Secrets of Dinosaur Footprints Revealed in Bird X-Rays

A detailed X-ray video of a modern bird foot is revealing the secrets of dinosaur tracks set down more than 250 million years ago.

The ancient tracks, made by the chicken-size dinosaur Corvipes lacertoideus, contain strange features that are likely the marks made when the dino withdrew its foot from the sediment — a process that is impossible to document without X-rays that reveal what's below the sediments.

The new video "is the first time anyone has been able to see a footprint being formed," said study author Peter Falkingham, a research fellow at the University of London's Royal Veterinary College. [See X-ray Video of Footprint Formation]

Falkingham and his colleagues used X-rays to videotape a bird called a guineafowl (a relative of chickens and pheasants) as it walked through a soft bed of poppy seeds. The researchers could see the foot plunging into the seeds, just as a dinosaur foot might have sunk into soft sand or dirt.

Frozen motion

Dinosaur prints are the only direct record of dinosaur movement, Falkingham told Live Science. But they are very tricky to interpret. A fossil print might come from the original land surface where the dinosaur stepped, or it might be an impression left over from several rock layers down, after the surface layer eroded away.

Prints are static impressions, Falkingham said, but a moving dinosaur foot is dynamic. Unlike celebrities carefully pressing their handprints into the cement on the Hollywood Walk of Fame, a walking dino didn't create an anatomically correct impression in the ancient soil. The dinos sank down, swept their toes through the dirt and flexed their foot muscles as they moved. Without an understanding of this process, scientists can only glean so much from dinosaur tracks.

Falkingham and his colleagues wanted to peer below the surface, second by second, as a track was being made. They chose guineafowl as their subjects because the birds are close in size to the dinosaur print fossils that the researchers wanted to use as comparisons. And, as birds, guineafowl are descendents of the avian dinosaurs.

"They are small dinosaurs without a tail, and that makes them a fantastic correlate for looking at the footprints produced by dinosaurs and produced by birds," Falkingham said.

In an X-ray machine, the birds trotted over solid surfaces and through dry poppy seeds, an analogue for sand or other soft sediment. Next, the researchers used computer simulations to model the movement of every single little poppy seed as the bird strode through. 

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