Thursday, March 16, 2006

Coolness alert: Dinosaur Locomotion

How did the dinos walk or run has often been depicted on the big screen, but never with much thought to accuracy. Now a couple of researchers have applied a degree of scientific rigor to the question and published their results in the journal Nature:
The availability of sophisticated animation and simulation software allows palaeobiologists to tackle fundamental questions about dinosaur locomotion in a new way. [...]

Having previously avoided complete reconstructions in our own research, we wondered whether we could legitimately present our 'result' as a scientific hypothesis rather than just a pretty moving picture. We found the process humbling — not because creating a walking stride for the model was difficult, but because (with the aid of the museum's animator, Scott Harris) inventing motion was all too easy. How would we know which limb poses were off the mark and which were closer to reality? We reluctantly settled on one animation that captured the basic principles of animal locomotion. Yet we could have produced thousands of animations that were no better or worse. Computational tools offer tremendous power to generate motion, but we now face the new problem of how to choose among countless hypotheses.

Palaeontologists have two main options when confronting redundancy in an articulated chain of bones. The first is to set arbitrary constraints on the system to reduce the number of possibilities. A dinosaur's limb orientation is often taken for granted by duplicating a mammalian, avian or some other intuitively pleasing pose. Other studies invoke simple geometric rules: for example, the knee and ankle can be restricted to identical angles, or the hips fixed midway between the planted feet. Such constraints simplify the redundancy enough to calculate a narrow range of solutions, but one risks answering a question that no longer has relevance to reality. Assuming a pose a priori is untenable.

The second, alternative option is to use demonstrably realistic constraints to exclude, rather than include, possible limb motions. Only by admitting that a single solution is unachievable can we begin to search for a set of movements that cannot be ruled out. A rigorous dynamic simulation of a moving dinosaur, one encompassing all motions and forces, cannot yet plausibly be done. But a major goal is to move towards an integrative analysis based on solid evidence and an adequate consideration of unknown factors. Because the limb skeleton is so redundant, this can be done by moving beyond the bones to include the forces (muscular, external, gravitational) that actually create limb motion.
Neat enough, but it gets better: they have animation.