Anatomical features of the neural canal in birds and other dinosaurs. A. MWC 9698, a mid caudal vertebra of Apatosaurus in posterodorsal view. Arrows highlight probable vascular foramina in the ventral floor of the neural canal. B. LACM 97479, a dorsal vertebra of Rhea americana in left anterolateral view. Arrows highlight pneumatic foramina inside the neural canal. C. A hemisected partial synsacrum of a chicken, Gallus domesticus, obtained from a grocery store. Anterior is to the right. The bracket shows the extent of the dorsal recess for the glycogen body, which only spans four vertebrae. Arrows highlight the transverse grooves in the roof of the neural canal for the lumbosacral organ. D. Sagittal (left) and transverse (right) CT slices through the sacrum of a juvenile ostrich, Struthio camelus. The bracket shows the extent of the lumbosacral expansion of the spinal cord. Indentations in the roof of the neural canal house the lumbosacral organ. In contrast to the chicken, the ostrich has a small glycogen body that does not leave a distinct osteological trace. Yellow arrows show the longitudinal troughs in the ventral floor of the neural canal that house the ventral eminences of the spinal cord. Wedel et al. (2021: fig. 4).

This is the second in a series of posts on our new paper about the expanded neural canals in the tail vertebrae of the Snowmass Haplocanthosaurus. I’m not going to talk much about Haplo in this post, though. Instead, I’m going to talk about chickens, and about how you can see a lot of interesting spinal anatomy in a living dinosaur for about two bucks.

You know by now that Academia Letters publishes peer reviews, which is one of the things that drew me to this fairly new journal. More on that in a later post, but in the meantime, the peer reviews for the Haplo paper are on the right sidebar here. I confess, I had a total forehead-slap moment when I read the opening lines of Niels Bonde’s review: 

This paper is interesting, and should be published and discussed by others with interest in dinosaur-bird relations. However, as these publications are also meant for the general public, I would recommend that 2 – 3 illustrations were added of the features mentioned for birds under nos. 3 – 6, because the general public (and many paleontologists) have no ideas about these structures, and what they look like.

The original submission only had figures 1 and 2. And this request is totally fair! If you are going to discuss six alternative hypotheses for some mysterious anatomical structure, it’s just responsible reporting to illustrate those things. That goes double if, as Niels Bonde noted, the anatomy in question is unfamiliar to a lot of people, even many paleontologists. Huxley’s quote after first reading Darwin’s Origin of Species flashed through my head: “How extremely stupid not to have thought of that.”

Slide 21 of my 2014 SVPCA talk on supramedullary diverticula in birds and other dinosaurs, illustrating pneumatic foramina in the roof, walls, and floor of the neural canal.

At the time I read that review, I already had images illustrating five of the six hypotheses. A juvenile ostrich synsacrum that Jessie Atterholt and I had CT scanned gave us three of them all by itself: the lumbosacral expansion of the spinal cord to run the hindlimbs, as in all limbed tetrapods and in some fish with sensitive fins; the transverse channels in the dorsal wall of the neural canal to accommodate the lumbosacral balance organ; and the paired troughs in the floor of the neural canal that house the ventral eminences of the spinal cord (Figure 4D in the image at the top of this post). I had good photos of pneumatic foramina in the walls and floor of the neural canal in a dorsal vertebra of a rhea from my 2014 SVPCA talk (Figure 4B), and some photos of small foramina, presumably for blood vessels rather than air spaces, in the floor of the neural canal in a caudal vertebra of Apatosaurus (Figure 4A).

What I did not have is a photo illustrating the fairly abrupt, dome-shaped space in the sacral neural canal that houses the glycogen body of birds. I mean, I had published images, but I didn’t want to wrestle with trying to get image reproduction rights, or with redrawing the images. Instead, I went to the grocery store to buy some chicken.

I don’t know how universally true this is, but IME in the US when you buy a quartered chicken, the vertebrae are usually nicely hemisected by the band saw that separated the left and right halves of the animals. So you can see the neural canal in both the dorsal and sacral parts of the vertebral column. Here are the hemisected dorsal vertebrae in the breast quarter from a sectioned rotisserie chicken:

That’s just how it came to lie on my plate, but it’s not in anatomical position. Let’s flip it over to sit upright:

And label it:

I could and probably should do a whole post just unpacking this image, but I have other fish to fry today, so I’ll just note a couple of things in passing. The big interspinous ligament is the same one you can see in transverse section in the ostrich dissection photos in this post and this one. Also, the intervertebral joints heading toward the neck, on the left of the image, have much thicker intervertebral cartilage than the more posterior dorsals. That’s because the posterior ones were destined to fuse into a notarium. You can see a diagram and a photograph of a chicken notarium in figures 4 and 5, respectively, here. And finally, the big takeaway here is that the neural canal is normal, just a cylindrical tube to hold the spinal cord.

The thigh quarter usually has the pelvis and the hemisectioned synsacrum attached. Here’s a lateral view of the left half of the pelvis and synsacrum:

And the same thing labeled:

And now flipped around so we can see it in medial view:

And now that image labeled:

And, hey, there are three of our alternative hypotheses on display: the long (many vertebral segments) lumbosacral expansion of the spinal cord, which is reflected in a gradually expanded neural canal in the synsacrum; the shorter, higher dome-shaped recess for the glycogen body; and finally the transverse spaces for the lumbosacral balance organ.

As a refresher, there’s nothing terribly special about the lumbosacral expansion of the spinal cord — you have one, labeled as the ‘lumbar enlargement’ in the above diagram. Where the spinal cord has adjacent limbs to run, it has more neurons, so it gets fatter, so the neural canal gets fatter to accommodate it. The cord itself doesn’t look very expanded in the chicken photo above, but that chicken has been roasted rotisserie-style, and a lot of lipids probably cooked out of the cord during that process. What’s more important is that the neural canal is subtly but unmistakably expanded, over the span of many vertebrae.

The lumbosacral spinal cord of a 3-week-old chick in dorsal view. The big egg-shaped mass in the middle is the glycogen body. Watterson (1949: plate 1).

That’s in contrast to the recess for the glycogen body, which is colored in blue in the chicken photo. Glycogen bodies, like the egg-shaped one in the young chicken in the image immediately above, tend not to go on for many vertebral segments. Instead they balloon up and subside over the space of just 4 or 5 vertebrae, so they leave a different skeletal trace than other soft tissues.

Finally, there are the transverse spaces for the lumbosacral balance organ, which I discussed in this post. Those are the things that look like caterpillar legs sticking up from the sacral endocasts in the above figure from Necker (2006). In life, the spaces are occupied by loops of meningeal membranes, through which cerebrospinal fluid can slosh around, which in turn puts pressure on mechanoreceptive cells at the edge of the spinal cord and gives birds a balance organ in addition to the ones in their heads. In the photo of the cooked chicken, the delicate meninges have mostly fallen apart, leaving behind the empty spaces that they once occupied.

