Fossae/foramina in the articular surfaces of turkey vertebrae

February 21, 2022

I was looking more closely at the turkey skeleton from my recent post, and zeroed in on the last two dorsal (= thoracic) vertebrae. They articulate very well with each other and with the first vertebra of the sacrum, with the centra and zygapophyses both locking in so that there can only have been very little if any movement between them in life. Here they are, in right lateral view:

Last two dorsal (= thoracic) vertebrae of a mature domestic turkey Meleagris gallopavo domesticus, in right lateral view.

Before we move on, it’s worth clicking through to the full-size version of this image and wondering at both the quality of modern phone cameras (a Pixel 3a in this case) and the variety of textures on these little bones. There is smooth, finished bone on the sides of the neural spines; very fine pits and bumps on the zygapophyseal facets where the thin layer of hyaline cartilage attached; rougher texture in the parapophyseal facets where thicker cartilage attached; and very rough texture on the ends of the transverse processes, where there was relatively thick cartilage.

And there is, unsurprisingly in a bird, pneumaticity everywhere. In the more anterior vertebra alone (to the right) the photo shows pneumatic openings (from bottom to top) low on the centrum (below the parapophysis), high on the centrum (below the lateral process),  in the hollow between the lateral process, the posyzyg and the centrum, on the lateral surface of the prezygapophyseal ramus, and on the rear surface of the lateral process. There are others that are obscured in this photo, including on top of the lateral process where it meets the neural spine. Here they are, pointed out for you (with the hidden one shown translucently):

Last two dorsal (= thoracic) vertebrae of a mature domestic turkey Meleagris gallopavo domesticus, in right lateral view. Pneumatic openings on penultimate vertebra highlighted with red lines; obscured opening above lateral process shown as translucent.

OK, that was the B-movie. Now to the main feature. The next photo shows the same two vertebrae, folded away from each other so that we see the anterior face of the posterior vertebra (on the left) and the posterior face of the anterior vertebra (on the right).

Last two dorsal (= thoracic) vertebrae of a mature domestic turkey Meleagris gallopavo domesticus. Left: last dorsal vertebra in anterior view; right: penultimate dorsal vertebra in posterior view.

Again, do click through to see the exquisite detail, especially the complex of pneumatic features on the anterior face of the neural spine of the last dorsal (on the left) and on the posterior face of the left lateral process of the penultimate dorsal (on the right).

And … in the articular facets of the centra?

Seriously, what the heck is going on here? It doesn’t make sense  that there would be pneumatic openings in articular surfaces, because by definition something else (in this case the adjacent vertebra) is abutted hard up against then, so there is no way for a diverticulum to get in. For the same reason, you don’t get vascular foramina in articular surfaces because there is no way to get an artery in there. And there is no hint in these vertebrae of channels along either articular surface that diverticula or arteries could  possibly have laid in.

And yet, there those big openings are. What are they?

I discussed this with Matt, in case it’s Well Known Phenomenon that I’d somehow not heard about but it seems it is not. What we know for sure is that these openings are present, and that they are not mechanical damage inflicted during preparation. So what are they?

What else even is there for them to be? What penetrates bone apart from diverticula and blood vessels? Nerves follow the blood vessels, so it can’t be nerves in the absence of blood vessels.

By the way, there are similar but smaller openings in the posterior face of the last dorsal (the one on the right in the photo), but none anywhere else along the postcervical column: not on the anterior surface of the penultimate dorsal, not on the front or back of the sacrum, and not in any of the other dorsals.

One possibility we considered is that the vertebrae were locked together in life and that a pneumatic space inside the centrum of the last dorsal worked right through into the penultimate one. But that doesn’t work: the openings are not aligned. Also, those in the penultimate dorsal are definitely blind (i.e. they do not connect to deeper internal air-spaces) and those in the last dorsal probably are, too.

We do not know what is going on here.

Help us! Is this kind of thing common in turkeys? Have people seen it in other taxa? Do we know what it is?

14 Responses to “Fossae/foramina in the articular surfaces of turkey vertebrae”

  1. llewelly Says:

    do neck vertebrae have synovial fluid between them?

  2. Mike Taylor Says:

    In general, yes they do — one of the big differences between the vertebral columns of birds and those of mammals and crocs is that they have synovial joints between the vertebrae rather than fibrocartilaginous discs — and one of the big unresolved questions about dinosaur vertebral columns is what kind of joints they had: see https://svpow.com/2013/11/08/did-sauropod-necks-have-intervertebral-discs/ and https://svpow.com/2014/09/27/does-anyone-want-a-project-how-can-we-understand-sauropod-neck-cartilage-better/

    But in the present case we’re talking about vertebrae with little or no range of motion between them, and I wonder whether they might have been in the process of fusing.

  3. llewelly Says:

    If there’s a pneumatic diverticulum in one vertebra, and it works through into the part of the joint cavity that’s filled with synovial fluid, could it spread out “sideways” (that is, approximately perpendicular to the main axis of the neck), while in the joint cavity, and then work its way into the next vertebra, in a way that leaves the the respective spaces in each vertebra displaced with respect to each other?

    Or is the joint cavity containing the synovial fluid so thin that’s unlikely?

  4. dale mcinnes Says:

    This may sound too simplistic BUT, did you check out the age of your specimens ?!? Is it possible that when young birds mature, sometimes their cartilage does not fully form into bone, leaving gaps in their vertebrae ?!? This might answer why those gaps appear in the strangest of places (articulate facets) for no apparent reason.

