ATTENBOROUGH: I'm here in Patagonia in the southern part of South America, because a few years ago, a man looking for one of his lost sheep found a simply gigantic bone sticking out of the rock.

A bone that was going to astonish science.

That first bone led to the discovery of over 200 others.

They were all huge, so big that they could only have come from a dinosaur.

One that seems to defy the laws of nature.

An international team of scientists assemble to piece together a gigantic prehistoric puzzle.

MAN: [ Speaking foreign language ] ATTENBOROUGH: Using the latest forensic technology, we'll compare it with how giant animals live today and build a full-size skeleton of this stupendous creature... Gosh!

Absolutely amazing!

...and find out if it really is the biggest animal to walk the earth.

Patagonia, in Southern Argentina.

Like many detective stories, this one began by chance.

A shepherd stumbled across the tip of a huge bone poking out of the ground.

Experts from Patagonia's premier paleontological museum confirmed it was part of a dinosaur.

MAN: [ Speaks foreign language ] ATTENBOROUGH: But they didn't realize at the time what a truly extraordinary one it would prove to be.

Dinosaurs of many kinds roamed all over these lands in the southern end of South America during what's known as the Cretaceous period, between 66 and 145 million years ago.

The largest were plant-eaters known as sauropods, and the largest of them were the titanosaurs.

Giant titanosaur bones are comparatively rare, so very little is known about these dinosaurs.

This new discovery could change all that.

Like many people, young and old, I'm fascinated by dinosaurs.

So the chance to join this investigation is just too good an opportunity to miss.

Oh, I'd love to have a go.

[ Laughs ] GARROD: I'm sure they'd let you.

[ Laughter ] ATTENBOROUGH: Of course, it's the giants in particular that capture the imagination.

The first sauropods to appear on earth were comparatively small creatures.

This is the cast of the thigh bone of one of them.

It's not even as big as my thigh bone.

But after about 20 million years, some had become pretty big.

This is a thigh bone from one of those creatures.

But then after that, our giant appeared.

This is its thigh bone.

It's the largest ever found.

Coming across such a bone in your backyard must be quite a shock.

Just ask farm owner Alba Mayo.

[ Chickens clucking ] Before long, the whole team of fossil-hunting scientists arrives and starts work.

The thigh bone proves to be 8 feet long, the longest ever found.

It's preserved in extraordinary detail, and detail will be critical to the forensic examination that will follow.

The research team soon turn the site into a vast quarry.

It proves to be one of the biggest dinosaur finds of the century.

Bone after bone emerge from the rocks.

[ Laughter ] [ All talking ] MAN: We just found another bone right here.

We wasn't expecting it at all.

We just started digging and found it.

ATTENBOROUGH: Until recently, giant titanosaurs have only been known from a dozen bones.

And our team have already found more than 10 times as many.

Doctor Diego Pol is the chief paleontologist leading the investigation.

POL: If you really want to know what a really gigantic dinosaur looked like, this quarry here has the potential to answer that question, and that's really exciting for us.

It's really impressive.

When you stand by one of these bones, you really feel tiny.

ATTENBOROUGH: With so much new evidence, there is a chance of discovering all kinds of new facts about the mysterious titanosaurs.

POL: It's like a paleontological crime scene.

It's a really unique thing that you will not find anywhere else in the world.

ATTENBOROUGH: Patagonia's harsh weather makes uncovering the fossils exhausting, but it also endangers the newly exposed fossils.

[ Indistinct talking ] POL: We've got a lot of damage from the rain, so we need to protect the bones that are at risk.

I'm really concerned that this already has some cracks.

ATTENBOROUGH: If the bones aren't protected, tiny details on their surface could be lost.

To protect the bones, they're covered with, of all things, wet toilet paper and plaster of Paris.

It's like putting a plaster cast on a broken leg.

There's a rush to get them back to the museum to begin examining them in minute detail.

MAN: [ Speaking foreign language ] ATTENBOROUGH: A new road has been specially built to enable them to be transported without too much jolting.

Once at the museum laboratory, the detailed detective work begins.

CARBALLIDO: Tremendo!

ATTENBOROUGH: It's a chance to start putting flesh on bones.