I really liked that chicken synsacrum, and I wanted to use it as part of Figure 4 of the new paper, but it needed a little cleaning, so I simmered it for a couple of hours on low heat (as one does). And it promptly fell apart. At least in the US, most of the chickens that make it to table are quite young and skeletally immature. That particular bird’s synsacrum wasn’t syn-anything, it was just a train of unfused vertebrae that fell apart at the earliest opportunity. I had anticipated that might be an issue, so I’d gotten a lot of chicken, including a whole rotisserie chicken and four thigh quarters from the deli counter at the local supermarket. Happily this fried chicken thigh quarter had a pretty good neural canal:

And it cleaned up nicely:

And with a little cropping, color-tuning, and labeling, it was ready for prime time:

I didn’t label them in the published version, for want of space and a desire not to muddy the waters any further, but the jet-black blobs I have colored in the lower part of that image are the exit holes that let the spinal nerves out of the neural canal so they could go serve the hindlimbs, pelvic viscera, and tail. We have them, too.

At my local grocery store, a fried chicken thigh costs about $1.65 if you get it standalone, or you can buy in bulk and save. You get to eat the chicken, and everything else I’ve done here required only water, heat, soap, and a little time. The point is that if I can do this, you can do this, and if you do, you’ll get to see some really cool anatomy. I almost added, “which most people haven’t seen”, but given how much chicken we eat as a society these days, probably most people’s eyes have fallen on the medial surface of a cooked chicken thigh quarter at one time or another. Better to say, “which most people haven’t noticed”. But now you can. Go have fun. 

Way back in January of 2019, I finished up “Things to Make and Do, Part 25b” with this line: “I have one more thing for you to look for in your bird vertebrae, and that will be the subject of the next installment in this series. Stay tuned!” Here we are, 2.3 years later, and I’ve finally made good. So if there’s a promised post you’ve been waiting for, stick around, we may get to it yet.

References

A. Recovered skeletal elements of Haplocanthosaurus specimen MWC 8028. B. Caudal vertebra 3 in right lateral view. C. The same vertebra in posterior view. Lines show the location of sections for D and E. D. Midsagittal CT slice. The arrow indicates the ventral expansion of the neural canal into the centrum. E. Horizontal CT slice at the level of the neural arch pedicles, with anterior toward the top. Arrows indicate the lateral expansions of the neural canal into the pedicles. B-E are shown at the same scale. Wedel et al. (2021: fig. 1).

New paper out today:

Wedel, Mathew; Atterholt, Jessie; Dooley, Jr., Alton C.; Farooq, Saad; Macalino, Jeff; Nalley, Thierra K.; Wisser, Gary; and Yasmer, John. 2021. Expanded neural canals in the caudal vertebrae of a specimen of Haplocanthosaurus. Academia Letters, Article 911, 10pp. DOI: 10.20935/AL911 (link)

The paper is new, but the findings aren’t, particularly. They’re essentially identical to what we reported in our 1st Paleo Virtual Conference slide deck and preprint, and in the “Tiny Titan” exhibit at the Western Science Center, just finally out in a peer-reviewed journal, with better figures. The paper is open access and free to the world, and it’s short, about 1600 words, so this recap will be short, too.

A. Photograph of a 3D-printed model of the first three caudal vertebrae of Haplocanthosaurus specimen MWC 8028, including endocasts of the neural canal (yellow) and intervertebral joints (blue), in right lateral view, and with the neural canal horizontal. B. Diagram of the same vertebrae in midsagittal section, emphasizing the volumes of the neural canal (yellow) and intervertebral joint spaces (blue). Anterior is to the right. Wedel et al. (2021: fig. 2).

John Foster and I described Museum of Western Colorado (MWC) specimen 8028, a partial skeleton of Haplocanthosaurus from Snowmass, Colorado, in late 2014. One weird thing about that specimen (although not the only weird thing) is that the neural canals of the tail vertebrae are bizarrely expanded. In most vertebrae of most critters, the neural canal is a cylindrical tunnel, but in these vertebrae the neural canals are more like spherical vacuities.

John and I didn’t know what to make of that back in 2014. But a few years later I started working with Jessie Atterholt on bird anatomy, which led me to do a little project on the whole freaking zoo of weird stuff that birds and other dinosaurs do with their neural canals, which led to the 1PVC presentation, which led to this. 

Caudal vertebra 3 of Haplocanthosaurus specimen MWC 8028 in left posterolateral (A), posterior (B), and right posterolateral (C) views, with close-ups (D and E). In A and B, a paintbrush is inserted into one of the lateral recesses, showing that the neural canal is wider internally than at either end. Wedel et al. (2021: fig. 3).

Of course there will be more posts and more yapping, as signaled by the ‘Part 1’ in the post title. Although I am extremely satisfied with the streamlined, 1600-word missile of information and reasoning that just dropped, there are parts that I want to unpack, that haven’t been unpacked before. But the paper launched at midnight-thirty, Pacific Daylight Time, I’m up way too late finishing this first post, and I reckon the rest will keep for a few hours at least.

Anatomical features of the neural canal in birds and other dinosaurs. A. MWC 9698, a mid caudal vertebra of Apatosaurus in posterodorsal view. Arrows highlight probable vascular foramina in the ventral floor of the neural canal. B. LACM 97479, a dorsal vertebra of Rhea americana in left anterolateral view. Arrows highlight pneumatic foramina inside the neural canal. C. A hemisected partial synsacrum of a chicken, Gallus domesticus, obtained from a grocery store. Anterior is to the right. The bracket shows the extent of the dorsal recess for the glycogen body, which only spans four vertebrae. Arrows highlight the transverse grooves in the roof of the neural canal for the lumbosacral organ. D. Sagittal (left) and transverse (right) CT slices through the sacrum of a juvenile ostrich, Struthio camelus. The bracket shows the extent of the lumbosacral expansion of the spinal cord. Indentations in the roof of the neural canal house the lumbosacral organ. In contrast to the chicken, the ostrich has a small glycogen body that does not leave a distinct osteological trace. Yellow arrows show the longitudinal troughs in the ventral floor of the neural canal that house the ventral eminences of the spinal cord. Wedel et al. (2021: fig. 4).

I have a ton of people to thank. John Foster, obviously, for initiating the line of research that led here. Julia McHugh for access to the MWC collections, and for being an excellent sounding board regarding the Morrison Formation, sauropod dinosaurs, and crafting ambitious but tractable research projects. Anne Weil for helping me be methodical in thinking through the logic of the paper, and Mike Taylor for helping me get it polished. Niels Bonde, Steven Jasinski, and David Martill for constructive reviews, which were published alongside the paper. We couldn’t take all of their suggestions because of space limitations, but figures 3 and 4 were born because they asked for them, and that’s not a small thing. Vicki and London Wedel for putting up with me at various points in this project, especially in the last few days as I’ve been going bonkers correcting page proofs. And finally, because I’m the one writing this blog post, my coauthors: Jessie Atterholt, Alton Dooley, Saad Farooq, Jeff Macalino, Thierra Nalley, Gary Wisser, and John Yasmer, for their contributions and for their patience during the unusually long gestation of this very short paper.

