No time for anything new, so here’s a post built from parts of other, older posts.

The fourth sacral centrum of Haplocanthosaurus CM 879, in left and right lateral view. This is part of the original color version of Wedel (2009: figure 8), from this page. (Yes, I know I need to get around to posting the full-color versions of those figures. It’s on my To Do list.)

Note the big invasive fossa on the right side of the centrum. The left side is waisted (narrower at the middle than the ends) like most vertebrae of most animals, but has no distinct fossa on lateral face of the centrum. What’s up with that? Here’s an explanation from an old post (about another sauropod) that still fits:

Now, this asymmetry is also weird, but it’s expected weirdness. Pneumaticity seems to just be inherently variable, whether we’re talking about human sinuses or the facial air sacs of whales or the vertebrae of chickens. It appears that the form of pneumatic features is entirely determined by local tissue interactions, with little or no genetic control of the specific form. Think of it this way: genes prescribe certain developmental events, and those events bring tissues into contact–such as pneumatic epithelium and bone. The morphology of the bone arises out of that interaction, and each interaction of bone and pneumatic epithelium has the potential to produce something new. In this case, the diverticula on the left side of the vertebral column come from the lungs or air sacs on the left, and those on the right side come from the lungs or airs sacs on the right, so it’s really two sets of diverticula contacting the bone independently. The wonder, then, is not that pneumatic bones are so variable, but that we see any regularities at all.