POL: Some really big muscle was going on here.

This animal was so big that it certainly needed really powerful muscles and really strong attachments into the bones.

[ Indistinct talking ] ATTENBOROUGH: This is a giant vertebra, one of the bones of the spine, and it's a very important find.

That's because it is likely to provide crucial evidence for identifying the species of our dinosaur.

[ Indistinct talking ] Despite weighing up to half a ton, these fossils are surprisingly fragile.

It's all rather nerve-wracking.

One bone like this has already cracked in half without warning.

Bravo!

Is it firm?

[ Laughter ] And so this is the position, as it was in life, with the center of the backbone there, then this is the crest on the top?

POL: Right, right, and this belongs to the middle part of the thorax.

ATTENBOROUGH: Right about here.

POL: Yes.

About that, yeah.

ATTENBOROUGH: Many more weeks of detailed examination will be needed before the backbones reveal all their secrets.

Surprisingly, perhaps, one of the first things the team is able to deduce about our titanosaur is its weight.

That's because after finding the thigh bone, they discover another huge bone from the front leg -- a humerus.

By measuring the circumference of each of these leg bones, it's possible to estimate how much weight they could support.

POL: Let's see how much it measures.

CARBALLIDO: 79.

POL: 79?

Wow!

I'm not sure how that translates to body weight.

CARBALLIDO: Yeah, around 70 tons or even more, probably.

POL: Wow.

That's really big!

CARBALLIDO: It's amazing.

ATTENBOROUGH: That evening, Dr. Jose Luis Carballido checks his calculations.

[ All cheering ] Until now, Argentinosaurus was the heaviest known dinosaur.

Ours already looks bigger.

[ All cheering ] Could this mean it was the largest animal ever to walk the earth?

Could it also be a new species?

We can't be sure...yet.

The rocks of Patagonia, so bare of vegetation, also contain astonishing evidence of how titanosaurs began their lives.

I've now come nearly 500 miles north from our Patagonian dinosaur excavation to a place called Auca Mahuevo.

This is the largest dinosaur nesting ground yet discovered.

The remains of their eggs and their nests are wherever I look.

In fact, it's quite difficult for me to take a step without walking on a dinosaur eggshell.

Over thousands of years, the wind and the rain have cleared away the soft rock that once enclosed these fragments.

And they can tell us quite a lot about how titanosaurs reproduced.

Careful excavation has shown that these dinosaurs laid eggs in clutches of up to 30 or 40 at a time.

They would have looked rather like these replicas because they lay on the surface of the ground, not covered by soil but in a shallow depression.

Sometimes, though, remains of vegetation have been found in some nests, which suggests that the dinosaurs might have used rotting leaves to help with the incubation.

The dinosaur that laid these eggs here were medium sized.

Our dinosaur that we are excavating probably laid eggs as big as that.

I'm shown around by Dr. Luis Chiappe who, with his team, discovered this remarkable site.

Dinosaur eggs here were laid on an old river plain.

Then the river flooded and covered the unhatched eggs, preserving them in mud.

CHIAPPE: You see, you know, many eggs... ATTENBOROUGH: Oh, there!

CHIAPPE: ...for kilometers and kilometers.

Here's a nice one.

ATTENBOROUGH: Oh, that's a huge piece!

CHIAPPE: Yep.

ATTENBOROUGH: And this is the actual surface of the egg?

CHIAPPE: Yes.

ATTENBOROUGH: Astounding!

Do you suppose they could have been colored, like birds' eggs?

CHIAPPE: They may.

Maybe they were off-white.

We can't tell, really.

ATTENBOROUGH: Yeah.

Well, we can see all the tiny pores on the surface.

CHIAPPE: And the texture, right?

ATTENBOROUGH: Yeah.

What a beautiful piece.

You must admit it's pretty romantic.

[ Both laugh ] CHIAPPE: I think it's incredible.

ATTENBOROUGH: I think it's absolutely extraordinary.

And I must put it back where I found it.

CHIAPPE: Thank you.

ATTENBOROUGH: The fragments can tell us quite a lot about how how the dinosaurs nested, but some, amazingly, can do even more than that.

