I’ve known who Peter Doson was since I was nine years old. A copy of The Dinosaurs by William Stout and William Service, with scientific content by Peter, showed up at my local Waldenbooks around the same time as the New Dinosaur Dictionary – much more on The Dinosaurs another time. Then when I started doing research as an undergrad at the University of Oklahoma, Peter’s chapter on sauropod paleobiology in The Dinosauria (Dodson 1990) was one of the first things I read. At the SVP banquet in 2000, I ran into Peter and he shook my hand and said, “Sauroposeidon rocks!” I managed not to swoon – barely.

When I was in Philadelphia this March, Peter invited me to the UPenn vet school for an afternoon. He gave me a tour of the building with its beautiful lecture halls and veterinary dissection lab, and then we spent a couple of hours rummaging around in his office. That was one of the highlights of the trip, because it turns out that Peter and I are both comparative anatomy junkies. He’s been at it for longer, and he has more regular access to dead critters and more space to display them, so his collection puts mine to shame. But he kindly let me play with study whatever I wanted.

 

In fact, he went farther than that: he quizzed me. A lot. I take it that it’s a right of passage for people coming through Peter’s office. It was an enjoyable challenge, and I got photos of a few quiz items so you can play, too. This transversely-sectioned skull was one of the first mystery specimens. I figured it out pretty quickly, for reasons I’ll reveal in a future post. Can you? Post your IDs in the comments.

I don’t remember all of the quiz items. One of them was the dark skull lying upside down behind the ratite skeleton in the photo up top. I had to figure that one out without picking it up, so you have about as much information as I did. We’ll call that one quiz item #2. Embiggenate for all the clues you’ll need.

This wasn’t a quiz item, just something cool: the skull of a large dog with the top of the cranium removed. In the paired cavities at the top, we’re looking down through the frontal sinuses to see the respiratory turbinates in the nasal cavities. The single large space behind is the braincase. At the very front, in the shadowed recess, you can see the cribriform plate of the ethmoid bone, perforated with dozens of holes to let the olfactory nerve endings through from the back of the nasal cavities. We have the same thing on a smaller scale a centimeter or two behind our brows, and oriented horizontally. But what really drew my attention were the linear arrays of paired foramina arcing across the floor of the braincase – holes to let cranial nerves and the internal jugular veins out of the skull, and the internal carotid arteries in. We have the same structures in our heads, of course, but the layout isn’t as neat – our big brains, bent forward at such a sharp angle from the spinal cord, have squished things around a bit.

Here are more skulls, garnished with a human femur and a ratite pelvis and synsacrum. Peter quizzed me on the Archaeoceratops (front) and Auroraceratops (back) skulls on the far right. I IDed them correctly, but only because I spent some quality time with the Alf Museum’s casts when I was reconstructing the skull of Aquilops. On the far left is an alligator skull with injected arteries, which is definitely worth a closer look.

Here’s a dorsal view of the injected alligator skull. The arteries have been injected with red resin, and then all of the soft tissue has been macerated away, leaving just the bone and the internal cast of the arterial tree. Some of the midline bone has been removed here to reveal the courses of the cerebral, ethmoid, and nasal arteries. Also note the artery looping around in the left supratemporal fenestra.

Here’s a look into the right side of the back of the skull, where the lateral wall of the braincase has been Dremeled away to show the course of the internal carotid artery. It’s a very cool demonstration of a bit of anatomy that I had never seen before. For more on cranial blood vessels in crocs, check out the obscenely well-illustrated recent paper by Porter et al. (2016).

To my chagrin, that’s all the good photos I got from Peter’s office – we were too busy passing specimens back and forth and frankly geeking out like a couple of kids. One of my favorite specimens from his office was the mounted foot skeleton of a horse, which Jessie Atterholt had prepared for him when she was his student at UPenn. It’s such a cool preparation that it captured my imagination, and when I got back I warned Jessie that if she didn’t get her own articulated horse foot posted soon, I was going to make something similar for myself and steal her thunder. A couple of months later, her horse foot is up on Instagram – I featured it in this post – and my cow foot is still sitting in pieces, waiting for me to put it together. Here’s a shot of Jessie’s, to hopefully prod me into action:

I didn’t get all of Peter’s quiz questions correct. I knew that the endocast of the pharyngeal pouch in a horse was an endocast, but of what I didn’t know, although I did correctly identify the hyoid apparatus of a horse, mounted separately. And there was a partial cetacean jaw that I misidentified as a shark (in my defense, it was from one of the small, short-faced weirdos). Still, Peter said that I’d done as well as anyone else ever had. That was nice to hear, but I was already happy to have gotten to see and talk about so many cool things with a fellow connoisseur. Thanks, Peter, for a wonderful afternoon, and for permission to post these pictures. I am looking forward to a rematch!