More to say about all that in the future. For now, yay, new paper. Have fun with it. Here’s the link again.

References

Here are some blank diagrams I whipped up for drawing in spinal cord pathways.

This one shows the whole cord, brainstem, thalamus, and cerebral cortex in coronal section, in cartoon form.

It’s for drawing in ascending sensory and descending motor pathways, as shown in this office hours sketch. DC-ML is dorsal column/medial lemniscus, which carries discriminative touch and conscious proprioception. ALS is anterolateral system, which carries pain, temperature, pressure, and itch. The lateral corticospinal tract carries fibers for voluntary control of major muscle groups. Each pathway differs in terms of where it decussates (crosses the midline, left-to-right and vice versa) and synapses (relays from one neuron to the next). The sensory pathways involve primary, secondary, and tertiary sensory neurons, and the motor pathways involve upper motor neurons (UMNs) and lower motor neurons (LMNs).

This one shows cross-sections of the cord at cervical, thoracic, lumbar, and sacral levels, for drawing ascending and descending pathways and thinking about how patterns of somatotopy come to exist.

Somatotopy is the physical representation of the body in the central nervous system. A common abbreviation scheme is A-T-L for arm-trunk-leg, as shown here for ascending sensory and descending motor pathways.

Finally, this one shows the spinal cord and spinal nerve roots at four adjacent spinal levels, for tracking the specific fates of sensory and motor neurons at each spinal level.

This is particularly useful when working out the consequences of an injury, like the spinal cord hemisection (Brown-Sequard syndrome) shown here in pink. The little human figure only shows the zone in which pain and temperature sensation are lost. There would also be losses of discriminative touch, conscious proprioception, and voluntary motor control on the same side as the injury.

Finally, since we’ve had a bit of a sauropod drought lately, here are a couple of photos of the mounted cast skeleton of Patagotitan in Stanley Field Hall at the Field Museum of Natural History in Chicago.

I gotta say, this mount beats the one at the AMNH in every way, because it’s well lit and you can move all the way around it and even look down on it from above. In fact, in terms of getting to move all the way around it, get well back from it to see the whole thing at once, and even walk directly underneath it (without having to ask permission to hop the fence), it might be the best-mounted sauropod skeleton in the world. The Brachiosaurus outside is also pretty great (evidence), but it loses points because you can’t walk around it on an upstairs balcony. Every other mounted sauropod I know of is either in more cramped surroundings, or you can’t get underneath it, or is less well-lit, or some combination of the above. Am I forgetting any worthy contenders? Feel free to make your case in the comments.

Incidentally, the spinal cord of Patagotitan was something like 120 feet long, and the longest DC-ML primary sensory neurons ran all the way from tail-tip to brainstem before they synapsed, making them among the longest cells in the history of life.

A belated thank-you to Josh Matthews and the rest of the Burpee PaleoFest crew for a fun day at the FMNH back in March. I got home from that trip about 3 days before the pandemic quarantine started, so it’s waaaaay past time for me to blog about how awesome that trip was. Watch this space. UPDATE: hey, look, it only took me a third of a year this time! Link.

Altounian et al. (2015: fig 6).

As has been discussed here before, the recurrent laryngeal nerve (RLN) does not only innervate the larynx, but also parts of the esophagus and trachea (see this post, and in particular this comment). You can see that in this cadaver photo, in which the RLN is sending nice big visible branches into both the esophagus and trachea on its way to the larynx. Why is it doing this? Because the embryonic gut tube, which gives rise to both the digestive and respiratory systems, is serially innervated by the nerves of the pharyngeal arches that the gut tube passes through. Parts of the esophagus and trachea pass through the 4th to 6th pharyngeal arches, so they are innervated by the nerve that serves those arches, which is the recurrent laryngeal nerve. As discussed in the post and comment linked above, the recurrent course of the RLN to the esophagus and trachea is just as dumb as its recurrent course to the larynx, and equally strong evidence of a developmental constraint.

Although all tetrapods have an RLN that innervates the larynx, the axons to the esophagus and trachea aren’t always bound up with it. In dogs and many other mammals, those nerve fibers to the esophagus and trachea form a second recurrent nerve, the pararecurrent nerve or recurrent pharyngeal nerve. In this wonderful, complicated figure by Lemere (1932), the recurrent laryngeal nerve is labeled ‘r’, and the pararecurrent nerve is labeled ‘pa’.

Here’s Lemere’s figure with the RLN and pararecurrent nerve highlighted for easier comparison. The pattern of axonal wiring here is the same as in humans–all the axons have the same connections at the brainstem end on one hand, and at the pharynx and larynx end on the other hand–but the bundling of axons into what we recognize as peripheral nerves is different.

Interestingly, Lemere (1932) mentioned that having the recurrent pathways split into two nerves was the most common pattern in dogs, but occasionally he saw a case in which all of the axons had been bundled into a single RLN that served both the larynx and the esophagus and trachea, as in humans.

Modified from Altounian et al. (2015: fig. 4).

That door of variation swings both ways: a few years ago in our lab, we had a cadaver in which the left RLN only went to the larynx, and the vagus fibers to the esophagus and trachea were carried in a second, variant nerve. I didn’t know what that nerve was for a long time, until I stumbled onto the work of Lemere. So it seems that two nerves is the usual pattern for dogs, with one nerve as a rare variant, and the opposite is true in humans. 

Incidentally, I didn’t find the variant nerve in our lab, my students did. We got as far as putting together a manuscript, which we posted as a preprint (here), but we haven’t gotten it formally published yet. One of my goals for this year is to get some of these old, stalled projects dusted off and properly published. Watch this space.

I also discussed the pararecurrent nerve in my “How to make new discoveries in (human) anatomy” talk from SVPCA 2019, which is also a PeerJ preprint (here).

Other posts on the recurrent laryngeal nerve, and on the peripheral nervous system in general:

References

Amazing diagram of the path of the vagus nerve and its branches in the neck, thorax, and abdomen, from Wilson-Pauwels et al. (1988). The gonads aren’t drawn here, but they do receive vagal innervation.

This isn’t new to science, it’s just one of the cool little quirks of human and comparative anatomy that more people should be aware of.

Quick-quick background: autonomic (unconscious, involuntary) innervation of the body comes in two flavors, sympathetic and parasympathetic. Sympathetic nerves mostly handle the fight-or-flight response, parasympathetics are feed-breed-and-read. You could also think of them as the “oh crap emergency” and “hum-drum housekeeping” branches of the nervous system. Sympathetic nerves to the whole body are derived from the spinal cord between T1 and L2, and parasympathetics come from certain cranial nerves and from the sacral part of the spinal cord. If a refresher on all of this would be handy, please see this.

