The world’s longest cells? Speculations on the nervous systems of sauropods

May 23, 2011

I have a new paper out:

Wedel, M.J. 2012. A monument of inefficiency: the presumed course of the recurrent laryngeal nerve in sauropod dinosaurs. Acta Palaeontologica Polonica 57(2):251-256.

Update June 6, 2012: the final version was formally published yesterday, so the rest of this paragraph is of historical interest only. Like Yates et al. on prosauropod pneumaticity, it is “out” in the sense that the manuscript has been through peer review, has been accepted for publication, and is freely available online at Acta Palaeontologica Polonica. Technically it is “in press” and not published yet, but all that formal publication will change is to make a prettier version of the paper available. All of the content is available now, and the paper doesn’t include any of those pesky nomenclatural acts, and so, as with the prosauropod pneumaticity paper, I don’t see any reason to pretend it doesn’t exist. Think of the accepted manuscript as the caterpillar to the published version’s butterfly: different look, but same genome.

This one came about because last summer I read a review of Richard Dawkins’s book, The Greatest Show on Earth: The Evidence for Evolution. The review mentioned that the book includes a lengthy discussion of the recurrent laryngeal nerve (RLN) in the giraffe, which is a spectacularly dumb piece of engineering and therefore great evidence against intelligent design creationism. It wasn’t the first time I’d heard of the RLN, of course. It’s one of the touchstones of both human anatomy and evolutionary biology; anatomy because of its clinical importance in thyroid surgery, known for more than two millennia, and evolutionary biology because it is such a great example of a developmental constraint. (Dawkins’s coverage of all of this is great, BTW, and you should read the book.)

No, the reason the book review inspired me to write the paper was not because the RLN was new to me, but because it was overly familiar. It is a cool piece of anatomy, and its fame is justly deserved–but I am sick and tired of seeing the stinkin’ giraffe trotted out as the ultimate example of dumb design. My beloved sauropods were way dumber, and it’s time they got some credit.

But first, let’s talk about that nerve, and how it got to be there.

No necks for sex? How about no necks for anybody!

Embryos are weird. When you were just a month old (counting from fertilization), you had a set of pharyngeal arches that didn’t look radically different from those of a primitive fish. These started out quite small, tucked up underneath your comparatively immense brain, and each pharyngeal arch was served by a loop of artery called an aortic arch. What we call the arch of the aorta in an adult human is a remnant of just one of these embryonic aortic arches, and as you’ve no doubt noticed, it’s down in your chest, not tucked up next to your brain. When you were in the embryonic stages I’m talking about, you didn’t yet have a neck, so your brain, pharyngeal arches, aortic arches, and the upper parts of your digestive system were all smooshed together at your front end.

One thing you did have at that stage was a reasonably complete peripheral nervous system. The nerve cell bodies in and near your central nervous system sent out axons into the rest of your body, including your extremities. Many of these axons did not persist; they failed to find innervation targets and their parent neurons died. Imagine your embryonic central nervous system sending out a starburst of axons in all directions, and some of those axons finding targets and persisting, and others failing and dying back. So the architecture of your nervous system is the result of a process of selection in which only some cells were successful.

Crucially, this radiation and die-off of axons happened very early in development, when a lot of what would become your guts was still hanging under your proportionally immense brain like the gondola on a blimp. This brings us to the recurrent laryngeal nerve.

Going back the way we came

The fates of your embryonic pharyngeal arches are complex and I’m not going to do a comprehensive review here (go here for more information). Suffice it to say that the first three arches give rise to your jaws and hyoid apparatus, the fourth and sixth form your larynx (voicebox), and fifth is entirely resorbed during development. Update: I made a pharyngeal arch cheat sheet.

There are two major nerves to the larynx, each of which is bilaterally paired. The nerve of the fourth pharyngeal arch becomes the superior laryngeal nerve, and it passes cranial to the fourth aortic arch. The nerve of the sixth pharyngeal arch becomes the inferior or recurrent laryngeal nerve, and it passes caudal to the sixth aortic arch. At the time that they form, both of these nerves take essentially straight courses from the brainstem to their targets, because you’re still in the blimp-gondola stage.

If you were a shark, the story would be over. The more posterior pharyngeal arches would persist as arches instead of forming a larynx, each arch would hold on to its artery, and the nerves would all maintain their direct courses to their targets.

The normal fate of the aortic arches in humans. From http://education.yahoo.com/reference/gray/subjects/subject/135

But you’re not a shark, you’re a tetrapod. Which means that you have, among other things, a neck separating your head and your body. And the formation of your neck shoved your heart and its associated great vessels down into your chest, away from the pharyngeal arches. This was no problem for the superior laryngeal nerve, which passed in front of the fourth aortic arch and could therefore stay put. But the inferior laryngeal nerve passed behind the sixth aortic arch, so when the heart and the fourth and sixth aortic arches descended into the chest, the inferior laryngeal nerve went with them. Because it was still hooked up to the brainstem and the larynx, it had to grow in length to compensate.

As you sit reading this, your inferior laryngeal nerves run down your neck into your chest, loop around the vessels derived from the fourth and sixth aortic arches (the subclavian artery on the right, and the arch of the aorta and ductus arteriosus on the left) and run back up your neck to your larynx. Because they do this U-turn in your chest and go back the way they came, the inferior laryngeal nerves are said to ‘recur’ to the larynx and are therefore more commonly referred to as the recurrent laryngeal nerves (RLNs).

An enlightening diversion

The RLN is the poster child for “unintelligent design” because it is pretty dumb. Your RLNs travel a heck of a lot farther to reach your larynx than they ought to, if they’d been designed. Surely an intelligent designer would have them take the same direct course as the superior laryngeal nerve. But evolution didn’t have that option. Tetrapod embryos could not be built from the ground up but had to be modified from the existing “sharkitecture” of ancestral vertebrates. The rules of development could not be rewritten to accommodate a shorter RLN. Hence Dawkins’s love affair with the RLN, which gets 7 pages in The Greatest Show on Earth. He also appeared on the giraffe episode of Inside Nature’s Giants, in which the RLN was dug out of the neck and the continuity of its ridiculous path was demonstrated–probably the most smack-you-in-the-face evidence for evolution that has ever been shown on television (said the rabid fan of large-tetrapod dissections).

Incidentally, the existence and importance of the RLN has been known since classical times. The RLN innervates the muscles responsible for speech, and on either side it passes right behind the thyroid gland, which is subject to goiters and tumors and other grotesque maladies. So a careless thyroidectomy can damage one or both of the RLNs; if one gets snipped, the subject will be hoarse for the rest of his or her life; if both are cut, the subject will be rendered mute. The Roman physician Galen memorably demonstrated this by dissecting the neck of an immobilized but unanesthetized pig and isolating the RLNs (Kaplan et al. 2009). One moment the poor pig was squealing its head off–as any of us would be if someone dug out our RLNs without anesthesia–and the next moment Galen severed the RLNs and the animal abruptly fell silent, still in unbelievable pain but now without a mechanism to vocally express its discomfort.

