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I was reading a post by Brian Switek on his blog Laelops—which by the way, if you aren’t reading it, you should—about interpreting injuries on dinosaur bones. It’s an interesting read, but what caught my eye was a problem that I have seen in more places than I can count. He included this picture in his post. Take at look at the front limbs.
The “hands” are pointed inwards. To get in this position, the elbows have to be turned outwards, using a rotation in the shoulders. But it also requires the hands to be pronated. To get what I mean by that, hold your hands forward with the palms up. When you do this, the two bones in your forearms—the ulna and the radius—are positioned side by side.
This position is called supination. Pronation requires the radius to rotate so that it crosses over the ulna. This can be done because of the construction of the elbow. The ulna is essentially the entire elbow joint, making it a hinge type joint. The radius is, for the most part, just along for the ride at the elbow. The head—the part at the elbow—is round, with a shallow indentation, which is surrounded by what is called the annular ligament. That ligament wraps around the radius, attaching it to the ulna, but never actually attaching to the radius itself, allowing the head to spin in the sling. It is that shape of the radius and the annular ligament that allows it to rotate freely, which makes our level of pronation possible.
Importantly, all tetrapods have the same bones. It was set in place from the first fish that developed bones to support their fins and remains that way all through the hundreds of millions of years to us today. However, not all animals have the same shape of the radial head. Some animals appear to not have both bones, but in reality, they do. They have just fused the bones together. But that fusion has consequences, just like altering the shape of the radial head.
Before we move on to dinosaurs and those consequences, it would be reasonable to ask about other animals to see if they show the same pattern. Let’s take a look at proboscideans, the family of elephants. They are large animals that have their palms facing downward.
This is an elephant skeleton on display at the Manchester Museum. They have a radius and ulna, just like humans. Theirs, however, do not swivel. Nevertheless, it is rotated so that the bones are not in parallel, but the two ends are twisted so that the radius is twisted over the ulna. Their forelimbs are in permanent pronation.
So what about dinosaurs? Let’s look at Dreadnaughtus, the giant sauropod.
This is Figure 2 from Lacovara et al. 2014. The radius and ulna are massive, as befits a giant quadruped. They are also incapable of rotating to pronate the foot. It has been said by some that some degree of pronation is required for efficient quadrupedal locomotion, but that is not really accurate. It does mean though, that the first digit, what would be our thumb, is going to face forward or at most slightly inward while the remaining digits will be angled outward (anterolaterally). They will not be situated directly forward without shifting the arms at the shoulder. As an example, here is a figure showing a sauropod trackway. Note the direction of the toes.
This is a figure from Falkingham, et al. 2010. As can be seen, the toes are not forward, but pointing outward. The level of pronation is minor and can be achieved with only rotation at the shoulder and a minimal shift or the forelimb. VanBuren and Bonnan (2013) found this was true in all quadrupedal dinosaurs.
This is a best case for pronation in dinosaurs and they can’t do the full pronation needed for bunny hands, much less the bizarre inward facing hands of the Plateosaurus above. So let’s look at theropods, where we see bunny hands all the time. For instance, in Jurassic Park pretty much all the dinosaurs have bunny hands.
The palms are downward with the arms close in, requiring a full pronation to achieve that position. So could they do it? To the bones. This is the radius of Neuquenraptor, as published by Novas and Pol in 2005. It is an unenlagiin, in the family Dromaeosauridae, along with Velociraptor, Deinonychus, and all those other raptor dinosaurs.
These radius is fairly straight, and if you look at the head, it is not the shallowly indented round cup we see in mammals. It is more angular, which is typical of theropods. That relative angularity would prevent the radius from rotating as ours do, in effect locking it into a small range of motion, preventing them from placing their hands palm down.
In point of fact, VanBuren and Bonnan didn’t just look at sauropods. They looked at all types of dinosaurs. They found that no dinosaur had the ability to cross the radius over the ulna, which means that at best, they had very limited ability to pronate their forearms. That means no known dinosaur could have characteristically held their arms close in with their palms facing downward, aka bunny hands.
It is a little more complicated than this. Studies have shown that if the arm is fully extended, the hands can be more pronated, by using the entire length of the arm to rotate, but even that is not going to be fully pronated like we can, or bunnies can. And if you think about the living dinosaurs, the birds, when was the last time you saw a bird put its wings flat on the ground in front of them? They can clap, but they can’t type or play basketball.
Of course, the wrist bones of the maniraptors—those dinosaurs leading up to birds—did have what is called a semilunate carpal bone, allowing them to move their hands to insane degrees side to side, which is what allowed them to develop the ability to fold their wings like they do, and they can flex and extend their hands to a remarkable degree. But they cannot rotate their wrist. Try it yourself. See what movements you can make with your hand without moving your forearm, just your hand. However far you can move your hand that way, these dinosaurs have you beat in spades. But once you move that forearm, you have a serious advantage over them.