The most awesome nerve in the body is cranial nerve X, the vagus nerve. “Vagus” means “wandering” in Latin; it’s the same root from which we get ‘vagabond’ and ‘vagrant’. As the name implies, the nerve gets around–various branches innervate just about all your viscera from the soft palate at the back of your mouth to the first half of your large intestine. And, as stated in the title of the post, your gonads.

That’s actually pretty weird, even for a nerve from your skull that innervates most of your digestive tract. The reason why it’s weird is that your embryonic hindgut (descending colon, sigmoid colon, and rectum) and most of your pelvic viscera and reproductive system get their parasympathetic innervation from the pelvic splanchnic nerves. The gonads are buried in all of that, and they may get some pelvic splanchnic innervation as well, but they also get innervated by the vagus nerves. (Note for fellow arch-pedants: yes, I know there’s evidence that the pelvic splanchnics should technically be considered sympathetic, but I’m not going down that rabbit hole right now.)

Like most of the weird stuff that goes on in our bodies, the reason why the gonads get vagal innervation is developmental. The gonads actually start developing pretty high up in the abdomen, not far below the diaphragm, and their nerve and blood supply are established at that point. High in the abdomen is firmly in vagus nerve territory, so the gonads get vagal innervation. Then later the gonads descend, in both sexes, and they drag their nerves and blood vessels along with them, which is why the gonadal vessels in both sexes come off the aorta near or with what end up being the renal arteries. (Kidneys do the opposite thing, developing down low and then climbing the aorta, swapping arteries as they go, but that’s a story for another day.) In males the testes take the final leap through the abdominal wall to descend into the scrotum, but ovaries descend almost as far, from up by the diaphragm down to the bowl of the pelvis, or at least to its rim. (Most of the time: just as males can have undescended testes, females can have incompletely-descended ovaries; they turn up now and then in the anatomy lab.)

Incidentally, fellas, this is why you feel sick to your stomach when you get kicked in the groin–the nerves to your testicles come out of the same plexus that serves your stomach and most of your intestines, and the pain fibers go back the same way.

For some super-interesting work on determining gonadal innervation using viral tracing, see Gerendai et al. (2005, 2009). I may quote some choice passages down in the comments.

Another great figure from Wilson-Pauwels et al. (1988), this time showing the path of vagal sensory fibers from the periphery to the brain (and again omitting the reproductive tract). Note the nucleus solitarius in the medulla oblongata, where vagal sensory fibers travel first on their way to the brain.

As is often the case in biology, things get stranger still. In females it’s not just the ovaries that are innervated by the vagus nerve, but part of the cervix and vagina as well. This was hypothesized by Komisaruk et al. (1997), based on the fact that some women with complete spinal cord injuries (‘complete’ here meaning ‘spinal cord cut all the way through’) could still experience genital sensation. It was confirmed by Komisaruk et al. (2004), who found that women with complete transection of the spinal cord could achieve orgasm from vaginal stimulation. Their fMRI study showed that the posterior part of the nucleus solitarius in the brainstem–which receives sensory fibers from the vagus nerve–was active in the process. I assume that there is some esoteric bit of embryology that explains how vagus fibers end up in the vagina, probably something to do with the mesonephric ducts. But I don’t know what that is off the top of my head, so I’ll have to go hit the books (again).

If you dig the diagrams I’ve used here, definitely go track down a copy of Wilson-Pauwels et al. (1988). There are newer versions of the same book that have full-color illustrations, but I think the single-color-nerve-on-black-and-white figures from the 1988 version are cleaner and more readable. It’s not just a good book on the cranial nerves, it’s a master class on clear visual presentation of complicated material. Unfortunately it is not cheap; even used paperback copies start around $40 unless you get lucky. UPDATE: However, through the generosity of Dr. Wilson-Pauwels, the illustrations and captions from the 2013 3rd edition are freely available for teaching purposes at this link. Go avail yourself of this phenomenal resource!

Anyway, the moral of the story is that, male or female, you have nerve fibers that exit your skull through the jugular foramen, pass down your neck behind your carotid arteries, follow your esophagus and stomach to the networks of nerves that run your guts, and run as impossibly slender fibers on the surfaces of the blood vessels that go to your testes or ovaries and vagina. You run your ‘nads from your brain, not just by way of the spinal cord but also by nerves that come straight out of your friggin’ head. Have fun with that thought.

References

  • Gerendai, I., Banczerowski, P. and Halász, B. 2005. Functional significance of the innervation of the gonads. Endocrine 28(3): 309-318.
  • Gerendai, I., Tóth, I.E., Boldogkői, Z. and Halász, B. 2009. Recent findings on the organization of central nervous system structures involved in the innervation of endocrine glands and other organs; observations obtained by the transneuronal viral double-labeling technique. Endocrine 36(2): 179-188.
  • Komisaruk, B.R., Gerdes, C.A. and Whipple, B., 1997. Complete’spinal cord injury does not block perceptual responses to genital self-stimulation in women. Archives of Neurology, 54(12): 1513-1520.
  • Komisaruk, B.R., Whipple, B., Crawford, A., Grimes, S., Liu, W.C., Kalnin, A. and Mosier, K. 2004. Brain activation during vaginocervical self-stimulation and orgasm in women with complete spinal cord injury: fMRI evidence of mediation by the vagus nerves. Brain Research 1024(1-2): 77-88.
  • Wilson-Pauwels, L., Akesson, E.J. and Stewart, P.A. 1988. Cranial Nerves: Anatomy and Clinical Comments. Toronto: BC Decker.

From Will’s Skull Page, here.

Here’s a skull of a wild boar. Note the loooong face, practically a straight line from the tip of the snout to the top of the back of the head.

We shall now proceed through a series of pig skulls with increasingly steep foreheads.

From the UCL Museums and Collections blog, here.

Some domestic pigs have a longish snout and nearly straight forehead, like their wild forebears. (Or foreboars, if you will.)

A cast skull from Carolina, available here.

But it seems–from a quick, unscientific, and in-no-way-standardized image search–that the vast majority of domestic pigs have at minimum a more steeply-inclined forehead.

This one was auctioned in New Zealand, at this site.

Foreheadization is becoming undeniable.

From skullbase.info, here.

Is this one any more pronounced than the one before? I’m not sure, and so far I’m too lazy to try superimposing the skulls. But they don’t even look like the same kind of animal as the wild boar shown at top.

From theweirdandwonderful.com, now apparently only available on Pinterest, here.

In my explorations so far, this appears to the ne plus ultra of short-faced, high-forehead domestic pigs, excluding truly pathological cases. The line from the inflection point of the forehead to the occiput is twice the length of the snout!

From theweirdandwonderful.com, now apparently only available on Pinterest, here.

Oddly enough, the high forehead in domestic pigs is not always associated with a super-short snout, as this skull demonstrates.

This figure from Owen et al. (2014) sums up the shape differences between domestic (left) and wild (right) Sus scrofa.