CHIAPPE: All these egg samples have something quite special.

But this one is my favorite.

And what you can see is a very large patch of baby dinosaur skin.

ATTENBOROUGH: How wonderful!

It's extraordinary!

And this is not just an impression.

This is the mineralized skin.

CHIAPPE: It is.

ATTENBOROUGH: Yeah.

Astounding!

Luis Chiappe has dozens of complete eggs in his museum, and he allows me to examine some of his most precious specimens for myself.

There are many other remarkable things in these astonishing time capsules.

This one has got perfectly clearly the limb bones.

Here is a skull -- That's the orbit of the eye, there's the lower jaw, there's the snout.

This one also has a skull, but on the tip of the snout, you can see a little spike, which is like the egg tooth that a bird embryo has to help it crack itself out of a shell.

And here is a replica of what the complete uncrushed shell must have looked like.

With all these details, it's possible to imagine how a baby titanosaur entered the world.

[ Dinosaur grunting ] To get an idea of how these youngsters might have lived, we can compare them with their closest living relatives -- birds.

Rather like baby ostriches, a young titanosaur would have been able to walk soon after hatching.

They may well have gathered into groups to give some safety from predators, as young ostriches do.

Microscopic analysis of dinosaur leg bones show rings, rather like tree rings, and these indicate that titanosaurs grew very swiftly early in their lives.

And they could have lived for some 50 years, plenty of time to become enormous.

The team now has 150 bones of our titanosaur, enough to get an idea not only of its weight but also its height and length.

Now the plan is to build a life-size reproduction of the complete skeleton.

But it's a challenge to find a place big enough to house an animal that is 121 feet long, over three times the length of a school bus.

But Diego thinks he's found one.

It's an old wool warehouse.

POL: One, two, three, four, five, six, seven, eight... We have been looking for a place that is big enough to fit our dinosaur.

This seems to be it.

This is a warehouse that we could use.

Not only in terms of length -- this is 70 meters long -- but also very important in terms of the height, so we need a place not only long but really high.

It really needs a bit of decoration, but I think it will do it.

It's going to be awesome.

ATTENBOROUGH: Putting the skeleton together will help us understand the particular challenges of being such a giant.

So next, an international team of skeleton builders arrive to scan the bones, ready to make a 3-D computer model of each of them.

Extraordinarily accurate 3-D scanning allows images of the bones to be placed in a virtual reality world so that they can now be examined from all points of view without needing eight people to lift them.

One of the mysteries surrounding our dinosaur is, how could an animal as big as it was actually move about?

The computer data allows us to put our dinosaur leg bones together in 3-D and then compare the arrangement with what we know about living animals.

Elephants are the largest land animal alive today.

They, like titanosaurs, have to move their massive bodies around without their bones shattering under the enormous weight.

I've come to meet Professor John Hutchinson here at ZSL Whipsnade Zoo.

He's studied elephants for many years and has joined the team that's investigating the internal workings of our titanosaur.

HUTCHINSON: We have about a one-meter-long pressure-sensitive mat out there with several thousand sensors in it.

And it's telling us in very high resolution what the pressure on an elephant's foot is like.

We can see on the elephant's foot here.

Oh, here she goes.

ATTENBOROUGH: Oh, yeah!

Great.

HUTCHINSON: Oh, that was a perfect one.

Make sure it hits the ground, rolls over, and then pushes off with its toenails.

So we can see there are some hot colors, or reds and oranges, on the toenails of an elephant's foot, indicating high pressure, and then some cooler colors back toward the heel pad in greens and light blue.

That's low pressure.

So elephants are supporting most of their weight on their toenails.

That pressure gets transmitted up to their toe bones and then up to their wrists and ankles and so forth.

ATTENBOROUGH: John's analysis suggests that our titanosaur's legs, like those of an elephant, were placed vertically beneath the body like strong, massive columns.

This arrangement transmits the weight to the toes and then spreads the force, using fatty pads in the back feet as shock absorbers.

But our titanosaur had one other adaptation to help them walk, one that elephant's lack.

A clue to this can be seen on the giant thigh bone.

GARROD: Hey.

How's it going?

JEFF: Good, good.