Galen versus pig. Figure 2 from Kaplan et al. 2009.

The name of the nerve also goes back to Galen, who wrote:

I call these two nerves the recurrent nerves (or reversivi) and those that come upward and backward on account of a special characteristic of theirs which is not shared by any of the other nerves that descend from the brain.

Like at least some modern surgeons, Galen does not seem to have been overly burdened by humility:

All these wonderful things, which have now become common property, I was the first of all to discover, no anatomist before me ever saw one of these nerves, and so all of them before me missed the mark in their anatomical description of the larynx.

Both of those quotes are from Kaplan et al. (2009), which is a fascinating paper that traces the knowledge of the recurrent laryngeal nerve from classical times to the early 20th century. If you’d like a copy and can’t get hold of one any other way, let me know and I’ll hook you up.

Share and share alike

By now you can see where this is going: all tetrapods have larynges, all tetrapods have necks, and all tetrapods have recurrent laryngeal nerves. Including giraffes, much to the delight of Richard Dawkins. And also including sauropods, much to the delight of yours truly.

Now, I cannot show you the RLN in a living sauropod, nor can I imagine a scenario in which such a delicate structure would be recognizably preserved as a fossil. But as tetrapods, sauropods were bound to the same unbreakable rules of development as everything else. The inference that sauropods had really long, really dumb RLNs is as secure as the inference that they had brainstems, hearts, and larynges.

Wedel (2012) Fig. 1. Course of the left vagus nerve and left recurrent laryngeal nerve in a human, a giraffe, and Supersaurus. The right recurrent laryngeal nerve passes caudal to the right subclavian artery rather than the aorta and ductus arteriosus, but otherwise its course is identical to that of the left.

Giraffes have necks up to 2.4 meters long (Toon and Toon 2003), so the neurons that make up their RLNs approach 5 meters in the largest indiividuals. But the longest-necked sauropods had necks 14 meters long, or maybe even longer, so they must have had individual neurons at least 28 meters long. The larynx of even the largest sauropod was probably less than 1 meter away from the brainstem, so the “extra” length imposed on the RLN by its recurrent course was something like 27 meters in a large individual of Supersaurus. Take that, Giraffa.

Inadequate giraffe is inadequate.

One way or another

It is possible to have a nonrecurrent laryngeal nerve–on one side, anyway. If you haven’t had the opportunity to dissect many cadavers, it may come as a surprise to learn that muscles, nerves, and blood vessels are fairly variable. Every fall in Gross Anatomy at WesternU, we have about 40 cadavers, and out of those 40 people we usually have two or three with variant muscles, a handful with unusual branching patterns of nerves, and usually half a dozen or so with some wackiness in their major blood vessels. Variations of this sort are common enough that the better anatomy atlases illustrate not just one layout for, say, the branching of the femoral artery, but 6-10 of the most common patterns. Also, these variations are almost always asymptomatic, meaning that they never cause any problems and the people who have them usually never know (ask Mike about his lonely kidney sometime). You–yes, you, gentle reader!–could be a serious weirdo and have no idea.

Variations in the blood vessels seem to be particularly common, possibly because the vessels develop in situ with apparently very little in the way of genetic control. Most parts of the body are served by more than one artery and vein, so if the usual vessel isn’t there or takes an unusual course, it’s often no big deal, as long as the blood gets there somehow. To wit: occasionally a person does not have a right subclavian artery. This does not mean that their right shoulder and arm receive no blood and wither away; usually it means that one of the segmental arteries branching off the descending aorta–which normally serve the ribs and their associated muscles and other soft tissues–is expanded and elongated to compensate, and looks for all the world like a normal subclavian artery with an abnormal connection to the aorta. But if the major artery that serves the forelimb comes from the descending aorta, and the 4th aortic arch on the right is completely resorbed during development, then there is nothing left on the right side to drag the inferior laryngeal nerve down into the torso. A person with this setup will have an inferior laryngeal nerve on the right that looks intelligently designed, and the usual dumb RLN on the left.

Can people have a nonrecurrent laryngeal nerve on the left? Sure, if they’ve got situs inversus, in which the normal bilateral asymmetry of the internal organs is swapped left to right. Situs inversus is pretty darned rare in the general population, occurring in fewer than 1 in 10,000 people. It is much more prevalent in television shows and movies, where the hero or villain may survive a seemingly mortal wound and then explain that he was born with his heart on the right side. (Pro tip: the heart crosses the midline in folks of both persuasions, so just shoot through the sternum and you’ll be fine. Or, if you’re worried about penetration, remember Rule #2 and put one on either side.) Anyway, take everything I wrote in the preceding paragraph, mirror-image it left to right, and you’ve got a nonrecurrent laryngeal nerve on the left. But just like the normally-sided person who still has an RLN on the left, a person with situs inversus and no remnant 4th aortic arch on the left (double variation alert!) still has an RLN looping around the aorta and ductus arteriosus on the right.

Bottom line: replumb the vessels to your arms, swap your organs around willy-nilly, you just can’t beat the aorta. If you have an aorta, you’ve got at least one RLN; if you don’t have an aorta, you’re dead, and no longer relevant to this discussion.

Nonrecurrent laryngeal nerves–a developmental Hail Mary?

But wait–how do we know that the inferior laryngeal nerve in embryonic sauropods didn’t get rerouted to travel in front of the fourth and sixth aortic arches, so it could be spared the indignity of being dragged into the chest later on?

First of all, such a course would require that the inferior laryngeal nerve take an equally dumb recurrent course in the embryo. Or maybe it should be called a procurrent course. Instead of simply radiating out from the central nervous system to its targets in the sixth pharyngeal arch, the axons that make up the RLN would have to run well forward of their normal course, loop around the fourth and sixth aortic arches from the front, and then run back down to the sixth pharyngeal arch. There is simply no known developmental mechanism that could make this happen.

Even if we postulated some hypothetical incentive that would draw those axons into the forward U-turn, other axons that took a more direct course from the central nervous system would get to the sixth pharyngeal arch first. By the time the forwardly-recurring axons finished their intelligently-routed course and finally arrived at the sixth pharyngeal arch, all of the innervation targets would be taken, and those axons would die off.

Also, at what point in the evolution of long necks would this forwardly-looping course supposedly be called into existence? Ostriches and giraffes have RLNs that take the same recurrent course as those of humans, pigs, and all other tetrapods. The retention of the recurrent course in extant long-necked animals is further evidence that the developmental constraint cannot be broken.

Finally, the idea that a non-recurrent laryngeal nerve would need to evolve in a long-necked animal is based on the perception that long nerve pathways are somehow physiologically taxing or otherwise bad for the animals in which they occur. But almost every tetrapod that has ever lived has had much longer neurons than those in the RLN, and we all get on just fine with them.