Okay, so domestic pigs have shorter snouts and steeper foreheads than wild pigs of the same species. But y tho? It seems to be part of the “domestication syndrome” present in many domesticated animals, which includes a shortened snout, smaller teeth, piebald coloration, floppy ears, a curly tail, and a host of other morphological and behavioral traits. Interestingly, pigs seem to show more aspects of domestication syndrome than any other domestic animals other than dogs, as shown in the figure below, from Sanchez-Villagra et al. (2016).

Okay, so domestication, but how? It’s not like the Domestication Fairy comes in the night and steals half your snout.

Wilkins et al. (2014: fig. 1)

The various morphological changes that go along with domestication syndrome seemed disconnected until 2014, when Wilkins et al. proposed a pretty nifty hypothesis, which goes like this:

  • Probably the most crucial aspect of domestication is selection for tameness, which is really selection for reduced adrenal gland and sympathetic nervous system activity, so the animals aren’t freaking out all the time.
  • The adrenal glands and sympathetic ganglia are derived from embryonic neural crest, which also influences the growth of the teeth, brain, skull, vertebral column, and ear cartilages, and the distribution of melanocytes in the skin and coat.
  • Selection for increased tameness (= reduced freaking out) is really selection for reduced neural crest activity in early development, and the smaller teeth, shorter snout, floppy ears, curly tail, patchy coloration, and so on, are unselected developmental consequences of reduced neural crest activity.

Wilkins et al. (2014: fig. 2)

So far, so good. The neural crest hypothesis seems to have genuine explanatory power, in that it lassos a disparate set of phenomena and provides a single, logical cause. Of course not everyone is convinced, and the neural crest hypothesis could be true without ruling out other complementary mechanisms and confounding effects. Along those lines, Sanchez-Villagra et al. (2016) is worth a read. It’s free at the link below, as is Wilkins et al. (2014).

The neural crest hypothesis might explain why domestic pigs have shorter snouts than their wild relatives, but I think there must be some other factors in play to explain pig foreheads. Which is fine, domestic dogs have a staggering variety of skull shapes that reflect thousands of years of strong artificial selection, and probably a healthy dose of unintended consequences and other knock-on effects. Given that pigs have been domesticated for a long time, were probably domesticated many times in many places, have had frequent infusions of wild-type genes (from possibly genetically disparate wild populations), and have been canalized into different breeds, it might actually be weirder if they all looked like short-snouted wild boars. All of which is a long way of saying that I’m not surprised that domestic pigs don’t all fall on some morphogenetic monocline from wild boars, but I’m still curious about how they got their foreheads.

I actually started writing this post before the very interesting discussion of pig domestication flared up in the comments on Mike’s pig skull post. Mike’s two skulls nicely illustrate the difference between forehead-less and, er, forehead-ful conditions, and the comment thread touched on a lot of related issues and is worth a read. In particular, I’d like to note again that domestic pig skulls are not notably paedomorphic with respect to wild boars, other than having short snouts–they’re on a different morphogenetic trajectory (Evin et al. 2016).

For a nice comparison of domestic pig and wild boar skulls, see Marcus Bühler’s post at Bestiarium, here.

UPDATE just a few days later: for a skeptical look at the very existence of domestication syndrome, see the new Lord et al. (2019) paper, “The history of farm foxes undermines the animal domestication syndrome”, freely available here.

References

A life-size silhouette of the Snowmass Haplocanthosaurus, with Thierra Nalley, me, and Jessie Atterholt for scale. Photo by Jeremiah Scott.

Tiny Titan, a temporary exhibit about the Snowmass Haplocanthosaurus project, opened at the Western Science Center in Hemet, California, last night. How? Why? Read on.

Things have been quieter this year on the Haplo front than they were in 2018, for many reasons. My attention was pulled away by a lot of teaching and other day-job work–we’re launching a new curriculum at the med school, and that’s eaten an immense amount of time–and by some very exciting news from the field that I can’t tell you about quite yet (but watch this space). Things are still moving, and there will be a paper redescribing MWC 8028 and all the weird and cool things we’ve learned about it, but there are a few more timely things ahead of it in the queue.

One of the things going on behind the scenes this year is that Jessie Atterholt, Thierra Nalley, and I have been working with Alton Dooley, the director of the Western Science Center, on this exhibit. Alton has had a gleam in his eye for a long time of using the WSC’s temporary exhibit space to promote the work of local scientists, and we had the honor of being his first example. He also was not fazed by the fact that the project isn’t done–he wants to show the public the process of science in all of its serendipitous and non-linear glory, and not just the polished results that get communicated at the end.

Everything’s better cut in half. Photo by Jessie Atterholt.

Which is not to say that the exhibit isn’t polished. On the contrary, it looks phenomenal. Thanks to a loan from Julia McHugh at Dinosaur Journey in Colorado, most of the real fossils are on display. We’re only missing the ribs and most of the sacrum, which is too fragmentary and fragile to come out of its jacket. As you can see from the photo up top, there is a life-size vinyl silhouette of the Snowmass Haplo, with 3D prints of the vertebrae in approximate life position. Other 3D prints show the vertebrae before and after the process of sculpting, rescanning, and retrodeformation, as described in our presentation for the 1st Palaeontological Virtual Congress last year (that slideshow is a PeerJ Preprint, here). The exhibit also includes panels on “What is Haplocanthosaurus” and its relationships, on pneumaticity in sauropods, on the process of CT scanning and 3D modeling, and on the unusual anatomical features of the Snowmass specimen. The awesome display shown above, with the possibly-bumpy spinal cord and giant intervertebral discs reconstructed, was all Alton–he did the modeling, printing, and assembly himself.

Haplo vs Bronto. Thierra usually works on the evolution and development of the vertebral column in primates, so I had to show her how awesome vertebrae can be when they’re done right. Photo by Brittney Stoneburg.

My favorite thing in the exhibit is this striking comparison of one the Snowmass Haplo caudals with a proximal caudal from the big Oklahoma apatosaurine. This was Alton’s idea. He asked me if I had any photos of caudal vertebrae from really big sauropods that we could print at life size to compare to MWC 8028, and my mind went immediately to OMNH 1331, which is probably the second-largest-diameter vertebra of anything from all of North America (see the list here). It was also Alton’s idea to fill in the missing bits using one of Marsh’s plates of Brontosaurus excelsus from Como Bluff in Wyoming. It’s a pretty amazing display, and it turns out to have been a vehicle for discovery, too–I didn’t realize until I saw the verts side-by-side that the neural canal of the Snowmass Haplo caudal is almost as big as the neural canal from the giant apatosaurine caudal. It’s not a perfect comparison, because the OMNH fossil doesn’t preserve the neural canal, and the Como specimen isn’t that big, but proportionally, the Snowmass Haplo seems to have big honkin’ neural canals, not just at the midpoint (which we already knew), but all the way through. Looks like I have some measuring and comparing to do.

