PROVIDENCE, R.I. -- A new study of theropod dinosaur prints shows that foot motions were preserved in three dimensions when the meat eaters slopped through mud 210 million years ago. Although the footprints are similar to those made by birds living today, there are significant differences in the position of the big toe, foot posture and hind limb movement.
"Living birds retain many features from their theropod ancestors, but hind limb anatomy and function did change," said lead scientist Stephen Gatesy, assistant professor of ecology and evolutionary biology at Brown University. "Birds do not move exactly like 210-million-year-old dinosaurs, but they are the closest thing alive today."
Gatesy and colleagues discovered and studied numerous trackways made by human-sized theropod dinosaurs that strode on hind legs across extensive mud flats in what is now Greenland. At the time the dinosaurs roamed, the region was more tropical than arctic.
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On firm ground, a theropod's foot produced shallow, three-toed impressions. On sloppier muds, the foot sank deeper and left behind unusually long, four-toed prints. Such deep tracks captured movements of the foot below the surface, preserving a three-dimensional record of locomotor behavior in Triassic-period dinosaurs.
Reporting in Nature, the researchers said that a spectrum of tracks similar to the Greenlandic series is produced when a bird, such as a turkey, walks through muds of different consistencies. In the tracks of both Triassic theropods and living birds, the foot plunged down and forward into the muck. The toes collapsed together below the surface as the entire foot emerged at the front of the track.