ATTENBOROUGH: Ben Garrod specializes in reconstructing skeletons, and he's joining the team to look at the bones in detail.

Marks on them show clearly where the muscles were attached.

GARROD: That's halfway down the femur, isn't it, that big lump there, for these massive muscle, I guess, tendon attachments.

ATTENBOROUGH: This lump is where a huge muscle was attached to the femur.

The other end of this muscle was connected to bones, like these in the tail.

It's this connection that helped our dinosaur to walk.

GARROD: They've got so much strength and so much rigidity up there, they actually used their tails to help move, to help their propulsion.

So they had massive muscles and tendons from -- ATTENBOROUGH: Helped?

GARROD: Yeah.

So the movement of the tail actually pulled the hind legs backwards and then released them forwards.

ATTENBOROUGH: Oh, I see.

I must try that sometime.

[ Both laugh ] The largest lizard alive today, the Komodo dragon, has a similar adaptation.

The swing of their tail helps their back legs move more efficiently.

Of course, our dinosaur was different, not least because it weighed over 500 times more.

And that makes John Hutchinson suspect that it would have had to deal with another problem, one also faced by passengers on long-haul flights.

HUTCHINSON: Pressure in the legs of big animals is a really big problem.

If blood stays down there too long, it's going in to pool and clot.

Much like airline socks that humans use, large animals again and again have evolved very thick elastic skin around their lower limbs that helps to keep that pressure very high.

Actually, I can empathize.

I have to wear those same kind of stockings to get my blood back up my long legs.

ATTENBOROUGH: Time to thank our helpful elephant.

You're a lovely thing.

Yes, you -- Oh, you want one of these, do you?

Okay.

In you go.

Thanks.

Thanks, pal.

[ Laughs ] That's all I've got.

A giant animal like an elephant also needs a huge heart to pump blood around its body.

And so did our titanosaur.

Its heart must have been immense.

From our new, detailed knowledge of the skeleton, John Hutchinson has calculated that it was more than six feet in circumference.

It probably weighed 230 kilos and would have had to shift 90 liters of blood with a single beat.

There's one.

And it would have had to repeat that beat every five seconds.

There it goes again.

Weighing more than three grown men, it would have been extraordinarily powerful.

And in order to pump blood around the body at high pressure and then into the delicate lungs at a lower pressure, it's thought that our titanosaur's heart had four chambers, more like that of a bird than a reptile.

So, a powerful heart pumped the blood to the extremities of the body.

But how did the blood get back?

As in an elephant, a combination of fatty foot pads and tight skin are thought to have forced the blood from its legs all the way back to its heart.

Toronto, Canada, and the world's biggest dinosaur-making factory.

The team is building a life-size skeleton of this vast creature to be unveiled in Diego's warehouse in Argentina in six months' time.

First they have to turn all the information from the 3-D scans into each individual bone.

State-of-the-art robots carve molds from polystyrene so that the bones can be cast in fiberglass.

Up until now, the fossil bones have been the main focus of the dig.

But the rock that surrounds the fossils also holds important information.

The nature of the layers of rock in which these fossils lie can tell us a great deal about how they got to be where they are and how old they are.

Some of these layers are volcanic ash, which must have come from a volcano erupting every now and then somewhere in the neighborhood.

And this ash around the bones can tell us how old the fossils are.

Scientists worked out that all these fossils dated from the Cretaceous period.

But better than that, they dated them precisely to 101.6 million years old.

In an investigation of this scale, sometimes the most important information comes not from the most eye-catching evidence but from quite tiny details.

Here is something that I really hoped the excavation was going to find.

It's a tooth, and it's tiny compared with the size of the huge animals from which it came.

Teeth can tell you a huge amount about an animal.

And if you look at the tip, you can see that it has been worn into two facets, one on either side.

And that tells us that this tooth engaged with the teeth on the other side in that alternate way, like that.

Not head-on but one on either side.

So this animal, like a pair of scissors, just nipped off the vegetation on which it was feeding.

Enormous though it was, just nipped off little leaves.

And here are fossils of some of the different kinds of plants on which it might have fed.

Cycads, ferns, and conifers.