(Oh, about the title: we don’t know if the Snowmass Haplo was fully grown or not, but not all haplocanthosaurs were small. The mounted H. delfsi in Cleveland is huge, getting into Apatosaurus and Diplodocus territory. Everything we can assess in the Snowmass Haplo is fused, for what that’s worth. We have some rib chunks and Jessie will be doing histo on them to see if we can get ontogenetic information. I’ll keep you posted.)

Brian’s new Haplocanthosaurus restoration, along with some stinkin’ mammals. Photo by Jessie Atterholt.

Brian Engh contributed a fantastic life restoration of Haplocanthosaurus pro bono, thanks to this conversation, which took place in John Foster’s and ReBecca Hunt-Foster’s dining room about a month ago:

Brian: What are you drawing?

Me: Haplocanthosaurus.

Brian: Is that for the exhibit?

Me: Yup.

Brian (intense): Dude, I will draw you a Haplocanthosaurus.

Me: I know, but you’re a pro, and pros get paid, and we didn’t include a budget for paleoart.

Brian (fired up): I’m offended that you didn’t just ask me to draw you a Haplocanthosaurus.

Then he went to the Fosters’ couch, sat down with his sketchbook, and drew a Haplocanthosaurus. Not only is it a stunning piece on display in the exhibit, there are black-and-white printouts for kids to take and color (or for adults to take to their favorite tattoo artists, just sayin’). [Obligatory: this is not how things are supposed to work. We should all support original paleoart by supporting the artists who create it. But Brian just makes so damn many monsters that occasionally he has to kick one out for the heck of it. Also, I support him on Patreon, and you can, too, so at a stretch you could consider this the mother of all backer rewards.]

One special perk from the opening last night: Jessica Bramson was able to attend. Who’s that, you ask? Jessica’s son, Mike Gordon, found the first piece of bone from the Snowmass Haplo on their property in Colorado over a decade ago. Jessica and her family spent two years uncovering the fossils and trying to get paleontologists interested. In time she got in touch with John Foster, and the rest is history. Jessica lives in California now, and thanks to John’s efforts we were able to invite her to the exhibit opening to see her dinosaur meet the world. Stupidly, I did not get any photos with her, but I did thank her profusely.

A restored, retrodeformed caudal of the Snowmass Haplocanthosaurus, 3D-printed at life size for the exhibit. Photo swiped from the WSC Facebook page.

I owe a huge thanks to Alton Dooley for taking an interest in our work, and to the whole WSC crew for their hard work creating and promoting the exhibit. You all rock.

The exhibit will run through the end of March, 2020, at least. I deliberately did not show most of it, partly because I was too busy having fun last night to be diligent about taking photos, but mostly because I want you to go see it for yourself (I will do a retrospective post with more info after the exhibit comes down, for people who weren’t able to see it in person). If you make it out to Hemet, I hope you have half as much fun going through the exhibit as we did making it.

I’ll have more to say about both of these in the near future, but for now suffice it to say that this (link):

and this (link):

are available for your perusal. Not just the abstracts, but the slide decks as well, just as Mike did for his talk on Jensen’s Big Three sauropods (link).

Jessie is also posting her talk a few slides at a time on her Instagram, with some helpful unpacking, so that’s worth a look even if you have the slides already. That stream of posts starts here.

The image I put together explaining the new discovery. Modified from Staples et al. (2019: fig. 6).

Today sees the publication of a new paper, “Cutaneous branch of the obturator nerve extending to the medial ankle and foot: a report of two cadaveric cases,” by Brittany Staples, Edward Ennedy, Tae Kim, Steven Nguyen, Andrew Shore, Thomas Vu, Jonathan Labovitz, and yours truly. I’m excited for two reasons: first, the paper reports some genuinely new human gross anatomy, which happens surprisingly often but still isn’t an everyday occurrence, and second, the first six authors are my former students. This isn’t my discovery, it’s theirs. But I’m still going to yap about it.

When the obturator nerve won’t stay in its lane

Your skin is innervated by cutaneous nerves, which relay sensations of touch, pressure, vibration, temperature, and pain to your central nervous system, and carry autonomic (involuntary) fibers to your sweat and sebaceous glands and the arrector pili muscles that raise and lower your hairs (as when we get goosebumps). Every inch of your skin lies in the domain of one cutaneous nerve or another. Known boundaries between cutaneous branches of different nerves are approximate, both because they vary from person to person, and because the territories of the nerves themselves interdigitate and overlap at very fine scales. That said, aside from complex areas where the domains of multiple nerves intersect (like the groin), most body regions get their cutaneous innervation from just one nerve.

The obturator nerve arises from the spinal levels of the 2nd-4th lumbar vertebrae (L2-L4), exits the pelvis through the obturator canal behind the superior ramus of the pubis, and innervates the adductor muscles of the medial compartment of the thigh. The cutaneous branch of the obturator nerve typically innervates a variable but limited patch of skin on the inner thigh. Here’s a diagram from Gray’s Anatomy, 40th edition, showing the common cutaneous distribution of the obturator nerve (Standring et al. 2008 fig. 79.17, modified):

In rare cases, however, the obturator nerve doesn’t stay in the thigh. I was teaching in the gross anatomy lab in the fall of 2013 when one of our podiatry students, Brittany Staples, called me over to her table. We were skinning the thigh and leg that day, and in her assigned cadaver, Brittany had found a nerve from the medial thigh running all the way down to the inner side of the ankle and foot.

I didn’t immediately freak out, because everyone has a nerve from the thigh running down to the inner side of the ankle and foot: the saphenous branch of the femoral nerve, which comes out of the anterior thigh (also highlighted in the above image). But when we traced back Brittany’s nerve, it wasn’t coming from the femoral nerve. Instead, it was coming from the anterior division of the obturator nerve, right behind the adductor longus muscle (when people do the splits, this is the muscle that makes a visible ridge from the inner thigh to the groin). We carefully cleaned and photographed the nerve, and then we hit the books. Our first question: was this a known variation, or had Brittany discovered something new in the annals of human anatomy?

Standing on the shoulders of giants

Virtually all introductory anatomy textbooks show the obturator nerve only going to the thigh. But a little digging turned up Bouaziz et al. (2002), which in turn reproduced a figure from Rouvière and Delmas (1973), a French textbook, which showed the obturator nerve passing the knee and innervating part of the calf. That was at least an advance on what we knew starting out. We found a similar written description in Sunderland (1968).

Bouaziz et al. (2002: fig. 1)

Then we discovered Bardeen (1906), a magnificent and magisterial work 130 pages in length. Titled, “Development and variation of the nerves and the musculature of the inferior extremity and of the neighboring regions of the trunk in man”, the paper delivers on its impressive title. Bardeen (1906: 285 and 317) reported than in 22 out of 80 cadavers, the cutaneous branch of the obturator nerve (CBO) reached the knee; in 10 of those cases it could be traced at least to the middle third of the calf; and in one case it reached “nearly to ankle”. Bardeen also commented on the difficulty of tracing out the limits of this tiny nerve (p. 285):

“How constant the cutaneous branch of the obturator may be I have been unable satisfactorily to determine. Students dissecting frequently fail to find it. Owing to the fact that this may often be due to its small size the negative records cannot safely be used in making up statistics.”

