David Attenborough presents the biggest dinosaur ever

It was, it soon transpired, the thigh bone of a dinosaur. And it was huge – nearly 8ft long. Cue much excitement from scientists around the world. And a dig that went on to unearth more than 200 massive bones, all of them from a hitherto unknown species.

Attenborough flew to Patagonia to meet the palaeontologists who had dug up the bones and he watched while they built a life- sized replica of the giant dinosaur’s skeleton. As he explains in the programme, our knowledge of physics and biology is now so advanced – and the haul of bones so comprehensive – that scientists can confidently predict the size of the creature.

Their conclusions are astounding. The animal, from the genus titanosaurus, would have weighed nearly 70 tons and measured about 20ft from toe to shoulder – about twice the height of an African elephant. And that’s before you factor in it's incredible 40ft-long neck.

From nose to tail the titanosaurus measured 120ft - four London buses put end to end!For comparison, "Dippy", the famous diplodocus at the National History Museum in London is 90ft-long (equivalent to merely three London buses)

Attenborough says he's been fascinated by fossils ever since he was a lad, when he would regularly cycle 20 miles in order to unearth them.

“I’ve always loved fossils. I grew up in Leicestershire, where there’s an iron limestone full of ammonites and belemnites (prehistoric sea creatures) and a few fish, if you were really lucky. When I was 12 years old I couldn’t think of anything more romantic than cycling to a quarry, taking a boulder, hitting it into two pieces... and there in your hand is the most beautiful and perfect shell you’ve seen in all your life. It’s the first time it’s seen the sunshine for 150 million years – and you’re the first human being ever to have seen it. I think that’s pretty exciting.”

Above,Young David excavating in Madagascar

The look of wonderment on his face in the programme as he first “meets” the 100m-year old dinosaur bones shows that his love of fossils hasn’t changed in the nearly eight decades that have passed since then.

But the journey to Argentina was rather more gruelling than his care-free days in Leicestershire. After a nearly 14-hour flight to Buenos Aires – the longest non-stop trip on British Airways’ schedule – he then had to travel a further 800 miles south, bringing the total journey time to 21 hours. And he did the return trip twice.

Isn’t that tough to contend with at the age of 89? “It’s just boring. But your doctor will tell you it doesn’t do your body a lot of good.” But he needed to travel there (rather than just report from London) so that he could see for himself the progress of scientists as they calculated and built a life-sized replica of the “new” dinosaur. And, in any case, the farmer’s family – who are relishing the attention of the world’s scientific community – were delighted to welcome him. “They were very kind to us. They let me sleep in their house.”

So how confident can we be about the palaeontologists’ claim that this new species is the biggest animal ever to have walked the Earth?

“Oh, you can’t be totally confident,” says Attenborough, “because somebody else made such a claim about ten years ago and they thought they were confident. But as to its actual size, it’s not just down to imagination. They’ve found more than 200 bones – that is extremely unusual for a dinosaur. And there are ways of deducing size from the bones – and we know that nature doesn’t build things that have no value.” In other words, nature would only create a thigh that can support a 70-ton animal if that’s what it needed to do.

But we don’t know whether the animals would have grown even larger than the scientists’ calculations, because “we can tell from the bones that were found that they weren’t fully grown”. What else do we know about this beast? Well, as a vegetarian, it would have needed to eat the equivalent of a builder’s skip full of branches and leaves every day. And how would a creature this big and cumbersome manage to mate? “You can get some idea from knowing how rhinoceroses and how elephants mate: not belly to belly, but belly to back.”

And why did these huge dinosaurs die in this one spot in Patagonia? We can’t know for sure, explains Sir David, but the bones that have been found were, in fact, from seven different animals, whose deaths most probably occurred hundreds of years apart. So this isn’t the site of some dino massacre.

Finally, a cheeky question to the grand old man of television. The word "dinosaur" is often used pejoratively to describe people. Would it be a fair description – in some aspects at least – for Sir David Attenborough OM CH CVO CBE FRS?

“I’m not a great fan of new technology. I don’t change my phone every time the bell rings,” he admits. “But no, I’m not a dinosaur. I may be past my time, that is quite true. But people think of dinosaurs as great big lumbering things. And I hope I’m not that.”