One thing these plants have in common is they're all very fibrous and hard to digest.

To get enough nutrients from such poor-quality foods, our Titanosaur would have had to eat them in vast quantities.

A descendant of one of these plants still grows in Patagonia today.

200 million years ago, when South America, Australia, and Antarctica were all joined together to form a super continent called Gondwana, a particular kind of vegetation was dominant.

They were conifers.

They continued to survive to 100 million years ago, when our titanosaurs were roaming the land.

And a few still survive today.

Here in the foothills of the Andes is one of them, the monkey puzzle tree, called Araucaria.

Trees like Araucaria show that the dinosaurs must have had another problem.

These conifers, apart from being poor-quality fodder, can grow to over 130 feet in height.

They would have been out of reach for many animals.

But not our titanosaur.

Here, boys, come on.

It's pretty clear why a long neck is useful for a land-living animal.

It enables it to reach vegetation that's growing high up at the top of trees that other ground-based animals couldn't reach.

And it must have been much the same for titanosaurs, except we know from the fossils that titanosaur's neck was very, very much longer.

And that enabled it to sweep its head in a great wide arc and even to reach between two tree trunks that happened to be growing close together to get other vegetation.

What about that?

[ Grunts ] This enormous reach would have saved our titanosaur a lot of energy.

It only needed to move its neck to feed, not its whole body.

But how did it eat enough of this poor-quality food to survive?

Elephants face a similar challenge today.

An elephant can collect and chew about 130 kilos -- that's 300 pounds -- of vegetation in a day.

But our titanosaur could have eaten five times that amount.

It's been estimated that a large titanosaur would eat enough plant material to fill a skip in a single day.

So how did they digest it all?

Elephants solve the problem by giving their food long preparatory chews.

But titanosaurs didn't bother.

They simply gathered leaves by nipping them off and then swallowing them whole.

But that, in turn, would mean that they needed a bigger and longer gut to digest all that unchewed food.

And it might well have taken 10 days for food to pass through their system.

A bigger gut needs a bigger body.

So titanosaurs grew bigger and bigger until they approached the limits of what their bones could support.

The 3-D data used to make the skeleton has also been used to create a computer model.

It means I can get a preview of what the final skeleton will look like.

GARROD: The first thing is if you look at these very, very lovely legs, if we turn it around, they're very, very column-like.

And this is like elephants, but interestingly, this titanosaur had slightly splayed legs at an angle -- about 5 degrees.

And this slight change would have really increased the ability to take all that extra weight.

ATTENBOROUGH: Can you see the splay because of the joint or because of the shape of the bone?

GARROD: A bit of both.

So you can tell from the shape of the bone and from where certain parts of the bones form and how they sit and then how the bones fit with one another.

You can really tell how it would have sat in real life.

Another thing you can see is a very, very long neck.

And we've just found out that ours had 15 bones in its neck.

Interestingly, some of them were five or six times longer than they were wide.

These incredibly long vertebrae, and there's lots of them.

ATTENBOROUGH: Why does it have such a long tail?

GARROD: Well, a couple of reasons.

If you've got an animal this big with a neck this long, the last thing you want to be is top-heavy.

And again, research has just shown that the center of gravity in this animal was somewhere right in the middle of the chest cavity.

ATTENBOROUGH: So the heavy tail counterbalances the exceedingly long neck.

But, judging from the size of the muscle attachments, the tail was also immensely strong.

GARROD: You'd have had huge muscles, from around here right down to about a third of the way down the tail, somewhere around here.

ATTENBOROUGH: So that would be solid flesh.

GARROD: Yep, the muscle tissue, other tissue, ligaments, tendons.

ATTENBOROUGH: Do you think they might have fought with it?

Thrashing it about?

GARROD: It could have been used as a defense mechanism.

So you're walking up to that as a predator, that last thing you want is to be on the receiving end of that.

ATTENBOROUGH: Don't put me into it!

[ Both laugh ] The long and painstaking examination of the backbone has now born fruit, and Ben has got some important news.

GARROD: Now, this is a vertebrae here from right high up in the back, right near the shoulder blades, and the most important thing is this little ridge that ends in this big lump.

And this is only found in this particular dinosaur.

