Radius, Not Ulna

As you may recall from your own background knowledge or from this tutorial, the forearm includes two bones: radius and ulna. One of the core analyses we’re going to be looking at is the proportions of the forearm relative to the arm (“upper arm”). Of course, we have to use bone length as a proxy for the true length of the different limb segments. For the arm, it’s a no-brainer. There’s only one bone, the humerus. For the forearm, we can choose between the radius and the ulna. At first glance, it might seem like the bones are interchangeable. But, it’s not really that simple.

Forelimbs of Othnielosaurus (left) and Triceratops (right), modified after the Marsh 1896 originals. In each, the radius is green, the main body of the ulna is blue, and the olecranon process of the ulna is yellow.

You see, some ornithischians (especially the big ones, it seems) have this giant sticky-outey thing on the proximal end of the ulna: the olecranon process. The triceps, among other muscles, attaches here. Because our spreadsheet only records maximum length of the ulna (and this is all most researchers ever report, anyhow), we encounter a complicated situation. The olecranon sticks up past the end of the humerus – so that a naive ulna : humerus ratio doesn’t really accurately describe forearm : arm length. It actually covers forearm + a little bit of the arm : arm length.

It doesn’t make much of a difference for some dinosaurs – for one Psittacosaurus, for instance, we’re talking ratios of 0.66 vs. 0.67. But what about an animal like Triceratops? In some specimen, it’s a matter of 0.99 vs. 0.71!  Obviously, we would get very different results if we apply this across the board.

For this reason, I would propose that we’ll want to use radius:humerus instead of ulna:humerus. In a quick look through the data, it also looks like we wouldn’t really lose out on any specimens, either; radius length is reported just as frequently as ulna length, if not more! And, of course, radius:ulna would be an interesting way to (indirectly) examine the relative size of the olecranon process in various bipedal and quadrupedal species. Which is an important issue in its own right. . .