March 6, 2009
Getting a Handle on Theropod Arms
A theropod trackway showing the palms-down hand strike. Image courtesy of Michael Skrepnick.
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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Well, I don’t agree that it necessarily shows anything about hand mobility, but it may say something about arm mobility. After all, birds can get their hands palms-down…but not by pronating at the wrist! Instead, it’s an effect of abducting the arm from the body. The same may be true of the theropod that made the BC tracks. Perhaps a semantic issue (hand vs. arm), but a somewhat important one lest someone reading this thinks that theropods were capable of pronation/supination the way humans are (see also Zach Miller’s excellent summary of this.)
Jerry is correct, and I mentioned in my initial correspondence to Brian that eversion of the arm complex from the ribcage and rotation at the shoulder could accomplish the same result. I remember Paul Sereno discussing radius and ulna in early theropods essentially locked in place, and with a simple carpal block, pronation / supination would be virtually eliminated. . . not so sure in regard to more advance theropods though. While the skeletal elements themselves may remain relatively rigid, soft connective tissue, ligaments,cartilage, etc. . . may allow for a limited range of rotation, being a little more plastic at the joint articulations.
Further, just as another point of interest, I recall Phil Currie recounting (pers. comm.), that many of the Gorgosaurus specimens recovered from Dinosaur Provincial Park, with skeletal elements in articulation, noted that most were found with both manus ( mani ? )oriented so that the palmer surfaces were actually directed in an antero-medial ( or dorso medial, depending on long axis placement of the arms ) positioning. . . so unless this was some sort of post depositional artefact in preservation, it might be indicative of further mobility at the wrist, in this group.
Any chance the therapod could have been ‘knuckle dragging’ or dragging its claws with the tops of the hands forward or angled toward the ground instead?