  5. Brad McFeeters Says:

    Bone disease?

  6. Matt Wedel Says:

    One quibble:

    For the same reason, you don’t get vascular foramina in articular surfaces because there is no way to get an artery in there.

    You do get vascular foramina in articular surfaces, especially in dinosaurs, where the arteries that are already inside the bone were coming to the surface* to nourish the overlying cartilage. In fact, you can see several such foramina in the diapophyses of both of these vertebrae in the lateral view shots, and I think that’s what the foramina in the parapophyses are as well.

    *The surface of the clean, dry bone, but the bone-cartilage junction in life, when it wasn’t really on the surface at all.

    So the problem isn’t that you can’t get vascular foramina on articular surfaces, it’s that I’ve never seen any that looked like the huge lakes of whatever-that-is that we’re seeing on the articular faces of the centra.

    I like dale’s suggestion that this is related to ontogeny — I wonder if those lakes (what I’m calling them) were blobs of cartilage that were still in the process of ossifying in life. That might explain why they fell out or fell apart during the cleaning process, and also why we’re seeing dark crud, which I suspect is blood, inside the lakes. The ends of endochondral bones are typically well-supplied with blood while they are developing. At least, that’s the least-unlikely thing I’ve thought of so far.

  7. Mike Taylor Says:

    @Llewelly, I can’t see something as fragile as a diverticulum surviving the variable forces inside a synovial joint.

    @Dale, ontogenty is worth thinking about, but no, this is a mature bird (and a big one, too). The difference in ossification between this an a chicken skeleton that I have is striking.

    @Brad, I wonder if you’re right. We tend to be so focussed on pneumaticity here that possibilities like disease don’t necessarily occur to us. Which is especially dumb given that as I am writing these turkey posts, Matt is alternating them vertebra-pathology posts.

    @Matt, sure, yes, we know about vacularization of the cartilage itself from within the bone. I’m talking about nerves getting into the bone via the cartilage-covered surface.

  8. Matt Wedel Says:

    I’m not sure how you’d tell one from the other from dry bone — foramina don’t often tell us which way they are ‘pointing’. But I take your point — not least because inside a synovial joint would be a stupid place to put either a vessel or a nerve (not talking about the fine nerve endings that innervate the synovial membrane itself).

  9. Mickey Mortimer Says:

    A minor point regarding ontogeny, but I was under the impression that domestic fowl were bred to grow large and quickly for meat purposes, but while technically mature their ossification isn’t comparable to a wild adult bird. If you compare your skeleton to e.g. Harvey et al.’s (1968) classic Meleagris osteology for instance, the sternum is less ossified, the preacetabular processes aren’t fused, the ilia and ischia aren’t fused distally, the posterior synsacral vertebrae are less fused with larger foramina remaining, the trochanteric crest of the femur isn’t as ossified, etc.. So I could see those ‘lakes’ as ossification issues that wouldn’t even necessarily be typical of a wild juvenile, but some product of forced quick growth that exists because humans never notice any negative side effects from it in the farming and dining process.

  10. Mark Evans Says:

    Hopley (2001; JVP Vol. 21, pp. 253-260) described something similar in a plesiosaur specimen and interpreted it as the first fossil occurrence of Schmorl’s nodes, effectively a herniated intervertebral disc caused by compressive stress. They are apparently quite common in humans due to our bipedal stance. Could this be something similar?

  11. Mike Taylor Says:

    Mickey, thanks for your observations on the ontogenetic state of my turkey, which I guess is less mature than I thought (though still very much more mature than a comparable chicken skeleton that also have, and which I should post for comparison).

    Mark, that Hopley paper is fascinating. But I don’t think it can be what we’re seeing here because Schmorl’s nodes are caused by herniation of the nucleus pulposus of the intervertebral disc, and bird torsos don’t have intervertebral discs.

  12. Mark Evans Says:

    Mike, true, but could synovial fluid herniate through the articular cartilage?

  13. Mike Taylor Says:

    Mark, I wouldn’t think so — I don’t think synovial joints contain pressure that could do that, and I suspect that if synovial fluid was somehow squeezed out of its capsule it would dissipate into other tissues rather than excavating bone and sitting there. But I will defer to people who understand more soft-tissue anatomy than I do.

  14. Matt Wedel Says:

    It would be super weird for synovial fluid to herniate through the hyaline cartilage on the articular surface of a bone, and I think it’s probably impossible unless the cartilage had suffered some other form of mechanical damage. Even then, I struggle to envision it. I’ve seen plenty of human joints where the articular cartilage was worn through or otherwise damaged, all the way down to the underlying bone, but those joints tend to have synovial fluid, and no indication that pressure in the fluid was a problem, as opposed to pressure on the bones themselves.

    Modern broiler chickens are typically slaughtered awfully young, like 5-7 weeks of age, when sexual maturity doesn’t hit until 5 or 6 months. I think about this a lot, because according to Hogg (1984), the cervical vertebrae of chickens start to pneumatize during week 5, but most cervicals aren’t fully pneumatized until week 8 or so. Thoracic and synsacral vertebrae pneumatize between weeks 10 and 11. So mass-produced grocery store chickens tend to be strikingly under-pneumatized compared to adults. My understanding is that they have to be slaughtered so early because too many will become lame or die of heart failure otherwise. Which sucks.

    Reference
    Hogg, D.A. 1984. The development of pneumatisation in the postcranial skeleton of the domestic fowl. Journal of Anatomy, 139: 105-113.


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