References

  • Dodson, P. 1990. Sauropod paleoecology. In: D.B. Weishampel, P. Dodson, P., & H. Osmolska, (eds), The Dinosauria, 402-407. University of California Press, Berkeley.
  • Porter, W.R., Sedlmayr, J.C. and Witmer, L.M., 2016. Vascular patterns in the heads of crocodilians: blood vessels and sites of thermal exchange. Journal of Anatomy 229(6): 800-824.
  • Stout, W., Service, W., and Preiss, B. 1984. The Dinosaurs: A Fantastic View of a Lost Era. Bantam Dell Publishing Group, 160pp.
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This is the second post in the “bird neural canals are weird” series (intro post here), and it covers the first of five expansions of the spinal cord or meninges in the lumbosacral regions of birds.

The lumbosacral expansion of the spinal cord is not unique to birds and doesn’t require any special explanation. As noted in the slide, all limbed tetrapods and some fishes with sensitive fins have adjacent segments of the spinal cord correspondingly expanded. These expansions house the extra afferent neurons needed to collect sensory inputs from the limbs, the extra efferent neurons needed to provide motor control to the limbs, and the extra interneurons needed for sensory and motor integration (including reflex arcs) – ‘extra’ here meaning ‘more than are required for non-limb neck, trunk, and tail segments’.

Humans have these, too, in our lower cervical vertebrae to run our forelimbs, and in our lower thoracic vertebrae to run our hindlimbs. Recall that the segmental anatomy of the adult human spinal cord corresponds increasingly poorly to the vertebrae the farther we are from the head because of our child-sized spinal cords (see this post for more).

So if the lumbosacral expansion is present in all tetrapods with hindlimbs, why bring it up? My goal is to develop a set of criteria to distinguish the various spinal and meningeal specializations in birds, in part because it’s an interesting challenge in its own right, and in part because doing so may help illuminate some unusual features in sauropods and other non-avian dinosaurs. If we want to be able to detect whether, say, a glycogen body is present, we need to know how to tell the impression left by a glycogen body from the more generalized lumbosacral expansion present in all limbed tetrapods. The key characteristics of the lumbosacral expansion are that the cord (and hence the canal) expands and contracts gradually, over many segments, and that the expansion is in all directions, radially, and not biased dorsoventrally or mediolaterally.

Numbering reflects spinal nerve count – 8 cervical, 12 thoracic, 5 lumbar, and 5 sacral spinal spinal nerves. Cervical expansion for the forelimbs is roughly C5-T1, and lumbosacral expansion for hindlimbs is L2-S3. Gray (1918 image 665).

The one way in which the lumbosacral expansion of birds is weird, at least compared to mammals, is that the magnitude of the change is so great in hindlimb-dominant flightless birds like the ostrich. Here’s a graph from Gray’s Anatomy showing the cross-sectional area of the human spinal cord in square mm, with the head on the left. Note that the swellings for the limbs bump up the cross-sectional area by a quarter to a third, relative to adjacent non-limb areas.

Streeter (1904: fig. 4)

Here’s the same diagram for an ostrich, again in square mm, again with the head to the left. The lines here are a little different – the “substantia grisea” is the gray matter (mostly neuron cell bodies), and the white matter (axons, mostly myelinated) is divided into the large ventrolateral funiculi (descending motor, ascending pain, temperature, and unconscious proprioception) and the much smaller dorsal funiculi (ascending touch and conscious proprioception). Here the lumbosacral expansion maxes out at more than double the cross-sectional area of the cord in the inter-limb torso segments – and this is just the white and gray matter, and does not include the glycogen body (which is proportionally small in the ostrich, as we’ll see in a future post).