In dire extremity

Probably you have seen enough pictures of neurons to know what one looks like: round or star-shaped cell body with lots of short branches (dendrites) and one very long one (the axon), like some cross between an uprooted tree–or better yet, a crinoid–and the Crystalline Entity. When I was growing up, I always imagined these things lined up nose to tail (or, rather, axon to dendrite) all down my spinal cord, arms, and legs, like boxcars in a train. But it ain’t the case. Textbook cartoons of neurons are massively simplified, with stumpy little axons and only a few to a few dozen terminals. In reality, each neuron in your brain is wired up to 7000 other neurons, on average, and you have about a hundred billion neurons in your brain. (Ironically, 100 billion neurons is too many for your 100 billion neurons to visualize, so as a literal proposition, the ancient admonition to “know thyself” is a non-starter.)

Back to the axons. Forget the stumpy little twigs you’ve seen in books and online. Except for the ganglia of your autonomic nervous system (a semi-autonomous neural network that runs your guts), all of the cell bodies of your neurons are located in your central nervous system or in the dorsal root ganglia immediately adjacent to your spinal cord. The nerves that branch out into your arms and legs, across your face and scalp, and into your larynx are not made of daisy chains of neurons. Rather, they are bundles of axons, very long axons that connect muscles, glands, and all kinds of sensory receptors back to the nerve cell bodies in and around your brain and spinal cord.

Indulge me for a second and wiggle your toes. The cell bodies of the motor neurons that caused the toe-wiggling muscles to fire are located in your spinal cord, at the top of your lower back. Those motor neurons got orders transmitted down your spinal cord from your brain, and the signals were carried to the muscles of your feet on axons that are more than half as long as you are tall.

Some of your sensory neurons are even longer. Lift your big toe and then set it down gently, just hard enough to be sure that it’s touching down on the floor or the sole of your shoe, but not hard enough to exert any pressure. When you first felt the pad of your toe touch down, that sensation was carried to your brain by a single neuron (or, rather, by several neurons in parallel) with receptor terminals in the skin of your toe, axon terminals in your brainstem, and a nerve cell body somewhere in the middle (adjacent to your sacrum and just a bit to one side of your butt crack, if you want the gory details). That’s right: you have individual sensory neurons that span the distance from your brainstem to your most distal extremity. And so does every other vertebrate, from hagfish to herons to hippos. Including, presumably, sauropods.

I had you set your toe down gently instead of pushing down hard because the neurons responsible for sensing pressure do not travel all the way from toe-tip to brainstem; they synapse with other neurons in the spinal cord and those signals have been through a two-neuron relay by the time they reach your brainstem. Ditto for sensing temperature. But the neurons responsible for sensing vibration and fine touch go all the way.

If you want to experience everything I’ve discussed in this post in a single action, put your fingertips on your voicebox and hum. You are controlling the hum with signals sent from your brain to your larynx through your recurrent laryngeal nerves, and sensing the vibration through individual neurons that run from your fingertips to your brainstem. Not bad, eh?

Wedel (2012) Fig. 2. The longest cells in the bodies of sauropods were sensory neurons that connected receptors in the skin of the extremities with interneurons in the brainstem, a pattern of neural architecture that is present in all extant vertebrates. The nerve cell bodies would have been located in the dorsal root ganglia adjacent to the spinal cord. The diagram of the neuron is based on Butler and Hodos (1996: fig. 2–1B).

Getting back to big animals: the largest giraffes may have 5-meter neurons in their RLNs, but some of the sensory neurons to their hindfeet must be more like 8 meters long. I don’t think anyone’s ever dissected one out, but blue whales must have sensory neurons to the tips of their flukes that are almost 30 meters (98 feet) long (subtract the length of the skull, but add the lateral distance from body midline to fluke-tip). And Supersaurus, Amphicoelias, and the like must have had neurons that were approximately as long as they were, minus only the distance from the snout-tip to the back of the skull. I could be wrong, and if I am I’d love to be set straight, but I think these must have been the longest cells in the history of life.

Oh, one more thing: up above I said that almost every tetrapod that has ever lived has had much longer neurons than those in the RLN. The exceptions would be animals for which the distance from brainstem to base of neck was longer than the distance from base of neck to tip of limb or tail, so that twice the length of the neck would be longer than the distance from base of skull to most distal extremity. In that case, the neurons that contribute to the RLN would be longer than those running from brainstem to tail-tip or toe-tip. Tanystropheus and some of the elasmosaurs probably qualified; who else?

Parting Thoughts

In this post I’ve tried to explain the courses that these amazingly long cells take in humans and other vertebrates. I haven’t dealt at all with the functional implications of long nerves, for which please see the paper. The upshot is that big extant animals get along just fine with their crazy-long nerves, and there’s no reason to assume that sauropods were any more troubled. So why write the paper, then? Well, it was fun, I learned a lot (dude: axoplasmic streaming!), and most importantly I got to steal a little thunder from those silly poseurs, the giraffes.

Department of Frivolous Nonsense: yes, I titled the paper after those TV ads for Chili’s hamburgers from a few years back. If you never saw them, the ads compared mass-produced, machine-stamped fast-food burgers with restaurant burgers painstakingly built by hand, and concluded with, “Chili’s Big-Mouth Burgers: monuments of inefficiency!”

Update: All of the following is out of date now that the paper has been formally published. Department of Good Karma: since the paper is at the “accepted manuscript” stage, I still have the chance to make (hopefully minor) changes when I get the proofs. As is always, always, always the case, I caught a few dumb errors only after the manuscript had been accepted. Here’s what I’ve got so far, please feel free to add to the list:

  • Page 1, abstract, line 3: pharyngeal, not pharyngial
  • Page 1, abstract, line 8: substitute ‘made up’ for ‘comprised’
  • Page 6, line 12: substitute ‘make up’ for ‘comprise’
  • Page 9, line 5: citation should be of Carpenter (2006:fig. 3), not fig. 2
  • Page 10, line 7: “giant axons of squid are”, not ‘ares’
  • Page 12, entry for Butler and Hodos should have year (1996)
  • Page 12, entry for Carpenter has ‘re-evaluation misspelled
  • Page 16, entry for Woodburne has ‘mammalian’ misspelled

(Notes to self: stop trying to use ‘comprise’, lay off the ‘s’ key when typing bibliography.)

References

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57 Responses to “The world’s longest cells? Speculations on the nervous systems of sauropods”

  1. Mike Taylor Says:

    LOL @ “sharkitecture”!


  2. I think this post is longer than the paper in question!

    Forgive me if the following sounds silly. My sources, including Grey’s, implies the laryngeal nerves are branches of the vagus, which do not split at the braincase, but branch close to the aortic region, from whence they course proximal to the thoracic cavity.

