Dinosaurs.

They've been cartoon characters, fire-breathing gorilla-provoking monsters and robotic movie sensations.

They've also been drawn, painted and sculpted from fossils for scientists and museums.

Graphic artists for movies work more from imagination, but paleoartists have a different mission. They draw, paint or sculpt their dinosaurs for science using fossils and relying on the paleontologist's discoveries.

There is always a certain amount of artistic license when paleoartists do scientific renderings of dinosaurs. Some things artists know for sure, such as the size of the dinosaur, while other features remain mysteries, such as the color of the skin.

Sealing the mouths of therapods, meat-eating dinosaurs that walked on two legs, with "lips" has been subject to debate for a number of years in the paleoartist community and is also a dinosaur detail that can't be scientifically backed up-- yet.

When paleoartists reconstruct these animals, their interpretations ranges from a "no lips," (the "crocodilian") look, to a partial lip covering the base of the upper jaw gum line, to lips that fully conceal the teeth.

"Lips are one of many 'gray areas' of interpretation, for which alternatives are freely expressed among the artists," said Michael Skrepnick, a paleoartist in Alberta, Canada.

Japanese artists seal the mouth while western artists tend to keep mouths open, like a crocodile who is constantly baring its teeth, said Tyler Keillor, a paleoartist and fossil preparator at Prof. Paul Sereno's lab at the University of Chicago.

Keillor has been experimenting with sealing therapod lips because he sees certain traits in animals that are closely related to dinosaurs, suggesting that they did have lips.

Unlike dinosaurs, crocodiles have a bony plate on the roof of their mouths and a valve that seals off their throats, so they can catch prey underwater without drowning.

This leaves Keillor relying on animals like Komodo dragons, whose big teeth are hidden behind their closed mouths, when he sculpts dinosaurs.

"If I can seal a dinosaur's mouth with lips like large lizards have, it allows dinosaurs to breathe in through the nose without losing air or getting dehydrated," he said.

Keillor's argument, that therapods would have had difficulty staying hydrated with a "no lips" model because they were more terrestrial, is a valid concern, Skrepnick said. But he also sees the other side.

"[Some dinosaur fossils] reveal when their jaws were shut, the length of their upper jaw teeth exceed the height of the lower jawbone," he said.

So, if the teeth were enclosed in a "lower lip" structure, he continued, the upper jaw teeth would extend below the lower jaw bone, piercing the flesh.

Many artists are wary of sealing dinosaurs' mouths with lips because big teeth define these predatory dinosaurs.

"The idea of killing teeth is so important and impressive, it's almost a selling feature of this animal," Keillor said. "I can imagine some resistance in wanting to cover the teeth."

There's a compromise to this debate: the upper teeth slide over the outer surface of the lower jaw, like a severe overbite, and a partial upper lip keeps the lower teeth moist. Soft tissue surrounding the lower jaw cushions and seals the space between upper and lower jaws when the mouth is closed, Skrepnick said.

"In this version, the crown tips are exposed externally and present no potential for inducing trauma or injury to the animal itself," he said. "Looking through popular books, you will note that the majority of theropod reconstructions seem to follow a variation on the model."

The only way to tell if dinosaurs had lips is hard evidence. Until paleontologists find a well-preserved specimen that indicates some sort of lips, or no lips.

"Until then we don't know and there's still room for interpretations," Keillor said.

Bones, Birds and Lizards

The Rugops primus, a meat-eating dinosaur with a short wrinkly snout, was a new dinosaur species unveiled in 2004 by Sereno.

Keillor, who reconstructed the bust, could have done a straight forward reconstruction with clay and a glass eye, but he chose to investigate what was on top of the animal's skull.

"There was an interesting network of ridges grooves and blood vessels imprinted in the bone," he said. "I went out to look at different animals to see what other animals had this same pattern of texture on top of the bone."

The texture on bone looked like it was covered with keratin, the hard tissue on a bird's beak, and he matched it to the crests on large birds.

"This dinosaur's snout was not as scaly, but it had hard tough armor plating," he said.

For the rest of the Rugops bust, Keillor looked at reconstructions of its closest dinosaur relatives to clue him in on how to make the scales and the texture of skin.

The artists only have bones. They can't just draw, paint or sculpt anything that comes to mind; they have to compare the bones to other animals and fossils.

"You can't visualize extinct animals without looking at living animals today," Keillor said.