Note that the ostrich does have a much smaller expansion of the spinal cord associated with the forelimbs, but one glance at the graph will tell you that the hindlimbs are a lot more important. This too has implications for fossils. Because the cross-sectional area of the neural canal tends to track the cross-sectional area of the spinal cord (despite the cord not filling the canal), it is possible to make inferences about limb use in fossil taxa based on the relative cross-sectional area of the neural canal along the vertebral column. Emily Giffin published several papers about this in the 1990s (e.g., Giffin 1990, 1995), all of which are worth reading.

Next in this series: the glycogen body.

References

Dorsal vertebra of a rhea from the LACM ornithology collection. Note the pneumatic foramina in the lateral wall of the neural canal.

If you’ve been here for very long you know I have a bit of a neural canal fixation. Some of this is related to pneumaticity, some of it is related to my interest in the nervous systems of animals, and some of it is pure curiosity about an anatomical region that seems to receive very little attention in proportion to its weirdness – especially in birds.

Human thoracic vertebrae in midsagittal section showing vertebral venous plexus. Gray (1918, image 579), available from Bartleby.com.

The neural canals of mammals are pretty boring. The canal is occupied by the spinal cord and its supporting layers of meninges, and the rest of the volume is padded out by adipose tissue and blood vessels, notably an extra-dural venous plexus. Aaand that’s about it, as far as I know. (If there are weird things inside mammalian neural canals that I’ve missed, please let me know in the comments – I’m a collector.)

But not so in birds, which have a whole festival of weird stuff going on inside their neural canals. Let’s start with pneumaticity, just to get it out of the way. Many birds have supramedullary diverticula inside their neural canals, and these can leave osteological traces, such as pneumatic foramina, in the walls of the neural canal. That’s cool but it’s a pretty well-known system – see Muller (1908) on the pigeon, Cover (1953) on the turkey, and these previous posts – and I want to get on to other, even stranger things.

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).

The spinal cords of birds have several gross morphological specializations not seen in mammals, as do their meninges, and most of these apomorphic structures can also leave diagnostic traces on the inner walls of the neural canal. In fact, birds have so many weird things going on with their spinal cords – at least five different things in the lumbosacral region alone – that I spent a week back in January just sorting them out. To crystalize that body of knowledge while I had it all loaded in RAM, I made a little slideshow for myself, and I’ll use screenshots of those slides to illustrate the morphologies I want to discuss. We’ll cover the vanilla stuff in the next post, and the really weird stuff in subsequent posts.

Stay tuned!

References

Frog RLN ventral view - Ecker 1889 plate 1 fig 115 - RLN highlighted

Just posting a few images from my impending talk at SVPCA this Thursday.

I’ve written about the recurrent laryngeal nerve before, in Wedel (2012) and in this post. It’s present in all tetrapods, from frogs and salamanders on up. The frog RLN is shown in ventral view above, and in lateral view below, both from Ecker (1889:plate 1, figures 114 and 115). I’ve highlighted the RLN in red in both. Perhaps not a monument of inefficiency, but still recurrent, and therefore dumb.

Frog RLN lateral view - Ecker 1889 plate 1 fig 114 - RLN highlighted

And in a giraffe – RLN in blue, nerve path to hindfoot phalanges in red. Hollow circles are nerve cell bodies, solid lines are axons.

Giraffe skeleton silhouette 1000 with nerves

And in the elasmosaur Hydrotherosaurus, same color scheme plus the nerve path to the tail in purple, base image from Welles (1943).

Hydrotherosaurus nerve pathways 4 - RLN pathway

That’s all for now!

References

The longest cell in Andy Farke is one of the primary afferent (sensory) neurons responsible for sensing vibration or fine touch, which runs from the tip of his big toe to his brainstem. (NB: I have not actually dissected Andy to confirm this, or performed any viral neuron tracing on him, this is assumed based on comparative anatomy.) Here’s a diagram:
Longest cell in Andy Farke

This is what happens when (a) I need to create a diagram to illustrate the longest cell in the human body for my students, and (b) my friends put stuff online with a CC-BY license.

Found this while I was checking out Aquilops art online:

Aquilops_scale

It’s a derivative work by Andy IJReid, from this Wikimedia page, based on two PhyloPic silhouettes Andy created (go here for the pathetically tiny lower vertebrate and here for Aquilops).

wedel-rln-fig2

From there it was pretty straighforward to mash up Andy’s silhouette with the nerve stuff from Wedel (2012: fig. 2).