    Forgive me a second statement. The nerve is hardly an example of “unintelligent” design. Bergman (not cited) responds by noting (as does Grey’s) that the laryngeal nerve (the RLN, rather) innervates the heart along with the cardiac branch of the vagus, which splits close to the laryngeal from the primary vagus. Your figures imply this, but the text implies that the nerve arises separately from the braincase.

    Asking for clarification. (Note: I do not support the ID argument–nonhypothesis, so this should not pepper the response as I think the subject irrelevent to a discussion of the biology of an animal, although I question why a discussion was included in the paper at all as it doesn’t seem to add much of value.)

  3. David Hone Says:

    My obvious question Matt would be how long do these things take to fire? I can’t remember if those ones are myelinated or not, but either way it should be possible to calculate the speed. If you touched a sauropod tail, how long would it take to realise, and how long would it take the signal to reach the larynx to bellow its disapproval?

  4. Mike Taylor Says:

    Jaime, you’re right about the relative lengths of the paper and the blog post. The text of the former (not counting the bibliography) is 3788 words. The text of the latter (also omitting the bibliography) is 4078 words, which is about 7.7% longer.

    Dave, for information on nerve conduction times, read the fine paper! :-)

  5. Matt Wedel Says:

    Forgive me if the following sounds silly. My sources, including Grey’s, implies the laryngeal nerves are branches of the vagus, which do not split at the braincase, but branch close to the aortic region, from whence they course proximal to the thoracic cavity.

    Not silly at all. But we have to make a distinction between what’s going on with the axons and what’s going on with connective tissue. The RLN is regarded as a branch of the vagus because it splits off from the vagus close to the aorta, as your sources say. But the RLN only appears to branch off there because that’s where the connective tissue that binds bundles of axons into nerves makes a fork. The axons themselves are separate, all the way back to the brainstem. If you wanted, you could conceivably slit the connective tissue holding the vagus nerve together above the branch point and pick out each of the axons that belong to the RLN. So the RLN as a connective-tissue-bound nerve that is visibly separate from the vagus does not (usually) branch off until the vagus is down in the superior mediastinum (the area above the heart with the great vessels). But the RLN as a separate nervous pathway from brainstem to larynx is distinct all the way along, it’s just that for part of the journey its axons are bundled with the rest of the vagus.

    Bergman (not cited) responds by noting (as does Grey’s) that the laryngeal nerve (the RLN, rather) innervates the heart along with the cardiac branch of the vagus, which splits close to the laryngeal from the primary vagus.

    Bergman is mistaken; now the distinction between axonal wiring and whatever is visibly bound together by connective tissue cuts the other way. The vagus does innervate the heart, the aorta, and most of the guts down to the middle of the large intestine (hence the name, Latin for “wandering”). The RLN loops under the aorta, and some of the vagal fibers are going to the aorta, and the latter might get bundled with the former until they branch off on the surface of the aorta. But that does not mean that the vagal axons serving the aorta are part of the RLN; their embryonic formation was completely different and they only got bundled with the RLN as a marriage of convenience. The RLN is defined as the nerve to the sixth branchial arch. Other axons may be seen to hitchhike along with it for part of the way, but that’s an illusion created by connective tissue.

    This idea of hitchhiking axons is not original with me, BTW. Many of the cranial nerves are promiscuous when it comes to letting their axons travel together. For a brief mention and a nice illustration showing how this works, go here. In the top illustration on that page, note that the chorda tympani is derived from the nervus intermedius, but it hitchhikes with both the facial nerve and the lingual nerve before it reaches its innervation targets in the tongue. Many textbooks list the chorda tympani as a branch of the facial nerve, either because they are focusing on the gross level rather than the neural wiring, or (more likely) because they only discuss the 12 classical cranial nerves and don’t mention the nervus intermedius, nervus terminalis, and other inconvenient complications. But at the level of neural wiring, the chorda tympani is not part of the facial nerve; it has its origin in a different part of the central nervous system and only hitchhikes with the facial nerve. Same story with the vagal fibers that travel with the RLN to the aorta.

    Thanks for the questions, both are good and they’ve given me a chance to hopefully clear the fog a bit.

  6. Matt Wedel Says:

    Dave, the answer is that some of the axons we’re talking about are myelinated and some are not. The unmyelinated axons might have been very slow indeed–rather than say just how slow, I’ll point you to the paper. ;-)

    But the myelinated ones would have had max conduction times of less than a second. I suppose it’s possible that a theropod that scored a tail-tip bite on a fully-grown Supersaurus with complete surprise might have had a whole extra second to work with before it got “Brontomerized”. Whether that extra second would have done it any good is another question.


  7. Matt, thank you for your responses.

    I brought Bergman up for only two reasons: It’s the only published source I can find that specifically addresses the ID crowd’s response to the “RLN as bad design” argument, and as such I wonder if, when there is a series of citations being used to discuss the role of the RLN in the creationism/evolution debate as in your MS, whether there is value in including rebuttals that you could then reasonably deconstruct. I understand Dawkins and Coyne both do this, but I’d wonder if an “in paper” treatment would serve the discussion better.

    The other thing is that Bergman is correct on a particular aspect: filaments arise from the laryngeal branch of the vagus and join the caridac complex from various other nerves, so it is true both that the laryngeal and a descending vagus both innervate the heart; but the laryngeal also innervates the esophagus on its ascent, as the vagus does on the descent, and these indicate that there would be an functional concern to the laryngeal’s current path, rather than just being a developmental remnant of piscine evolution.

    I take your point about the laryngeal being part of the bundle of the original vagus, but this is generally not how nerves are said to arise. Otherwise, we’d (correctly, in this case) say that all nerves arise from the brainstem, yet we’d also (correctly) say that all nerves arise from where they branch. The laryngeal is traditionally and effectively bundled with the vagus (or pneumograstric, in Grey’s), as with other sections of the nerves; even though the vagus is both sensory and motive, they are nonetheless a “single” cranial nerve, and passing over this essential constraint seems confusing.

    Again, thank you for your consideration.

  8. Mike Taylor Says:

    By the way, when this paper was in development, I asked Matt whether it was really true that sauropod cells were the longest in the history of life, and whether there might not be longer cells in tall trees. Matt’s illuminating response:

    “Big trees have no comparable cells. Plants of any size are just stacks of small cells, and they grow by adding cells rather than elongating what they’ve got. So do animals, for the most part, but the neurons make their connections when we’re tiny embryos and whatever connections get made, must be maintained, which explains the ridiculous telescoping in big animals. So while the skin, muscles, blood vessels, etc., get longer by adding more cells, the neurons are constrained to stretch out. Weird, but true.”

    Very cool.

  9. Mike Taylor Says:

    Jaime, what is the reference for this Bergman publication?

    In fact, is there an actual publication? Or are you just referring to the web-page, Recurrent Laryngeal Nerve Is Not Evidence of Poor Design? If so, I am not convinced that it warrants attention in the actual literature.