All of us on the paper can back up Bardeen’s comments here: by the time they reach the skin, cutaneous nerves might be as big around as a pencil lead, or a strand of dental floss, or a human hair, but they won’t be much bigger. Sometimes they run just under the skin, sometimes down in the subcutaneous fat and fascia (with vanishingly small extensions spidering out to the underside of the skin), always variable in their courses and often devilishly hard to find, preserve, and trace.

If there is a prior report in the literature of a CBO passing the ankle, we haven’t found it, and neither have the numerous podiatric physicians who commented on the manuscript before we submitted, nor the reviewers and editors of the Journal of Foot & Ankle Surgery. I feel pretty safe saying that this is truly new (and if you know otherwise, please let me know in the comments!).

The second case, and the long silence

Every year since 2013, I’ve warned our medical and podiatric students to be on the lookout for anomalously long branches of the obturator nerve. The very next year, a group of summer anatomy students found a second example (they’re authors 2-6 on the paper). Since then, nada, in over 200 more bodies as of this summer. Either we got crazy lucky to find two examples in back to back years, or long CBOs are more common than we think, just really hard to find and identify. More on that in a minute.

A quick aside: we didn’t deliberately hold up the paper while we were looking for more examples, we’ve all just been busy. Brittany and the other student authors were occupied with passing med school and their board exams, surviving clinical rotations, and applying to residency programs. I’m happy to say “were occupied” with all those things because they’re all graduated now, and in residency training. Anyway, that’s why the paper had a 5-year gestation: med school doesn’t leave a lot of time for research and writing. Kudos to Brittany for giving all of us regular kicks to keep things moving along. In every sense, the paper would not exist without her skill and dedication.

So what’s going on here?

There are two sides to this: what happened to produce the variants we found in 2013 and 2014, and why variants like that escaped detection for so long, and I’ll tackle them in that order.

We found both of the long CBOs in the territory normally occupied by the saphenous branch of the femoral nerve. The saphenous nerve is so named because it runs along the great saphenous vein, the major superficial vein of the medial leg and thigh. Sometimes the saphenous nerve has only a single main trunk, but more commonly it splits into two parallel branches, one on either side of the saphenous vein, as illustrated here by Wilmot and Evans (2013: fig. 3):

In both of our cases, the saphenous branch of the femoral nerve was present, but it only had one branch, in front of the big vein, and the long CBO ran behind the vein, in the place usually occupied by the posterior branch of the saphenous nerve. In effect, the posterior part of the saphenous branch of the femoral nerve had been replaced by a sort of saphenous branch of the obturator nerve. This has interesting implications.

Suppose you were a surgeon, harvesting the distal portion of the saphenous vein for a coronary artery bypass graft, and you saw two nerves accompanying the vein, one in front and one behind. You would probably assume that both branches arose from the femoral nerve, because that is what happens most commonly. But if the posterior branch actually came from the obturator nerve, you’d have no way of knowing that, without tracing the nerve back to its origin in the inner thigh. The watchwords in surgery these days are “minimally invasive” and “patient outcomes” — smaller openings in the body mean less pain, fewer complications, faster recoveries, and happier patients. So surgeons aren’t going to flay patients open from ankle to groin just to chase down a nerve that might be coming from the normal place after all.

If you only get to look inside the box, these two things look the same.

We suspect that long CBOs may be fairly common, just hard to recognize, because who is going to find them? Medical students dissecting human cadavers have the opportunity to trace long cutaneous nerves back to their origins, but since it’s the students’ first time cutting, they usually haven’t yet developed the experience to recognize weird versions of tiny nerves, nor the skill to preserve them. Surgeons have the experience and the skill, but not the opportunity, because they can’t go around filleting their patients to see where all the nerves come from. So long CBOs probably fall into a perceptual blind spot, in which almost no-one who cuts on human bodies has both the opportunity to find them, and the skill to preserve them — my former students excepted (he said with no small helping of pride).

That’s pretty darned interesting, and it makes me wonder what other perceptual blind spots are out there, in both anatomy and paleontology. I know of at least one: the true nature and extent of the fluid-filled interstitial tissues that pervade our bodies (and those of all other vertebrates at least) were not fully appreciated until just last year, because the first step in the production of most histological slides is to dehydrate the tissues, which collapses the fluid-filled spaces and makes the interstitium look like regular connective tissue (Benias et al. 2018). That is a spooky kind of observer effect, and it makes me wonder what else we’re missing because of the ways we choose — or are constrained — to look.

What next?

What’s the fallout from this study? For me, two things. First — obviously — we’re going to keep looking for more examples of long CBOs, and for other similar cases in which one nerve may have been replaced by its neighbor. This is more than trivia. Knowing which nerves to expect and where to find them is important, not only for surgeons but also for anaesthetists and pain management physicians doing nerve blocks. The decks may be stacked against med students for some of these discoveries, but clearly “difficult” does not mean “impossible” or I’d have nothing to write about. Lightning has already struck twice, so I’ll keep flying this particular kite.

Second, this case, a few other odd things we’ve found in the lab over the years, and other recently-reported discoveries in human anatomy have caused me to wonder: could we formulate predictive maxims to help guide future discoveries in human anatomy, or in anatomy full stop? I think so, and provided my abstract is accepted, I’ll be presenting on that topic at SVPCA in a couple of months. More on that in due time.

Finally — and this cannot be overstated — without the keen eyes, skilled hands, sharp minds, and hard work of the student authors, there would be no discovery and no paper. So congratulations to Brittany, Edward, Tae, Steven, Andrew, and Thomas. Or as I’m happy to address them now, Drs. Staples, Ennedy, Kim, Nguyen, Shore, and Vu. Y’all done good. Keep it up.

References

  • Bardeen, C.R. 1906. Development and variation of the nerves and the musculature of the inferior extremity and of the neighboring regions of the trunk in man. Developmental Dynamics 6(1):259-390.
  • Benias, P.C., Wells, R.G., Sackey-Aboagye, B., Klavan, H., Reidy, J., Buonocore, D., Miranda, M., Kornacki, S., Wayne, M., Carr-Locke, D.L. and Theise, N.D. 2018. Structure and distribution of an unrecognized interstitium in human tissues. Scientific Reports, 8:4947.
  • Bouaziz, H., Vial, F., Jochum, D., Macalou, D., Heck, M., Meuret, P., Braun, M., and Laxenaire, M.C. 2002. An evaluation of the cutaneous distribution after obturator nerve block. Anesthesia & Analgesia 94(2):445-449.
  • Rouvière, H., and Delmas, A. 1973. Anatomie humaine, descriptive, topographique et fonctionnelle: tome 3—membres-système nerveux central, ed 11, Masson, Paris.
  • Standring, S. (ed.) 2008. Gray’s Anatomy: The Anatomical Basis of Clinical Practice, 41st ed, Elsevier Health Sciences, London.
  • Staples, B., Ennedy, E., Kim, T., Nguyen, S., Shore, A., Vu, T., Labovitz, J., and Wedel, M. 2019. Cutaneous branch of the obturator nerve extending to the medial ankle and foot: a report of two cadaveric cases. Journal of Foot & Ankle Surgery, advance online publication.
  • Sunderland, S. 1968. Nerves and Nerve Injuries. Churchill Livingstone, Edinburgh.
  • Wilmot, V.V., and Evans, D.J.R. 2013. Categorizing the distribution of the saphenous nerve in relation to the great saphenous vein. Clinical Anatomy 26(4):531-536.