So if you want the full deets on licensing – which I am obligated to provide whether you want them or not – the image up top is a derivative image by me, based on work by Andy published at PhlyoPic under the Creative Commons Attribution 3.0 unported (CC-BY 3.0) license, and based on my own image published in Acta, also under a CC-BY license.

If you’d like to know more about the science behind very long nerves in vertebrates, please see these posts:

Also, keep making stuff and putting it online under a license people can actually use. It’s beneficial for science and education, and hugely entertaining for me.

Reference

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.

I’m scrambling to get everything done before I leave for England and SVPCA this weekend, so no time for a substantive post. Instead, some goodies from old papers I’ve been reading. Explanations will have to come in the comments, if at all.

Streeter (1904: fig. 3). Compare to the next image down, and note that in birds and other reptiles the spinal cord runs the whole length of the vertebral column, in contrast to the situation in mammals.

Streeter (1904: fig. 3). Compare to the next image down, and note that in birds and other reptiles the spinal cord runs the whole length of the vertebral column, in contrast to the situation in mammals.

Nieuwenhuys (1964: fig. 1)

Nieuwenhuys (1964: fig. 1)

Butler and Hodos (1996: fig. 16.27)

Butler and Hodos (1996: fig. 16.27)

For more noodling about nerves, please see:

References

  • Butler, A.B., and Hodos, W. 1996. Comparative Vertebrate Neuroanatomy: Evolution and Adaptation. 514 pp. Wiley–Liss, New York.
  • Nieuwenhuys, R. (1964). Comparative anatomy of the spinal cord. Progress in Brain Research, 11, 1-57.
  • Streeter, G. L. (1904). The structure of the spinal cord of the ostrich. American Journal of Anatomy, 3(1), 1-27.

 

being eaten 600

My 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 UK (you can buy prints starting at £18.99 here).

I know that this isn’t the only image in the “oblivious sauropods getting eaten” genre. There’s a satirical drawing in Bakker’s The Dinosaur Heresies showing a sleeping brontosaur getting its tail gnawed on by some pesky mammals. I’ll scan that and post it when I get time (Update: I did). I’m sure there must be others in a similar vein–point me to them in the comments or email me and I’ll post as many as I can get my hands on.

I wouldn’t post stuff like this if I didn’t think it was funny. But if you want the real scoop on whether sauropods could have responded quickly to injuries to their distant extremities, here’s the deal:

First of all, sauropods really did have individual sensory nerve cells that ran from their extremities (tip of tail, soles of feet)–and from the rest of their skin–to their brainstems. In the longest sauropods, these cells were probably something like 150 feet long, and may have been the longest cells in the history of life. We haven’t found any fossils of these nerves and almost certainly never will, but we can be sure that sauropods had them because all vertebrates do, from hagfish on up. That’s just how we’re built. (This is all rehash for regular readers–see this post and the linked paper.)

Wedel RLN fig2 480

So how long does it take to send a nerve impulse 150 feet? The fastest nerve conduction velocities are in the neighborhood of 120 meters per second, so a signal from the very tip of the tail in a 150-foot sauropod would take about half a second to reach the brain.

Is it possible that sauropods had accelerated nerve conduction velocities, to bring in those distant signals faster? To the brain, probably not. The only ways to speed up a nerve impulse are to increase the diameter of the axon itself, which some invertebrates do, and to increase the thickness of the myelin sheath around the axon, which is what vertebrates tend to do (some invertebrates have myelin-like tissues that apparently help accelerate their nerve impulses, too). Fatter axons mean fatter nerves, and for at least half the trip to the brain, the axons in question are part of the spinal cord. And we know that sauropod spinal cords were pretty small, relative to their body size, because the neural canals of their vertebrae, through which their spinal cords passed, are themselves small–Hatcher wrote about this more than a century ago. So there’s a tradeoff–sauropods could have had very fast, very fat axons, but not very many of them, and therefore poor “coverage” at their extremities, with nerve endings widely spaced, or better coverage with more axons, but those axons would be skinnier and therefore slower. We don’t know which way they went.

Incidentally, you can experiment with the density of sensory nerve endings in your own body. Close your eyes or blindfold yourself, and have a friend poke you in various places with chopsticks. Seriously–start with the two chopsticks right together, and gradually spread them out until you can feel two distinct points (or, if you want to get really tricky, have your friend mix up the close and widely spread touches so there’s no direction for you to anticipate). The least sensitive part of your body is your back–over your back and shoulders, you’ll probably have a hard time distinguishing points of touch that are less than an inch apart. On your hands and face, you’ll probably be able to distinguish points only a few millimeters apart; in fact, for fingertips you’ll probably need finer instruments than chopsticks–maybe toothpicks or pins, but I take no responsibility for any accidental acupuncture!