  10. Mike, the website actually has the citation.

    Bergman, J. 2010. Recurrent laryngeal nerve is not evidence of poor design. Acts & Facts 39(8):12-14.

    Acts & Facts is apparently a publication of the Institute for Creation Research, and I am not aware of whether it has a print version. The PDF is unfortunately embedded in an whole-issue file, http://www.icr.org/i/pdf/af/af1008.pdf .

  11. Matt Wedel Says:

    The other thing is that Bergman is correct on a particular aspect: filaments arise from the laryngeal branch of the vagus and join the caridac complex from various other nerves, so it is true both that the laryngeal and a descending vagus both innervate the heart; but the laryngeal also innervates the esophagus on its ascent, as the vagus does on the descent, and these indicate that there would be an functional concern to the laryngeal’s current path, rather than just being a developmental remnant of piscine evolution.

    Aaargh, no, this is more baloney from Bergman (now I’ve seen the page in question and the PDF, thanks to Mike and Jaime). Well, not full-on invention, just carefully sown confusion. The RLN does not innervate the entire esophagus. The upper part of the esophagus is derived from the 4th through 6th branchial arches (reference). Guess which part the RLN innervates? That’s right, the part of the esophagus derived from the 6th branchial arch. Ditto for the trachea. And the fact the RLN takes a recurrent course to those parts of the trachea and esophagus is just as dumb as the recurrent course to the larynx, and just as indicative of our basal vertebrate ancestry.

    To put it another way: the nerve of the 6th pharyngeal arch (RLN) passes caudal to the 4th and 6th aortic arches (right subclavian artery, arch of the aorta, and ductus arteriosus) on its path from the brainstem to the tissues of the 6th pharyngeal arch (larynx and associated muscles, part of the trachea, part of the esophagus). In the embryos this course is direct; in all adult tetrapods it is recurrent. The recurrent course, whether to the larynx or the trachea or the esophagus, is an inefficiency imposed by a developmental constraint, namely the descent of the posterior aortic arches into the chest during development.

    Bergman’s page on the RLN is a nifty but misleading piece of creationist propaganda, not a reliable source. I’m saddened (and, I must admit, somewhat amused) that his misrepresentations of the RLN are so transparent–the reference linked above is literally the very first hit on a Google search for ‘esophagus innervation’–but happy that I’ve had the chance to smack them down. I might have to put all of this into a separate post where it will be more easily accessible, especially for people looking for a refutation of Bergman’s distortions.

    I take your point about the laryngeal being part of the bundle of the original vagus, but this is generally not how nerves are said to arise.

    That’s fine, but I’m not speaking generally, nor am I describing the gross appearance of the nerve (which is just connective tissue packaging). There is a lot of informal description of the gross appearance of nerves that makes no sense on the level of axonal wiring. For example, nerves branch, but axons don’t; anyone who is concerned with the paths of axons will perceive the branching of a nerve as the division of two or more bundles of axons by a fork in the connective tissue sheath.

    The importance of the RLN for the purposes of the paper is not the gross appearance at all, but specifically the path taken by the individual axons from brainstem to larynx. And so that is the level on which I was forced to discuss things. If some people find the discussion in the paper confusing, I can only hope that this post and the comments have provided some clarification–that’s was my motivation for writing it up here.


  12. Matt, I am not pronouncing Bergman correct; I am using Bergman as a source of the ID-ish (the publication is a full-blown Creationism venue) crowd that produced a document rebutting (however unsuccessfully) the laryngeal nerve as bad design.

    As I said, however, “but the laryngeal also innervates the esophagus on its ascent, as the vagus does on the descent,” and thus am not incorrect when you write:

    Well, not full-on invention, just carefully sown confusion. The RLN does not innervate the entire esophagus. The upper part of the esophagus is derived from the 4th through 6th branchial arches (reference). Guess which part the RLN innervates? That’s right, the part of the esophagus derived from the 6th branchial arch. Ditto for the trachea.

    Bergman may simply not understand, or be deliberately misleading, I know not, but I also never cited him in my statements regarding innervation. For that I used Grey’s.

    “The recurrent laryngeal, as it winds around the subclavian artery and aorta, gives off several cardiac filaments, which unite with the cardiac branches from the pneumogastric and sympathetic. As it ascends in the neck it gives off oesophageal branches, more numerous on the left than on the right side, which supply the mucous membrane and muscular coat of the oesophagus; tracheal branches to the mucous membrane and muscular fibres of the trachea: and some pharyngeal filaments to the Inferior constrictor of the pharynx.” [Grey's Anatomy, Running Press edition (Philadelphia & London), 1974, pg.752]

    [I don't have the dominant literature on anatomical studies of this nature at hand, so I make do with old technical works like Grey's, so please forgive me.]

    My primary reason for bringing up Bergman in this was not to use him as a source for the multiple innervation element, however confusing it would be, but because there was an essential rebuttal argument from “that side,” rather than the one-sided series of citations used. It should be noted that, in your citations in that section, six papers are cited, and each of them only once in the entire paper, which seems rather askew in the normal practice of fairly balancing such things. Adding in bergman as an off-hand “so-and-so said this,” could have allowed you to split citations up from primary arguments for, rebuttals, and Dawkins and Coyne firmly laying the law in final. That’s if I would have left the section in at all.

    Finally, Matt, I think the paper is fine, as Mike said. You cover a lot of ground and extrapolate it against sauropods. My only worry is that it goes places or deals with situations that don’t seem necessary, or are one-sided when covered, and that seems odd. I wonder if adding in a hypothesis to the argument, such as placement of the larynx in the saurpod neck, could have permitted a more absolute measure of laryngeal nerve length estimation, and thus render this a true paper doing science, rather than something that reads like a “Ain’t it Cool?!” article from SciAm. This is, then, more a curiosity regarding the venue, not the content.

  13. Mark Robinson Says:

    Great paper (and blog post) Matt, definitely learnt some cool stuff there. And +1 “sharkitecture”!

    With regard to the response time for a Supersaurus suffering a tail-tip bite, would there not likely be some sort of reflexive reaction generated from the base of the spine (nerve cell nuclei?), akin to when someone stabs your hand with a pin and you are unable to stop from spilling the hot beverage it was holding? I guess that this would pretty much at least halve the time required for a directed response from the brain.

  14. Matt Wedel Says:

    Bergman may simply not understand, or be deliberately misleading, I know not, but I also never cited him in my statements regarding innervation. For that I used Grey’s.

    Right. Sorry if I mischaracterized what you were saying. I’m not denying that RLN provides innervation to part of the esophagus and part of the trachea–I mention that in the paper. Just pointing out that the innervation of those other 6th-branchial-arch derivatives by the RLN is just as goofy as the path to the larynx, and equally strong evidence of a developmental constraint. And, contra Bergman, equally strong evidence of evolution.