So from that and a few other physical differences, we think we've got a brand-new, exciting species.

ATTENBOROUGH: So our titanosaur is not only a giant, but it's a new species of dinosaur... ...one that had to face many dangers, as it wasn't the only giant here.

[ Dinosaur roars ] This was a dangerous world, where meat-eaters were giants, too.

[ Dinosaur roars ] Fresh evidence of large carnivores has been found close to our dinosaur bones.

Wow.

POL: So these are some of the over 80 teeth we found on the dig site, and you can feel how sharp they are.

ATTENBOROUGH: Oh, yes, its serrated, just like a shark's tooth, in fact.

POL: Yeah, absolutely.

They actually belong to a family known as shark-toothed dinosaurs.

We can identify the teeth at the family level.

We know of one species that belong to that family.

It's called Tyrannotitan chubutensis.

ATTENBOROUGH: Tyrannotitan.

POL: Yeah, Tyrannotitan.

ATTENBOROUGH: That means a ferocious giant, ferocious beast.

-Yeah.

Good name.

-POL: Yeah.

ATTENBOROUGH: Chubutensis is because of the area it comes from?

POL: Yeah, this is the Chubut Province.

ATTENBOROUGH: Yeah.

Great!

Tyrannotitan must have been a ferocious-looking beast.

With large eyes, sharp flesh-eating teeth, and strong legs, it was a fast, alert, meat-eating dinosaur.

POL: And it was as big as T-rex.

ATTENBOROUGH: Really?

Not as famous.

POL: Yeah, not as famous.

ATTENBOROUGH: Tell that to Hollywood!

[ Both laugh ] POL: I have some sauropod bones over there I would like to show you.

So this is one of the tail vertebrae we found at the dig site.

There is something really interesting here.

You can see this groove?

Well, this groove is probably a bite mark made by one of the carnivores.

ATTENBOROUGH: By one of these teeth.

POL: Right.

ATTENBOROUGH: So it was -- What do you mean?

Like that?

POL: Exactly.

Taking the flesh out of the tail.

-ATTENBOROUGH: Really?

-POL: Yeah.

ATTENBOROUGH: The tender bits.

[ Both laugh ] They would be, too.

POL: Yeah.

Absolutely.

ATTENBOROUGH: Can you tell whether it was a scavenger or it was a hunter?

POL: We don't know if they were dead.

I mean, they were scavenging on the carcasses.

Or if they were actually hunting and killing them.

ATTENBOROUGH: Well, it didn't make much difference to the old dinosaur.

POL: Yes.

ATTENBOROUGH: In a detective story, to close the case, you really want to know how the victim met its end.

If our titanosaur didn't perish in the jaws of a Tyrannotitan, how did it die?

Clues can be found by the detailed three-dimensional mapping of the location of every fossil bone small and large.

That shows that the dig site contains the remains of not just one but seven different individuals, all of the new species.

And the first thing to notice is that they are on three different levels.

That's to say the animals must have come here on at least three different occasions.

But why should they have done that?

There are several theories as to why seven bodies should have all ended up at this one particular place.

The first is that this was a seasonal climate and that as the dry season preceded, this was one of the last remaining pools of water.

And when this went, the sauropods that happened to be here died here.

The second is that these bodies were swept down by great rivers during the rainy season and then where the land leveled out so those bodies were dumped.

Analysis of the sediments around the bones shows that there were rivers gently flowing across this site at the time of their death.

There was no shortage of water to drink.

What's more, the rivers were not moving fast enough to shift such huge bodies, so the corpses weren't washed here by flood waters, either.

Could there be another reason why they all died in one place on three different occasions?

We know from layers of ash around the bones that there were volcanoes erupting in the neighborhood.

So doubtless, there were areas where the ground was warmed by volcanic fumes, just as they are here today.

We also know that dinosaurs regularly laid their eggs in such places, doubtless taking advantage of the volcanic warmth to help incubate their eggs.

So maybe that was the reason why they kept returning to the same place.

Certainly the excavation at the dinosaur egg site seems to support this.

Nests like these have been found at four quite widely separated layers in the rocks, showing that dinosaurs came back to this particular site again and again and again over a long period of time.