Back to sauropods. Could predators have taken advantage of the comparatively long nerve conduction velocities in sauropods? I seriously doubt it, for several reasons:

  • Simple reflex arcs are governed by interneurons in the spinal cord. The tail-tip-to-spinal-cord distance was a lot shorter than the tail-tip-to-brain route. Even over the round trip of “sensory impulse in, motor impulse out”, it would have been at worst equal, and that’s assuming the nerve impulse had to go all the way to the base of the tail.* Call it half a second, max.
  • It gets worse: the peripheral nerves outside the spinal cord are not limited by the size of the neural canal, so they can be more heavily myelinated, with faster conduction times. For example, each of the sciatic nerves running down the backs of your thighs is much larger in cross-section than your entire spinal cord. If sauropod peripheral nerves were selected for fast conduction, they might have been bigger and faster than anything around today.
  • Half a second is not much time for a theropod to formulate a plan, especially if Step 1 of the plan is “grab 150-foot sauropod by the tail”.
  • This assumes that said theropod was able to sneak right up to the sauropod without being detected. You go try that with a big wild herbivore and let me know how it works out. (Also, a big animal tolerating your presence, because you are pathetically small and weak, is not the same as it being unaware of your presence.)
  • All of this assumes the theropod only went for the bony whip-lash at the tip of the tail–the fastest-moving extremity, and the least-nourishing single bite anywhere on the target. If the theropod went for a meatier bite closer to the base of the tail, it would have to sneak closer to the sauropod’s head (better chance of being spotted), and the nerve conduction delay would be shortened.
  • A 150-foot sauropod would probably mass somewhere between 50 and 100 tons, and would be capable of dealing incredible damage to even the largest theropods, which maxed out around 15 tons. There’s a good reason predators go after the young, sick, and weak. Smaller sauropods would be less dangerous, but they’d also have faster tail-to-central-nervous-system-and-back reaction times.
  • A theropod big enough to go after a 150-foot sauropod would also be subject to fairly long nerve-conduction delays, which would limit whatever trifling advantage it might have gotten from such delays in the sauropod.

So, although I have no doubt that in their long history together, giant theropods did occasionally tackle full-grown giant sauropods–because real animals do all kinds of weird things if you watch them long enough, and lions will take on elephants when they get desperate–I am extremely skeptical that the theropods enjoyed any advantage based on the “slow” nervous systems of those sauropods.

* Some relevant hard-core anatomy for the curious: sauropods have neural canals in their tail vertebrae, and usually far down their tails, too. But that doesn’t mean much–you have neural canals to the bottom half of your sacrum, but your spinal cord stops around your first or second lumbar vertebra. From there on down, you just have nerve roots. So the shortest reflex arc from your big toe has to go up to your lower back and return. Why is your spinal cord so short? Basically because your central nervous system stops growing when you’re still a child–it will add new connections after that, and a few new cells in your olfactory bulbs and hippocampus, but it won’t get appreciably bigger or longer. After mid-childhood, your body keeps growing but your spinal cord stays the same length, so you end up with this freaky little-kid spinal cord tucked up inside your grown-up vertebral column. Weird, huh?

So did sauropod spinal cords stop at mid-back or go all the way into the tail? We have several conflicting lines of evidence. In extant reptiles, the spinal cord does extend into the tail in at least some taxa (I haven’t done anything like a complete survey, just read a couple of papers). Birds are no help because their tails are extremely short, but their spinal cords do extend into the synsacrum (and expand there, thanks to the glycogen body, which was probably also present in sauropods and responsible for the inaccurate “second brain” meme). But then birds grow up very fast, with even the largest reaching full size in a year or two, so they don’t share our problem of the body outgrowing the nervous system. We know that sauropods grew pretty quickly, but they also took a while to mature–somewhere between one and three decades, probably. Did that protracted growth period give their vertebral columns the time to outgrow their spinal cords? I have no idea, because the division of the spinal cord into roots happens inside the dura mater and doesn’t leave any skeletal traces that I know of. Someone should go figure it out–or at least figure out if it can be figured out!