Birds have little blobs of tissue sticking out on either side of the spinal cord in the lumbosacral region (solid black arrow in the image above). These are the accessory lobes of Lachi, and they are made up of mechanosensory neurons and glycogen-rich glial cells (but they are not part of the glycogen body, that’s a different thing that lies elsewhere — see this post).

These accessory lobes have been known since at least 1889, when they were first described by Lachi. But the function was mysterious until recently.

Starting in the late 1990s, German anatomist and physiologist Reinhold Necker investigated the development, morphology, and function of the lumbosacral canals of birds. These are not pneumatic spaces, they’re fluid-filled tubes that arch above (dorsal to) the spinal cord in the lumbosacral regions of birds. In a sacral neural canal endocast they look like sets of ears, or perhaps caterpillar legs (below image in the above slide).

Here’s the same slide with the top image labeled, by me.

In our own bodies, the meningeal sac that surrounds the spinal cord is topologically simple, basically a single long bag like a sock with the spinal cord running through the middle. In the lumbosacral regions of birds, the meningeal sac is more like a basket in cross-section, with dorsally-arching loops — the lumbosacral canals — forming the basket handles (lower image in the above slide). Evidently cerebrospinal fluid can slosh through these meningeal loops and push on the accessory lobes of Lachi, whose mechanosensory neurons pick up the displacement. This is essentially the same system that we (and all other vertebrates) have in the semicircular canals in our inner ears, which give us our sense of equilibrium.

Evidence that the lumbosacral canals function as organs of equilibrium comes not only from anatomy but also from the behavior of experimentally-modified birds. If the lumbosacral canals are surgically severed, creating the ‘lesion’ mentioned in the above figure, the affected birds have a much harder time controlling themselves. They can do okay if they are allowed to see, as shown on the left side of the above figure, but if they are blindfolded, they don’t know how to orient themselves and flop around clumsily. Meanwhile, blindfolded birds with their lumbosacral canals intact can balance just fine.

All of this is documented in a series of papers by Necker and colleagues — particularly useful are Necker (1999, 2002, 2005, 2006) and Necker et al. (2000). Necker (2006) seems to be the summation of all of this research. It’s very well-documented, well-reasoned, compelling stuff, and it’s been in the literature for over a decade.

So why is no-one talking about this? When I discovered Necker’s work last spring, I was stunned. This is HUGE. In general, the central nervous systems of vertebrates are pretty conserved, and animals don’t just go around evolving new basic sensory systems willy-nilly. Minimally I would expect congressional hearings about this, broadcast live on C-SPAN, but ideally there would be a talk show and a movie franchise.

I was equally blown away by the fact that I’d never heard about this from inside the world of science and sci-comm. Necker’s discovery seemed to have been almost entirely overlooked in the broader comparative anatomy community. I searched for weaknesses in the evidence or reasoning, and I also searched for people debunking the idea that birds have balance organs in their butts, and in both cases I came up empty-handed (if you know of counter-evidence, please let me know!). It’s relevant to paleontology, too: because the lumbosacral canals occupy transverse recesses in the roof of the sacral neural canal, they should be discoverable in fossil taxa. I’ve never heard of them being identified in a non-avian dinosaur, but then, I’ve never heard of anyone looking. You can also see the lumbosacral canals for yourself, or at least the spaces they occupy, for about three bucks, as I will show in an upcoming post.

Incidentally, I’m pretty sure this system underlies the axiomatic ability of birds to run around with their heads cut off. I grew up on a farm and raised and slaughtered chickens, so I’ve observed this firsthand. A decapitated chicken can get up on its hind legs and run around. It won’t go very far or in a straight line, hence the jokey expression, but it can actually run on flat ground. It hadn’t occurred to me until recently how weird that is. All vertebrates have central pattern generators in their spinal cords that can produce the basic locomotor movements of the trunk and limbs, but if you decapitate most vertebrates the body will just lie there and twitch. The limbs may even make rudimentary running motions, but the decapitated body can’t stand up and successfully walk or run. Central pattern generators aren’t enough, to run you need an organ of balance. A decapitated bird can successfully stand and run around because it still has a balance organ, in its lumbosacral spinal cord.

You may recognize some of the slides that illustrate this post from the Wedel et al. (2018) slide deck on the Snowmass Haplocanthosaurus for the 1st Palaeontological Virtual Congress. Those were stolen in turn from a much longer talk I’ve given on weird nervous system anatomy in dinosaurs, which I am using piecemeal as blog fuel. Stay tuned!

So, birds have balance organs in their butts. We should be talking about this. The comment thread is open.

References

  • Lachi, P. 1889. Alcune particolarita anatomiche del rigonfiamento sacrale nel midollo degli uccelli. Lobi accessori. Att Soc Tosc Sci Nat 10:268–295.
  • Necker, R. 1999. Specializations in the lumbosacral spinal cord of birds: morphological and behavioural evidence for a sense of equilibrium. European Journal of Morphology 37:211–214.
  • Necker, R. 2002. Mechanosensitivity of spinal accessory lobe neurons in the pigeon. Neuroscience Letters 320:53–56.
  • Necker, R. 2005. The structure and development of avian lumbosacral specializations of the vertebral canal and the spinal cord with special reference to a possible function as a sense organ of equilibrium. Anatomy and Embryology 210:59–74.
  • Necker, R. 2006. Specializations in the lumbosacral vertebral canal and spinal cord of birds: evidence of a function as a sense organ which is involved in the control of walking. Journal of Comparative Physiology A, 192(5):439-448.
  • Necker, R, Janßen A, Beissenhirtz, T. 2000. Behavioral evidence of the role of lumbosacral anatomical specializations in pigeons in maintaining balance during terrestrial locomotion. Journal of Comparative Physiology A 186:409–412.
  • Wedel, M.J., Atterholt, J., Macalino, J., Nalley, T., Wisser, G., and Yasmer, J. 2018. Reconstructing an unusual specimen of Haplocanthosaurus using a blend of physical and digital techniques. Abstract book, 1st Palaeontological Virtual Congress, http://palaeovc.uv.es/, p. 158 /  PeerJ Preprints 6:e27431v1