Ash from a volcanic eruption can sometimes fall in such quantities that the whole vegetation is blanketed by it and killed.

So life in the aftermath of a big eruption can be very difficult for a plant-eater.

Whatever the explanation, individuals over several generations came to this one place and died here.

The dig is coming to an end, and the team have assembled a record-breaking number of bones, but they are still hoping to find one last piece of the puzzle -- the skull.

So, what number is this, 203 or...?

POL: Well, actually, it's 223.

ATTENBOROUGH: 223!

POL: Yeah.

ATTENBOROUGH: Between the seven individuals?

POL: Yeah, between all the seven individuals we found here in this site.

ATTENBOROUGH: If these are neck vertebrae, could they be leading towards the skull?

POL: Well, yeah, that's what we are hoping for.

We just found another neck vertebrae over there, so... ATTENBOROUGH: That would be a great triumph if you found a skull, wouldn't it?

POL: Oh, yeah, yeah.

There are only three titanosaur skulls known so far.

-ATTENBOROUGH: Really?

-POL: Yeah.

So they are very rare.

ATTENBOROUGH: And that's because they are very fragile.

POL: Yeah, they're very delicate bones, and they have very light sutures between each of the bones.

ATTENBOROUGH: Okay, well, let's hope you find number four!

POL: Yeah.

ATTENBOROUGH: It could be under there.

POL: It could be.

We're going for that.

ATTENBOROUGH: Wonderful.

Alas, it was not to be.

So I gather you haven't yet found the skull.

POL: Sadly not.

The only thing we have found out of the skull is this tooth.

ATTENBOROUGH: So to complete the skeleton, the team have to reconstruct one... MAN: Take that piece out of there.

ATTENBOROUGH: ...basing it on the three skulls of other titanosaur species to produce one which most suits the single tooth that we have.

The scientific team has discovered, collected, cleaned, scanned, and copied 220 bones of our giant.

Soon it'll be possible to put those copies together to get some idea of what the living animal actually looked like.

But the fossil bones themselves still have many secrets that are waiting to be revealed.

All the theory can now be put to the test.

We can finally get the most accurate estimate of our dinosaur's weight and true size.

It takes two weeks working day and night to fit all the bones together.

Wow!

Gosh!

Absolutely amazing!

Good gracious!

[ Chuckles ] Well, Diego, are you pleased?

POL: Yes, we are very pleased.

It has been a lot of work.

It has taken 40,000 man-hours to get here.

But we are really, really happy with it.

ATTENBOROUGH: And does it answer some of your questions about the animal?

POL: Oh, yeah, absolutely.

It answers a lot of questions.

But the good thing is it raises more questions.

So we have a lot of research to continue on this animal.

GARROD: It's clear that this thing still wasn't fully grown.

It's massive, but it still had room to go.

ATTENBOROUGH: You mean the structure of the bones look as though they were still growing?

GARROD: Yeah, yeah.

ATTENBOROUGH: So that raises the really big question -- Is it the biggest so far discovered?

POL: Well, according to our estimates, this animal weighed 70 metric tons.

ATTENBOROUGH: That's 77 short tons.

What would that compare with?

POL: Well, that is like 15 African elephants.

ATTENBOROUGH: 15 African elephants.

POL: We are now sure that this animal was 10% larger than Argentinosaurus.

ATTENBOROUGH: The previous record holder?

POL: The previous record.

So, yes, we now -- We think we have the largest dinosaur ever known.

ATTENBOROUGH: Fantastic!

I can quite believe it!

-Congratulations to you.

-POL: Thank you.

ATTENBOROUGH: Congratulations to he, she, or it.

Wonderful!

Marvelous, marvelous thing to see.

Piecing this complex jigsaw puzzle together has been a fascinating adventure.

It all started with the discovery of one enormous thigh bone.

And then a team of 40 worked for over two years to excavate and put together the near complete skeleton of the largest land animal yet discovered.

And so added one further marvel to the astonishing history of life on Earth.

What a thrill it must have been to see it when it was alive.

[ Dinosaur grunts ] To learn more about what you've seen on this "Nature" program, visit pbs.org.