Ask SV-POW! #1: hollow bones and bitten verts
January 6, 2012
We’re starting the new year with a new feature, in which we answer questions that have come our way. We never had a policy about not answering questions, it’s just that previous ones have tended to arrive in the comments section and have been dealt with there. But suddenly in the last few days I’ve gotten two questions from extrabloggular sources, and rather than hide the replies I thought I’d make them available to all.
One of my cohort at Berkeley texted me the other day with the following questions:
OK, phylobuddy: can you suck the marrow from a chicken bone? If they have hollow bones, where’s the marrow?!? Google is getting me nowhere.
Short answer: yes, one can get marrow from chicken bones, from those bones that contain marrow rather than air. In most fully mature chickens, the pneumatic bones include the braincase, the cervical, dorsal, and most or all synsacral vertebrae, some of the dorsal ribs, the central portion of the sternum, the coracoids, and the humeri (if you’re not a regular and some of these terms are unfamiliar, check out these handy guides [1, 2] to the vertebrate skeleton). That leaves marrow in everything else, although the only bones with large marrow cavities–as opposed to tiny trabecular spaces, which also house marrow–are the radii, ulnae, femora, tibiotarsi, and tarsometatarsi. So if you want to actually see large amounts of chicken marrow, or suck the marrow out of chicken bones, you’re basically stuck with the big distal bones of the wing, the thigh, and the drumstick (tibiotarsus). If you are boiling chicken bones to get stock for soups or stews, might as well throw them all in; even the pneumatic bones will still have bits of adhering meat, cartilage, and ligaments that will give up molecules and flavor to the stock.
The long answer is that the expression “hollow bones” has caused no end of confusion, because there are at least two ways to interpret hollow: filled with air, or not filled with bone (the former is a subset of the latter). If you mean “not filled with bone”, then the bones of almost all amniotes* are hollow, and the spaces inside are occupied by marrow (most commonly) or air. If filled with air, the bones are referred to as pneumatic, and an accessible introduction to them is here.
* At least; I know less about amphibians and fish, although at least one osteoglossomorph (IIRC) pneumatizes its vertebrae from its swim bladder!
The reasons it gets confusing are twofold. First, sometimes authors describe bones as hollow and mean only that they have chambers inside, but later readers see ‘hollow’ and infer ‘pneumatic’. Not all hollow bones are pneumatic; in fact, the vast majority of them are not, including the long bones of your arms and legs. The criteria for inferring pneumaticity from dry bones are more strict, and are explored in this paper and this one. Anyway, this point is just confusion caused by an ambiguous term.
The second case is more interesting, because it involves real unknowns. In the fossil record we can almost always tell if a bone is hollow, sensu lato, but sometimes it is not possible to say for certain whether the hollow space(s) inside were filled with marrow or air. Particularly vexing and intriguing examples include the humerus of Eotyrannus and the iliac chambers of some sauropods, which are discussed in this paper. My guess is that the iliac chambers of sauropods are genuinely pneumatic, because they only occur in sauropods that already have sacral pneumaticity, and we know from broken ilia of more basal sauropods and sauropodomorphs that large marrow-filled chambers are not present in those taxa. Conversely, I suspect that the humerus of Eotyrannus was apneumatic (marrow-filled), given that humeral pneumaticity is otherwise unknown in non-avian theropods, although the pneumatic furcula of Buitreraptor at least shows that the necessary clavicular air sac was present in some.
Next question! This one came to me on Facebook, from ReBecca Hunt-Foster, whom you may know from her awesome Dinochick Blogs. You should also envy her and hubby John Foster for getting the most awesome wedding present of all time: a 1/12 scale skeleton of Apatosaurus sculpted by Phil Platt, which you can read about here. That’s cool enough that I am stealing it for this otherwise picture-challenged post.
ANYWAY, ReBecca wrote on my FB wall today to ask:
Random question: Have you seen many tooth marks on sauro cervical verts? I am debating on whether something I have is a dessication crack or really some tooth marks. Thanks :)
In all the 15 years that I have spent looking at sauropod remains in the bowels of many, many museums, I have never seen a single tooth mark on a sauropod vertebra.
[Update the next day: Er, except for the bitten Apatosaurus tail on display in the AMNH! Many thanks to reptilianmonster and steve cohen for reminding me about this in the comments. I’m going to go hide for a while now.]
Now, that doesn’t mean that they aren’t there. Truth be told, I’ve never looked for them, and my usual mental search pattern for pneumatic traces (large, irregular) would probably exclude tooth scratches (small, linear) as noise. But I’ve certainly never seen any vertebrae with easily recognizable signs of predation or scavenging or with obvious bites removed.
People also sometimes ask me what kinds of healed traumas I’ve seen in pneumatic sauropods bones. That’s easy: apart from vertebral fusions, most of which probably have nothing to do with trauma, I’ve seen zip. Nada. Null set. The wingspan of the average tadpole. I’ve seen some pretty cool pneumatic bones from extant birds that were broken and later healed, including a eagle femur in the UCMP comparative collection that is now shaped like the letter Z, but nothing in sauropods.
I can think of three possible reasons for this, which sort of flow into each other. The first is that apart from the very solid and blocky centra of apneumatic vertebrae, sauropod verts were pretty fragile, and prone to getting distorted and busted up even when they started out intact, and those verts that started out broken just had a tougher time with the taphonomic lottery.
The second is that pneumatic sauropod bones would been nothing to most predators other than a mouthful of relatively dry bone shards, so either carnivores left them alone, or if they were osteovores like T. rex, they ate the shards and whatever is left over is unrecognizable. I have seen, and mostly ignored, plenty of vert-shrapnel in quarries and in collections, and maybe sharper eyes than mine could have discerned evidence of predation from those bits. To me it mostly looked like trampling, hydraulic transport, erosion, and other mundane ways to explode a vertebra.
The third is that in addition to a preservation bias against half-destroyed verts, there is probably also a collection bias against them. I’m probably not the only one would pass up a few shards of excellence to dig out the complete fibula sitting next to them in the quarry, and I love this stuff. That said, we did get a LOT of blasted vert bits out of the Wolf Creek quarry in the Cloverly, so if you want to pore over sauropod shards looking for tooth marks, visit the OMNH.
And, if you do know of tooth marks on sauropod vertebrae, please let us know in the comments. And consider publishing them, given the apparent vacuum of such things.