

And you can see the fracture pattern here,

here's very tight fractures that fit together.

Here, a pair of fractures, one occurring against the next.

A straight fracture.

The pieces on either side are the same thickness. Same density.

But when we get to the edge of the block,

this piece is a little bit thicker

and denser than the piece it's glued against.

As we move to the tail, to the critical area,

we can see that it's completely surrounded by grout,

and that there are no natural ties between the tail piece

and this piece to the right or left.

In fact, it's just swimming in this ocean of grout here.

And as we map through the entire specimen, we found no verifiable fits

between the tail and any of the other parts anywhere else in the specimen.

The scan clearly showed what the naked eye couldn't see.

There was no natural skeletal link between the all-important tail

and the rest of the fossil.

It had simply been glued on with grout.

The vital evidence that seemed to prove the link between birds

and dinosaurs was a fake.

The dinosaur fake was a dreadful blow for supporters of the bird theory.

But scientists who were committed to the idea refused to give up.

They were determined to keep looking for proof.

Although the fossil had been a fake,

its front half was a new kind of primitive bird.

Fossil hunters flocked to the region where it had been found.

And they struck gold.

Extraordinary, well preserved fossils revealed dinosaurs

and birds not only shared features like downy feathers,

but also hollow bones and similar pelvises and hind limbs.

On a remote farm in Colorado,

palaeontologist Brent Breithaupt presented even more proof

of the close relationship between the ancient fossils and birds.

Very good. That should make an excellent track.

Here we have two tracks that we recently made.

This one here is from the Red Gulch Dinosaur Tracksite.

This one here preserves the three-toe impressions,

tridactyl impressions, of the foot of the dinosaurs.

The small to medium sized theropod dinosaurs that lived up there.

Now, over here, we have one that we just got from this site.

Again, a nice tridactyl footprint.

Again, very well preserved.

If we compare both casts,

we can see these particular tracks look very, very much the same.

But these footprints are not of a theropod that died 65 million years ago...

These are only a few hours old.

There's dinosaurs in them there hills.

In fact, dinosaurs are everywhere.

For the first time on network television,

palaeontologist Julia Clarke is about to perform an autopsy on a dinosaur.

Only you are more likely to know it...

as a roast turkey.

Because you see, birds ARE dinosaurs.

So today we're going to dissect the evidence that birds

are living dinosaurs from this turkey.

What we're pulling off here is the major flight muscle,

supracoracoideus that is in velociraptor and oviraptor.

One of the features you see is that the second finger is the longest.

This is a feature we see going back as far as early dinosaurs,

even Triassic forms.

We're all familiar with wishbones, from any kind of turkey meal.

Wishbones actually are one of the most intuitive pieces of evidence

that birds are living dinosaurs, because we have wishbones now

from a variety of theropod dinosaurs, including relatives of tyrannosaurus

and velociraptor, and even earlier dinosaurs such as coelophysis.

Yes, just as we share 98% of our DNA with chimps,

turkeys - in fact all modern birds -

are direct descendants of theropod dinosaurs.

And the freshly-made dinosaur tracks in the hills of Colorado?

Emus.

Inwardly, outwardly, even in the way they move,

the similarities between theropod dinosaurs and birds are numerous.

But, being warm-blooded, their ultimate success

was in an evolutionary solution to the need to keep warm.

Large dinosaurs really don't have a problem with body heat.

If they have a problem, it's getting rid of excess body heat.

But small dinosaurs have this problem.

They're losing their heat all the time.

So it would be a good thing if a small dinosaur was

warm-blooded, for it to have some kind of insulation on its body.

It started with the development of thin, downy filaments.

In time, those filaments strengthened and thickened.

As non-flying birds, emus are one of the best examples

of feathers as they were originally designed. As an insulating layer.

Once you have those long feathers, then of course it does give you

an aerodynamic advantage as well.

And if you have that advantage, then selection starts working on that advantage.

And it may well be that that was forcing these feathers to become longer

and longer until finally that animal not only jumped across the ditch,

it actually flapped its arms and flew across the ditch.

And so it seems that flight, far from being

the reason for the evolution of feathers, may have been a by-product.

But with it, some dinosaurs were already adapting in ways

that would equip them for life after the meteorite impact.

The fact of the matter is that the age of the dinosaurs never actually ended.

Dinosaurs DID survive the cataclysmic event of 65 million years ago.

So when we talk about dinosaurs living with us today,

and the fanciful notion of what it would be like, it's not so much fantasy.

They're right there.

Dinosaurs have not only survived, there are far more species of them

on the Earth today than there are mammals. They're not the biggest animals any more,

but still there's over 10,000 living species of descendents of dinosaurs.

They didn't actually go extinct at the end of the Cretaceous period

like everybody thinks. They're outside flying around.

You can't go into a forest without hearing dinosaurs.

In that sense, maybe they won out,

and we just think we're on top.

Over the last half-century,

scientists have hunted all over the world for new clues

to help them piece together the fragments which reveal the life of the dinosaurs.

They've come up with ingenious new ways of working out

how the dinosaurs lived and behaved,

made extraordinary discoveries,

and battled to answer some of the oldest, most vital questions of all.

But there are still things we don't know,

mysteries to be solved, and one of the exciting things

about palaeontology is that, in an instant,

perhaps with just the tiniest of discoveries,

everything we think we know about dinosaurs today

could all change again.

There are always new discoveries out there...

waiting to be found.