  15. Matt Wedel Says:

    With regard to the response time for a Supersaurus suffering a tail-tip bite, would there not likely be some sort of reflexive reaction generated from the base of the spine (nerve cell nuclei?), akin to when someone stabs your hand with a pin and you are unable to stop from spilling the hot beverage it was holding? I guess that this would pretty much at least halve the time required for a directed response from the brain.

    Yes, absolutely, sorry I didn’t make that clear in my earlier comment. One thing I have had in the back of my mind is that people may be interested in the implications of these long nerve pathways for behavior, particularly agonistic encounters with predators. The actual time for that reflex arc to kick in would almost certainly be well under a second. I just wanted to point out that, even being as pessimistic as possible and allowing for millisecond delays in the receptors, interneuron decision time (as sufficient impulses come in to trigger the all-or-none response), and muscular contraction, the time between injury and response would not be more than a second. Just to set things straight in case anyone is visualizing allosaurs happily devouring sauropod tails while the heads remain blissfully unaware. :-)

  16. David Hone Says:

    Sorry, I’m a bit late getting back. Fair enough Matt, I’ll go read it. That was an instinctive question and I must confess, I didn’t think to check.

    Cheers!

  17. Nathan Myers Says:

    You shouldn’t let the grammar nannies dominate you. (No, don’t imagine that image.) Your original use of “comprise” goes back to the word’s beginnings. It has always been ambiguous. The (typically barren) grammar nannies have long tried to conceive a new meaning that excludes some of the old ones, but in natural languages, meanings evolve additively. As in all languages, the English lexicon is full of ambiguities. All languages evolve ways to disambiguate cases when needed. Here, it’s not needed, because it’s perfectly clear what is the assembly and what are the parts.

  18. Matt Wedel Says:

    Grammar nannies–I love it! Thanks for the reassurance.

  19. filippo Says:

    page 10, line 7: “The giant axons of squid areS widely used in studies of neuron function…”

  20. Mark Konings Says:

    What would maximum axon length be in Lineus longissimus?

  21. Mike Taylor Says:

    That’s Grammar Police, thank you!

    In science communication (and indeed in communication in general), the goal is to communicate (clue’s in the question). So it’s always better to use unambiguous words than ambiguous words. Since, as you say, “comprise” has been compromised by widespread misuse, Matt is absolutely right to avoid using it completely.

  22. Matt Wedel Says:

    page 10, line 7: “The giant axons of squid areS widely used in studies of neuron function…”

    Thanks, filippo. Argh, once again with the superflous s! My left ring finger is out of control.

    What would maximum axon length be in Lineus longissimus?

    Good question! If bootlace worms really do get up to 200 feet, then they are probably longer than any sauropod, including Amphicoelias fragillimus. But there is some speculation that 180-200 ft reports are based on stretched specimens. Whether the animals can really be that long in life, I don’t know. Also, I know zip about the nervous systems of nemerteans. If they’re wired like vertebrates, with single nerve cells stretching from brain to tail-tip, they might also have stupid-long neurons, possibly even longer than those of the longest sauropods. But there are too many unknowns for me to be able to say anything more with confidence.

    Thanks for bringing them up, though–hopefully someone who knows more will chime in. I am amazed that nemerteans get to more than a few meters long. I would think that that much dumb, defenseless biomass would be an instant smorgasbord for anything that happened along. Maybe they have awesome powers of regeneration. Or maybe most predators get sick of worm after eating 10 or 20 meters and go do something else. I honestly don’t know–but I’d like to.

  23. Nathan Myers Says:

    “Comprise” has not been compromised by misuse. Rather, grammar nannies have falsely accused more-competent speakers of misusing it. (Were you to look up the early meaning of “compromise”, by the way, you might find yourself able to use it, or to misuse it, again.) Furthermore, “comprise” used in any sense is no more ambiguous than most of the rest of English. If English has a strength, it is in its readiness to express precisely the degree of ambiguity intended by the writer, no more and no less. Wasting precision on distinctions of no importance squanders precious attention. To so signal in one word, in the abstract, his intent to give distinctions precisely the attention they merit demonstrates an enviable degree of native linguistic authority.

    But this is all off-topic.

    Of real, lasting importance is whether the one-way transit time of a nerve impulse offers more than a hint at the actual time needed to coordinate a complex response. Unless that glycogen store near the sacrum houses a deep-seated reflex to punt raptors, your hyper-elongated might-have-been Gentle Reader will need several round-trip transit times (i.e. n * 2t) to coordinate lofting her assailant. Stepping on it sells fewer issues, but may offer surer protection for her somewhat less elongated protegé.

  24. Matt Wedel Says:

    Of real, lasting importance is whether the one-way transit time of a nerve impulse offers more than a hint at the actual time needed to coordinate a complex response.

    For sure. The initial reflex would only be for jerking the tail away from the source of pain, like yanking your hand back from a hot pot-handle on the stove (I do this a lot). Any actual combat would require coordination with the brain, lo these tens of meters away. But my guess is that we’re still talking about very short intervals of time, maybe handfuls of seconds at most. And this is assuming complete surprise on the part of the predators. Also, there’s gotta be an element of trade-off here. A 40-meter sauropod might have long enough physiological delays to give a predator an extra second or two to work with, but it would also be able to bring a lot more force to bear when it did respond. I am skeptical that any non-avian theropod had the cognitive capacity to knowingly take advantage of the minimally delayed response. But, as you point out, it’s an interesting question.

  25. Mike Taylor Says:

    “To so signal in one word, in the abstract, his intent to give distinctions precisely the attention they merit demonstrates an enviable degree of native linguistic authority.”

    And this is an example of clear communication, I suppose.

  26. Doug Henning Says:

    Page 1, sentence 2 pharyngial for pharyngeal

  27. Matt Wedel Says:

    And this is an example of clear communication, I suppose.

    Hey, he’s paying me a compliment–gerroff!

    Doug, thanks for the catch on ‘pharyngial’. *cringe* This is turning out to be a humbling experience. But better caught and fixed than left to fester.

    Keep ‘em coming.

  28. Nathan Myers Says:

    And this is an example of clear communication, I suppose.

    Evidently it was unwise to have hoped that the pointed mention of envy in the referenced sentence might give you pause. I do not care, I will continue to hope against all reason, for the alternative is despair.

    For authority on the foregoing, by the way, allow me to direct your attention to

    http://languagelog.ldc.upenn.edu/nll/?p=3136

  29. anne Says:

    Sir, you continue to awe me with your science and your writing. Keep up the good work.

  30. Dave Godfrey Says:

    I am most disappointed that you didn’t call it “The recurrent pharyngeal nerve of giraffes is inadequate” ;-) Congratulations on another paper!

  31. heteromeles Says:

    I thoroughly enjoyed this, except for the title. Since I worked on fungi, I’d point out that some of the most common fungi, the arbuscular mycorrhizal fungi in glomeromycota, are coenocytic. While they rarely extend more than a few centimeters, they are highly branched, and they are all one cell, more or less.

