When the American Museum of Natural History wanted to create a digital walking Tyrannosaurus rex for a new dinosaur exhibit, it turned to dinosaur locomotion experts John Hutchinson and Stephen Gatesy for guidance.
The pair found the process humbling.
With powerful computers and sophisticated modeling software, animators can take a pile of digital bones and move them in any way they want. This part is easy; but choosing the most likely motion from the myriad of possibilities proved difficult.
"We kind of took a step back and said 'Whoa, boy! Limbs are very complicated.' We can't just take a limb, connect the bones together and figure out how the animal moved," Hutchinson told LiveScience. "It's absolutely impossible."
Hutchinson is from the Royal Veterinary College at the University of London; Gatesy is from Brown University. The pair writes about the problems involved in recreating T. rex's walking pose in the March 16 issue of the journal Nature.
Millions of possibilities
A computer model that assumes T. rex's hip, knee, ankle and main toe joints all flex and extend through a 90-degree arc yields more than 67 million possible poses. Most can be ruled out as structurally or biologically unlikely, but that still leaves thousands of possible configurations.
For the museum's "Dinosaurs: Ancient Fossils, New Discoveries" exhibit, the team reluctantly settled on one animation that captured the basics of T. rex's movements.
"Yet we could have produced thousands of animations that were no better or worse," the researchers write.
Traditional approaches not enough
Scientists wanting to learn how dinosaurs moved have traditionally looked to living animals, especially birds, which many paleontologists agree are direct descendents of dinosaurs. This approach forces scientists to make a lot of assumptions, however, and there is no way to test how closely the movements of a living animal resemble that of one that's been dead for millions of years.
Scientists can also estimate a dinosaur's gait and speed from fossilized footprints, but these estimates often have wide margins of error.
The researchers think that one way to narrow down the possibilities of dinosaur movement is to use more rigorous physical constraints in computer models. These constraints fall into two broad categories: kinematic (motion-based) and kinetic (force-based). One simple kinematic constraint, for example, is that the ankle and knee cannot bend backwards.
"This approach can be used to reveal how dinosaurs did not move, moving us progressively closer to reconstructing what kinds of motions they may have used," the researchers write.