How do you build a dinosaur?

Building a baby T. rex skeleton is hard when you have only a few bones

We all think that we know what dinosaurs look like, but no human has actually seen one. But recent palaeontological breakthroughs mean that scientists are now able to create the most accurate reproductions ever seen.

A full Tyrannosaurus rex skeleton has never been found, so how do we know what they looked like?

In 1854, the world's first dinosaur exhibition opened in Crystal Palace, South London. But by the end of the 19th Century the show had lost credibility, as scientific discoveries superseded these early impressions of dinosaurs.

Now the world's leading dinosaur scientists are working on a groundbreaking exhibition in Los Angeles, California, which aims to be the most scientifically accurate representation of dinosaurs so far.

Luis Chiappe, director of LA's Museum of Natural History, says they aim to show how "we translate the evidence that we find in the field into scientific knowledge".

Dinosaur excavation in Utah Discoveries of fossilised bones are the first step in a long scientific process of gathering new information

The centrepiece of the exhibit will be a "growth series" of three T. rex skeletons, and it will also feature a model of a baby T. rex.

To get both models and skeletons exactly right, a huge amount of science goes into each exhibit.

The palaeontologists' starting point remains the fossilised bones uncovered on digs around the world, but it's not always fresh excavations that yield new information.

Darren Naish, a palaeontologist based at the University of Portsmouth, looks for new dinosaurs in the back rooms of museums.

"You don't necessarily have to go out in the field and look for dinosaurs, you can just rummage through museum drawers and you will find something new," he says.

He and a colleague only recently found a new species tucked away in storage, and he says there are a huge number of specimens which could become new discoveries.

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Dr Alice Roberts

Watch How to Build a Dinosaur presented by Dr Alice Roberts on BBC Four Wednesday 21 September at 21:00

"We're in a golden age of dinosaur discovery, there's about 50 new species of dinosaurs named every year," he says, adding that "about 90% of all named dinosaurs have been named since about 1990".

The next step is to work out exactly what the unearthed fossils are. It can take years to clean off sediment from an entire skeleton.

Once the skeleton is ready, it needs to be pieced together and hung true to life.

Paul Zawisha is in charge of creating a custom-made steel frame for the T. rex, which needs to be strong enough to support the enormous weight of the fossils.

He says: "Most of the bones are real which makes them extremely heavy.

"We're estimating that the total weight of the bones is a little over a tonne.

"The femurs probably are a good 200-250lbs (90-115kg) apiece, we have to set those in place with special rigging devices.

"Heaven forbid one of them falls because it would take quite a bit of time to get those back together."

Paul is in charge of putting together the lead T. rex in the exhibition - known as "Thomas".

Contractor working on T. rex skeletons in LA Rebuilding a skeleton takes many contractors months

Thomas is one of the best T. rex specimens discovered, but is still only 70% complete. His missing bones will be made by Paul's team, based on those belonging to over 30 other partial T. rex that have been found.

The steel frame will be a work of art in itself, millimetre perfect, and subtle enough not to draw attention away from the dinosaur.

The pose in which the dinosaur is hung, while being true to science, will also involve a degree of artistic interpretation, to really bring the exhibit to life.

"We might change the toes just a little bit to give this thing a sneaking feeling, or a pausing feeling. But it's very, very subtle," says Paul.

"You might move one toe just one inch in one direction, and that changes how you visualise this whole thing," he says.

Most experts now believe that although many dinosaurs died out 65 million years ago, a few survived and evolved into modern birds.

"What it means is that because dinosaurs have living descendants, dinosaurs are not extinct, they did not become extinct at the end of the Mesozoic era," explains Luis.

"You know you have 10,000 living species of birds that are providing you an enormous amount of information that you can use to understand the biology."

One of the world's leading experts on dinosaur movement, Dr John Hutchinson is being consulted by Luis to make sure the T. rexes reflect the latest theories on musculature and physiology.

By comparing bird anatomy with dinosaur anatomy, estimating the muscle sizes of extinct animals, and inputting them into computer models, Dr Hutchinson is able to get an insight into how dinosaurs actually moved.

Dr Hutchinson says: "We've found using our computer models that a human sprinter which can do 25 miles an hour (40km/h) or a little faster would probably be pretty well matched for a muscular Tyrannosaurus.

"An average human who can run about 15 miles an hour would probably be a pretty good match for a skinnier version of a T. rex."

Also being constructed for the exhibition for the first time is a model of a tiny, chicken-sized dinosaur called Fruitadens, the smallest dinosaur to be found in North America.

Man painting Fruitadens Scientists did not initially know what colour the model Fruitadens should be

Fossils and imaging have enabled scientists to reproduce it faithfully, but one mystery remains: its colour.

How do we know what colour dinosaurs should be?

Doyle Trankina, one of Luis's artists who is working on the Fruitadens model, says: "If you push things too far, you go with polka dots, purple and pink, your audience simply won't believe it.

"If you draw upon the examples of living animals, we can actually gain a lot just by looking at crocodile skin, colouration, maybe some lizards and fish even, it will remain believable."

But there is another way. Prof Mike Benton recently came across the remains of a dinosaur that were so exquisitely well-preserved that feathers, as well as bones, had fossilised.

When scanned with an electron microscope which magnifies objects 9,000 times, the secrets of their pigmentation were able to be unlocked.

By comparing their structure to living feathers, colours could be identified - ginger, black, dark brown or grey.

It's a finding that has helped scientists create even more accurate portrayals of dinosaurs.

"Who on earth would have thought a dinosaur is close to a bird? But there we are. You know it's kind of proved in the skeletons, and now if you like, proved in the anatomy of the feathers," says Prof Benton.

Palaeontology is a constantly developing field, where new discoveries are changing the game all the time.

And that is a burden Luis is well aware of: "It is our responsibility to make sure that people understand that things are not written in stone, and our scientific conclusions change as we gather more evidence."

How to build a dinosaur is broadcast on BBC Four on Wednesday 21 September at 21:00 or catch up on BBC iPlayer.

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