Paleoartists don't look at just any animal, they have to look at the animals that are closely related to dinosaurs.

There is the extant phylogenetic bracket, the basic family tree of life, that helps scientists and artists see how organisms are related to each other.

In the case of dinosaurs, the extant (living animals) on this tree are birds and crocodiles, and many artists use that as a starting point for a reconstruction.

"We can compare them to other dinosaurs they are aware of or what animals they are closely related to to draw some analogies," Skrepnick said, "If their legs are not there I can make a comparison and draw conclusions to what length or size these limbs were."

But birds have beaks, no teeth and can fly and crocodiles have specializations unique to their aquatic lifestyle so they can catch prey underwater without drowning.

Dinosaurs don't have that, so artists sometimes have to look to animals that are not as closely related.

"I find looking at large terrestrial reptiles very useful as well," Keillor said. "The Komodo dragon which is a large animal, has a complete set of teeth, lives in a terrestrial environment and is very similar to dinosaurs."

The way muscles were networked on top of each other, the way the soft tissue is padded and different features of different animals help the artists accurately portray dinosaurs, and make them more realistic.

"The most influential things that will effect a reconstruction are the reports on new anatomy, either a discovery that is revealed, something interesting or different about the anatomy," Keillor said.

The Changing Posture of the T-Rex: From Godzilla to Jurassic Park

The Tyrannosaurus rex used to be a lizard-like, lumbering king of the dinosaurs, dragging its tail on the ground behind it.

"Artists started out rendering images that didn't look anything at all like what dinosaurs looked like today," said John Lanzendorf, a Chicago resident who owned the largest collection of dinosaur art until he sold it to the Indianapolis Children's Museum. "Now they are as close to real as humanly possible."

A century ago Charles Knight, the prolific dinosaur artist, painted the T-Rex in an upright posture pose, which was the accepted stature of that dinosaur.

That changed in the 1970s when scientists figured out that it if the T-Rex stood that way, it would dislocate its hipbones. The T-rex stands horizontally now, balancing its massive tail with its massive head.

Artists followed suit, and began rendering the T-Rex in that same "teeter-totter" pose.

The fascination with the T-Rex skull hasn't changed with the pose and the teeth are still the focal point of that animal.

"People will readily accept the change in pose in skeletons," Keillor said. "It brings down the skull into a more ominous pose and the skull is kind of leaning down at you."

The T-Rex isn't the only dinosaur whose looks have changed over the last century.

"The art mirrors what goes on in science," Skrepnick said. "The early ideas they had a century ago, [were in the] art generated a century ago."

This change in art isn't uncommon; new discoveries and better technology has changed the way dinosaurs are depicted in art.

"Scientists are finding things they couldn't find before," Lanzendorf said. "New machinery digs out the fossils easier and paleontologists know how to protect the bones easier."

Whatever the science brings to the different looks of dinosaurs, paleoartists don't need to let their imagination run away with them.

"Dinosaurs are so beyond our regular experience. I hear this stuff, and just want to shake my head. You don't believe what comes out of the ground," Skrepnick said. "Dinosaurs are such a thrill, so wonderful and bizarre, but real, and not made up you'd swear they were made up."

Launching pad into science

Keillor has a personal cause when it comes to paleoart. He wants everyone to understand all of the new species discovered.

There are at least two major announcements every month in the paleontological field, like a new species or a new discovery. Few get widespread notice because there might not be an artist on hand to help the public understand what the dinosaur looked like, he said.

"Researchers might not have a lot of resources left over for a wonderful piece of artwork to convey this new dinosaur to the public," Keillor said. "There's no shortage of specimens for paleo artists to reconstruct, but there's a shortage of resources for researchers to be able to do reconstructions."

Paleoartists are the link between scientists and people. They scientifically render dinosaurs into sculptures, paintings or illustrations so people can understand what dinosaurs could have looked like.

"If it weren't for the artist, people wouldn't know what dinosaurs would look like," Lanzendorf said.

Without the artist's rendering, dinosaur science risks being stuck in the gridlock of pop culture and movie monsters.

"I have the responsibility to be accurate, I'm an advocate for the dinosaur," Skrepnick said. "Between the paleontologist and myself, it's our job is to bring animals back to life so the public can make sense of it."

Artists play a big role in reeling children into the world of science; if it weren't for them, dinosaurs would be a pile of bones.