February 2 0 , 2017:
by Alisa Alering
For millions of years, dinosaurs thundered across the earth. An extinction event about 65 million years ago wiped them out, leaving behind only a few bones and some tracks in the mud to mark their existence.
European scientists first became interested in these massive beasts when quarry workers discovered the bones of a Megalosaurus in the English countryside in the 1670s. Scientists coined the term ‘dinosaur ’ in 1842, and soon life-size models of these mysterious creatures were displayed to the delight of a curious public.
Unfortunately, those first reconstructions looked nothing like the popular conception of dinosaurs today. And it wasn’t until the late 20th century that paleontologists realized that many of these ancient reptiles may actually have been more closely related to modern-day birds.
But these are understandable mistakes, considering that the animals studied have been extinct for millions of years. After all, with only bones and fossil footprints to draw upon, how do you accurately reconstruct the appearance of something that no longer exists?
X-ray video, for one. Peter Falkingham, senior lecturer in Vertebrate Biology at Liverpool John Moores University, was the winner of the ARCHER 2016 image competition for his reconstruction of dinosaur movement from fossil footprints. To model the movement of the ancient and absent beasts, he examined the motion of living animals.
“Because the leg is one big linkage system, if we know what the foot is doing, we can work up the leg from there,” says Falkingham. “This information can be used to support or reject hypothesis about how extinct animals moved.”
Tracks in the sand
Fossil footprints offer an exciting amount of information to scientists because they are the direct record of an animal in motion.
“Even the most exceptionally preserved dinosaur mummy is still just a carcass that may have been washed some distance from where the animal lived,” says Falkingham. “But a footprint was made by the living animal’s motions, and is always preserved in rocks that record the environment in which that animal lived and breathed.”
To create the movement of a bird-like dinosaur, Falkingham and fellow scientist Stephen Gatesy combinied bi-planar X-ray data of a guineafowl walking over a sand-like material with a discrete element simulation of the substrate.
They first used X-ray Reconstruction of Moving Morphology (XROMM) to capture and animate the skeleton and then opted for the open source software package LIGGGHTS to model the substrate, simulating grains of 1 mm diameter.
Even the most exceptionally preserved dinosaur mummy is still just a carcass that may have been washed some distance from where the animal lived. A footprint was made by the living animal’s motions, and is always preserved in rocks that record the environment in which that animal lived and breathed. ~ Peter Falkingham
“Using a CT scan of the animal’s bones, we can match the bones to both cameras, and reconstruct the 3D motions of those bones,” says Falkingham. “Because everything is calibrated in 3D space, it means we can view the motion from any angle.”
Beneath the surface
The scientists used simulations to see what was happening below the surface of the sand as the bird moved across it. But as you might imagine, simulating every grain of sand in a large box with a complex foot walking over it is computationally intensive.
The team calculated each particle’s interaction with neighboring particles, and the information-dense operation resulted in millions of calculations per second of simulation.
“We use very small particles to accurately capture the behavior of the ‘sand’,” says Falkingham, “but this means that there are an awful lot of particles!”
Running even one of the smaller simulations required 12-24 hours on nearly 10,000 processors, using the ARCHER supercomputer at the University of Edinburgh.
“The datasets created are one of the hardest parts of the research to manage – outputting data at 250 frames per second, containing position and force data for over two million particles can quickly lead to datasets of about 2TB for a single run,” says Falkingham. “Even just a few years ago, this was pretty unthinkable.”
Evolving our picture of dinosaurs
By simulating the entire movement of the bird’s foot on sand, from the initial strike to the final scrape of the toes, Falkingham and Gatesy gave us the world's first visualization of a track’s formation both at and below the surface.
Dinosaurs as a group contain the largest land animals that have ever existed, including giant two-legged carnivores unlike anything alive today.
Comparing tracks created by an animal with a foot similar to that of ancient dinosaurs, Falkingham hopes to extract more reliable inferences from existing fossil tracks and shed light on previously enigmatic features in dinosaur physiology.
“What evolutionary constraints might produce similarities in, for example, the way an 8-ton T. rex moved and the way a modern bird does?” he asks. “Conversely, how differently can two bipedal animals move and still be able to walk or run?”
Falkingham's research breaks new ground and sets in motion a new way of understanding dinosaurs. These insights are important steps to bring evolution and dinosaurs into sharper focus.