Getting a Handle on Theropod Arms

Earlier this week a new paper in the journal PLoS One reported a set of fossilized impressions that showed how theropod dinosaurs held their hands. Scientists were able to confirm that theropods' palms faced each other. But paleo-artist Michael Skrepnick reminded me of another trackway that confirmed the mobility of theropod hands.

At the 2002 Society of Vertebrate Paleontology meeting, paleontologists reported on a set of early Cretaceous (about 146 to 100 million years old) tracks found in British Columbia made by a large theropod dinosaur. The tracks showed that it was walking at a uniform speed, but for some unknown reason the theropod raked the ground with both of its hands, making two sets of slash marks. To do so it would have had to have held its hands palms-downward, as shown in the wonderful illustration by Skrepnick.

The findings of these studies are not mutually exclusive. The neutral position, or the position at rest, of theropod hands was so the palms were facing each other, as shown in the PLoS One paper. Theropod hands were not locked in this position, however, and the earlier 2002 report reveals that they did have enough of a range of motion so that they could hold their arms with elbows out and palms down. Both studies show why trackways can be so important to paleontology: they are snapshots of anatomy and behavior preserved in stone.

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Riley Black is a freelance science writer specializing in evolution, paleontology and natural history who blogs regularly for Scientific American.