    There’s also plasmodial slime molds, and Physarum polycephalum has been grown to be rather enormous in the lab.

    While I’m not going to argue that sauropod neurons cover the greatest linear length, if you want total cell length along all (fractal) branches, the record is probably held by something that’s lowly and coenocytic.


  32. [...] a diagram from Wedel’s paper, which is very clear and well written (he also explains it in a post at his website, Sauropod Vertebra Picture of the [...]

  33. NickMatzke Says:

    Great stuff Matt — there is another creo-response on the RLN issue, from Casey Luskin & a German creationist he likes named Loennig:

    http://www.evolutionnews.org/2010/10/wolf-ekkehard_lonnig_under_neo039191.html

    I haven’t read it in awhile, but interested in your thoughts…cheers!
    Nick


  34. [...] Wedel talks about the world’s longest cells in the nervous systems of sauropods. A great example of blogging your own [...]

  35. Matt Wedel Says:

    Great stuff Matt — there is another creo-response on the RLN issue, from Casey Luskin & a German creationist he likes named Loennig

    Thanks for the kind words, and thanks for the heads up on the other ID RLN treatment. I am seriously considering writing another paper on the RLN, just to debunk the ID BS.

  36. Blake Nield Says:

    Matt,

    Great read! I’m about to complete my medical degree and move onto internship. I have always found that to really get my anatomy to stick I need to read something or see a patient with something interesting in relation to it. We continuously hear about the RNL in thyroid and throacic surgery but your piece solidified the embryology and more detailed anatomy for me. So… thanks, I guess!

    I learnt the anatomy of a few other areas in similar ways:
    – The different venous drainage of the testicles through nutcracker syndrome (renal vein entrapment syndrome) where the aorta and SMA clamp down on the L renal vein.
    – A paediatric case of Total anomalous pulmonary venous return (TAPVR), where the right pulmonary veins drained into the innominate veins and the left into the portal vein to give him high PV pressure in one lung and low PV pressure in the other, gave me a more detailed understanding of the mediastinal architecture.
    – Malrotation volvulus also is a great way to learn about the embryological development of the gut.
    – I am always quick to discuss with other doctors the nervus intermedius and place as the 13th cranial nerve as well.

    Thanks again! Keep up the good (and interesting) work.

  37. Matt Wedel Says:

    Thanks for your comments. I knew about malrotation volvulus but hadn’t heard of TAPVR or nutcracker syndrome. I’ll look ‘em up. Can’t have too many weird clinical correlations to throw at the students on slow dissecting days. Good luck with your internship!

  38. Wang-Lo Says:

    It seems within the realm of possibility that a Supersaurus could reach maturity without a right subclavian artery. Such an individual would present a 25 to 27 meter difference between his left and right inferior laryngeal nerves. Unless his brain somehow learned to compensate, he would vocalize with a distinctive built-in echo chamber effect.

    I am both unable and unwilling to visualize the effect this would have on his fellow Supersaurs.

    -Wang-Lo.

  39. Matt Wedel Says:

    It seems within the realm of possibility that a Supersaurus could reach maturity without a right subclavian artery.

    Possibly, possibly. We don’t really know the fates of the 4th aortic arches in extinct archosaurs. In mammals, the one on the left persists as the aorta, and the one on the right almost always becomes the right subclavian artery–it’s the “almost” that opens the door for a nonrecurrent laryngeal nerve on the right. In birds, the 4th aortic arch persists on the right, and the subclavian arteries originate from the 3rd aortic arches. Those are even farther up the line in the embryonic stack of pharyngeal arches and their associated arteries and nerves, so I reckon birds have RLNs that go around the (single) aorta on the right, and around the subclavian on the left, but with a much reduced option for alternative blood flow to the forelimb opposite the aorta, and thus less opportunity for the RLN on that side to slip off the hook, as it were.

    Crocs are worse still: they retain the paired aorta of basal vertebrates, meaning that the 4th aortic arch persists on both sides, so neither of the RLNs would have the option of being nonrecurrent unless something went very, very wrong.

    I don’t know where along the archosaur tree the ancestors of birds went from the primitive dual aorta system to the derived single aorta system we see in extant birds. I don’t know if anyone else does, either. It seems like the sort of thing that might have happened anywhere between the base of Ornithodira and the last common ancestor of the crown group. If anyone else has any more insight, I’m all ears.

    If the descriptions of the various schemes of aortas and subclavians and so on made your head hurt, or were at least unclear without diagrams, check this out.

    ANYWAY…

    Such an individual would present a 25 to 27 meter difference between his left and right inferior laryngeal nerves. Unless his brain somehow learned to compensate, he would vocalize with a distinctive built-in echo chamber effect.

    An amusing thought, but the brain almost certainly would have learned to compensate. That’s exactly the sort of compensation that brains are crazy good at, although admittedly a big sauropod with one RLN and one nonrecurrent would be the ultimate expression (that I can think of) of such compensation. Anyway, as pointed out in the paper, all sauropods faced the problem of coordinating swallowing with their short, sensible superior laryngeal nerves and long, dumb RLNs. The thing to remember is that as hatchlings the length difference between the nerves was basically imperceptible (to the conduction times of the nerves, not to a hypothetical time-traveler dissecting a baby sauropod), and the incremental growth of the nerves over ontogeny would have given the brain plenty of time to get used to the necessary regulation.

    But who knows? Human kids going through growth spurts put up with bouts of being uncoordinated. Maybe teenage sauropods had similar problems with their swallowing and spent a lot of time barfing. This could even have been advantageous for even younger sauropods if they could spot the puke and gobble it up. Sorry, I know that’s nasty, but we’re quite keen on the nourishing vomit hypothesis here at SV-POW! and never miss a chance to bring it up (so to speak! Evidence). Pigeons do it, why not Puertasaurus?

  40. Mike Taylor Says:

    “Pigeons do it, why not Puertasaurus?”

    Dude, please. The correct expression is: “What’s sauce for the pigeon is sauce for the Puertasaurus“.

  41. Nathan Myers Says:

    Some of us prefer not to let the possibility of sauropod juvenile coprophagy fall by the wayside (as it were).


  42. [...] was reminded of this by a blog post by a biologist called Wedel. I enjoyed reading it. He studies sauropods. Sauropods were a type of [...]

  43. DDeden Says:

    Were giant sauropods just supersized “bugs”?

    See the evidence here:

    http://the-arc-ddeden.blogspot.com/2011/06/vertebrates-are-invertebrates.html


  44. [...] My post on the long nerves of sauropods was chosen as one of ten blog posts for the Science Writer Tip Jar at Not Exactly Rocket Science, [...]

  45. Jim Says:

    Great discussion and great website. I’m going to suggest it to our students as a way to learn AND remember the RLN and the aortic arches.


