What happened when the dinosaurs disappeared?

Around 50 million years ago, such a fantastical world was a reality.

And this week, TV viewers in the UK will be able to see such bizarre creatures as these, and many others, resurrected in the first episode of the BBC's Walking With Beasts series, thanks to advanced computer-animation technology.

The programmes, which were two years in the making and cost �7m, pick up the story where the hit series Walking With Dinosaurs left off. Beasts chronicles the rise and rise of mammals and birds after dinosaurs cleared the stage.

Series producer Jasper James comments: "The period after the dinosaurs is like the Dark Ages of prehistory, populated by a staggering number of weird, wonderful and totally unique animals that people just don't know about. We are lifting the lid on a mind boggling, forgotten era."

Living relatives

Lifting the lid - sort of. Before it has even begun, Walking With Beasts is provoking controversy about the liberties it takes with the facts. The series gives the animals behavioural traits and colour markings that could not have been established from the fossil records.

I think it is perfectly reasonable for a documentary to say, "we are guessing", I don't think that would be spoiling the magic

Prof Richard Dawkins, zoologist

Of course it would be a dull set of programmes indeed without such speculations, but to what extent should the viewer be left to untangle fact from fiction?

Palaeontologist Dr Adrian Lister is confident the audience will not be left in an information vacuum. "I think the BBC has responded very well to that criticism because, along with the programmes, there is a huge amount of additional material that the viewer can access, where you get text on your screen saying what is known and what is not," he told BBC News Online.

Palaeontologists look to the present to understand the past

This is one of the drivers of digital television - the option viewers have to call up additional information as they watch. The interactive version of Beasts - available to some digital users in the UK this week - has already won a prestigious TV industry award.

"Also, with Walking With Beasts, the BBC had a slightly easier task than in the previous series because many of the species portrayed have living relatives. This means the guess work is likely to be more accurate," Dr Lister said.

"I mean really, you either accept you have got to guess certain things or you abandon the whole exercise. I think the programme makers have done as much as they possibly can."

Maintaining the magic

But while "faking it" is getting more and more easy, thanks to computer technology, zoologist Professor Richard Dawkins is getting more and more nervous. He feels that care should be taken to control this new breed of filmmaking, and that some semi-formal guidelines might even be called for.

What we have is a wonderful series, which will tell us a hell of a lot about prehistoric animals

Dr Adrian Lister, palaeontologist

"The computer tools for faking it have become so good that we need a new code of conduct," the Professor of the Public Understanding of Science at Oxford University told BBC News Online.

"Perhaps some new guidelines should be put together by a committee of documentary producers and scientists that act as a point of reference for programme makers.

The series is seen as a way of bringing science to life

"Of course the individual producer doesn't want to mess it up by breaking the spell, but you have to be clear. I think it is perfectly reasonable for a documentary to say, 'we are guessing'. I don't think that would be spoiling the magic."

Just quite how to handle the brave new world of computer-generated documentaries remains a grey area. But that creatures like Andrewsarchus, a giant wolf-like mammal with a one-metre-long (3ft) jaw, can be brought to life at all is undeniably remarkable.

"What we have is a wonderful series, which will tell us a hell of a lot about prehistoric animals," said Dr Lister. "I am very much in favour of the enterprise."

Digital viewers can go deeper into the science

Walking With Beasts is broadcast in the UK on BBC One on Thursday, 15 November, at 2030 GMT.