Dinosaur handprints reveal birdlike arm anatomy

Fossil handprints made by a crouching theropod reveal that those meat-eating bipedal dinosaurs had palms that always faced inward, a sign that these creatures abandoned the use of their forelimbs as legs early in their evolution.

Theropods include the tiny-armed Tyrannosaurus rex, which lived between 68 million and 65 million years ago, and Allosaurus, which lived about 150 million years ago and was also poorly endowed in the bicep department. Because most theropods’ arms were very small, there are very few scenarios where those forelimbs could have touched the ground, says Andrew R.C. Milner, a vertebrate paleontologist at the St. George Dinosaur Discovery Site at Johnson Farm in Utah.

Of the half-dozen examples known where theropods crouched down and left impressions in soft ground — dents that were ultimately preserved as fossils — only two also include possible handprints, and those are faint or smudged, he notes.

Now, Milner and his colleagues describe what they say is a clear set of theropod handprints, two inch-deep impressions preserved among hundreds of dinosaur footprints at the Utah site. The rocks also contain the fossils of fish, invertebrate burrows and the footprints of crabs, as well as evidence that the sediments had cracked and dried in the sun. Those are signs that the trackway, a fossilized trail of footprints made by a single animal, was made in the muddy sand of an ancient lakeshore about 198 million years ago, the researchers report March 4 in PLoS ONE. The now-hardened rocks preserve many details of the lakeside topography of that time, says Milner.

The theropod handprints, discovered by a volunteer at the site in 2004, are part of a trackway that appears to have been made when the creature walked up a slight incline and then crouched to rest. The size and spacing of the footprints suggest that the theropod, of an unidentified species, was about 4.5 meters long. Each handprint was made by the edge of the hand, not its palm, and indicates that the fingers on each forelimb curled inward, says Milner. This configuration backs up anatomical studies of later theropods, suggesting that those creatures couldn’t rotate their palms to face downward, he notes.

Thomas R. Holtz Jr., a vertebrate paleontologist at the University of Maryland, College Park, agrees. “If there’s any situation when you’d expect a theropod to plant its palm facedown, this is it,” he suggests, and the fact that it didn’t in this case is strong evidence that it couldn’t. The new research, he adds, “is a really neat paper” that shows that even early theropods had abandoned the use of their forelimbs as legs.

The fossil record of theropods from this era, early in their evolution, is sparse, and not a lot is known about those creatures’ anatomy, says Anthony Martin, a vertebrate paleontologist at Emory University in Atlanta. The newly described handprints are “yet another example of how trace fossils can provide better insights about dinosaur behavior and anatomy than mere bones,” he adds.