  46. [...] Las neuronas. ¿Cuál es el tamaño máximo de una neurona? Más de 40 metros En un fascinante artículo, Mathew J. Wedel revisa los datos existentes para llegar a tan asombrosa [...]


  47. [...] friend, colleague, and sometime coauthor Dave Hone sent the above cartoon, knowing about my more-than-passing interest in sauropod neurology. It was drawn by Ed McLachlan in the early 1980s for Punch! magazine in the [...]

  48. Michael Says:

    I actually think the RLN is a very clever way of dealing with a huge problem. I don’t see why nobody else can see it. The RLN controls vocal cord activity. For the vocal cords to produce sound they need a body of air moving over them. Sounds are controlled by varying tension of the vocal cords and controlling precisely the speed of the moving column of air. If the RLN has a nerve conduction speed of 0.5 metres per second and the phrenic nerve, controlling diaphragm activity has a similar conduction speed this could cause huge problems for a giraffe or sauropod trying to communicate. For example, if the sauropod wanted to greet a potential mate saying the equivalent of ‘hi gorgeous’ and it did not have a RLN that went down to the chest and back, the nerve signal from brain to larynx would take less than a second, the vocal cords would twitch and then stop moving. By the time the phrenic nerve signal reached the diaphragm, and nerve signals reached other muscles controlling expulsion of air from the lungs. The muscles contract, the column of air moves up the very long trachea and reached the vocal cords there would have been a delay of about 40-80 seconds between the vocal cords twitching and the moving air reaching them. Just imagine, the first 40-80 seconds of any conversation would be mute. Any conversation lasting less than that would be a lot of mouth movement followed by breathing out in the mates face. Having a recurrent laryngeal nerve solves that problem. As the animal grows the timing of signals remains synchronised so the animal doesn’t have to continually learn how to time speech during adolescence. Genius solution!

  49. Matt Wedel Says:

    Thanks for writing! You raise some interesting points. However, the idea that sauropods could not have made noises without having long RLNs won’t fly, for reasons that I’ll explain.

    Just imagine, the first 40-80 seconds of any conversation would be mute. Any conversation lasting less than that would be a lot of mouth movement followed by breathing out in the mates face. Having a recurrent laryngeal nerve solves that problem.

    Your solution assumes that the sauropods were incapable of learning coordination. Actual mismatches between the nervous system and the muscular system happen all the time, especially when animals (including us) are growing rapidly. This is why human teenagers go through bouts of being uncoordinated–they’re going through, or have just gone through, growth spurts, and their old nervous system “maps” of where all the parts are and when to send out the messages to get them all working together correctly are out of date. All animals go through this–and all animals get over it, because their nervous systems learn.

    We can be quite sure that sauropods were capable of syncing things so that very short nerves and very long ones could produce coordinated actions. As discussed in the linked paper, swallowing involves coordination between the superior laryngeal nerves, which go straight from the brain to the larynx (via the vagus nerves) and would have been less than 1 meter long in even the largest sauropods, and the RLNs, which were crazy-long. (It’s fun to speculate that fast-growing juvenile sauropods might have gone through spells of choking and barfing during growth spurts, if this SLN/RLN coordination got thrown off.) Anyway, if sauropods could coordinate long RLNs with short SLNs, then their RLNs didn’t need to be long to slow down the signals, because sound production or modification using the larynx* would already have involved coordination between the short SLNs and the long nerves to the thorax that controlled breathing.

    * IF sauropods used their larynges to produce or modify sounds. We don’t have any direct evidence, and not all animals do. But enough do that it falls under the heading of reasonable speculation–like pretty much everything in this post!

    Incidentally, the breathing-controlling nerves for sauropods were probably not phrenic nerves, since sauropods had bird-like air sac systems and almost certainly lacked muscular diaphragms. Sauropods probably ventilated their lungs with rib motions produced by intercostal muscles, like birds (and almost everything else; even crocs and mammals supplement diaphragm action with rib motions).

    So the moral of the story is, if you don’t see why no one else can see it, it might mean you’re not seeing the whole picture. ;-)

  50. DDeden Says:

    “The RLN innervates the muscles responsible for speech, and on either side it passes right behind the thyroid gland, which is subject to goiters and tumors and other grotesque maladies”.

    “If you want to experience everything I’ve discussed in this post in a single action, put your fingertips on your voicebox and hum. You are controlling the hum with signals sent from your brain to your larynx through your recurrent laryngeal nerves, and sensing the vibration through individual neurons that run from your fingertips to your brainstem.”

    Ok Matt, I just read in ‘Natural Awakenings’ an article by medical anthropologist Sydney Ross Singer which claims that the physical act of humming (vibration, presumably speaking and singing as well) is beneficial for the thyroid gland.

    He says “simply humming “mmmmmmmmm” a couple of minutes a day can stimulate the thyroid and increase the production of thyroid hormones of those with an underactive thyroid. The butterflyshaped gland wraps around the larynx, or voice box, which Singer contends is part of nature’s elegant design, meant to be stimulated by sound.”

    I would guess normally mute or non-vocal healthy animals also have thyroid glands around their larynx, which might invalidate Singer’s statement, but I don’t know. I’d appreciate any opinion on this, since I’m curious about the role of laryngeal air sacs, larynx, thyroid, RLN etc. in hominid communication and the change from quadrupedal to bipedal upright posture.


  51. [...] day (Dec. 3) and publication day (Feb. 12). My fastest turnaround before this was 73 days for my sauropod nerve paper, but that was from submission to posting of the accepted manuscript, not publication of the final [...]


  52. […] The world’s longest cells? Speculations on the nervous systems of sauropods. Ja alkuperäisartikkeli Acta Paleontologica […]


  53. […] worked. At heart, I’m still a wannabe chrononaut, and all my noodlings on pneumaticity and sauropod nerves and neural spines and so on are just baby steps toward trying to understand sauropod lives. Safari […]


  54. […] mercifully short version of this much longer post, in which I consider the consequences of the world’s largest animals having the world’s […]


  55. […] * There is a sort of an arrow-of-inevitability thing here, in that reviewers almost always ask you to cite more papers rather than fewer. Only once ever have I been asked to cite fewer sources, and that is when I had submitted my dinosaur nerve paper (Wedel 2012) to a certain nameless anatomy journal that ended up not publishing it. One of the reviewers said that I had cited several textbooks and popular science books and that was poor practice, I should have cited primary literature. Apparently this subgenius did not realize that I was citing all of those popular sources as examples of publications that held up the recurrent laryngeal nerve of giraffes as evidence for evolution, which was part of the point that I was making: giraffe RLNs are overrated. […]


  56. […] The world’s longest cells? Speculations on the nervous systems of sauropods […]


  57. […] Las neuronas. ¿Cuál es el tamaño máximo de una neurona? Más de 40 metros En un fascinante artículo, Mathew J. Wedel revisa los datos existentes para llegar a tan asombrosa […]


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