The Magi present gifts to the Christ child

This tutorial is based on all the things that I stupidly forgot to do along the way of tearing down the juvenile giraffe neck that Darren, John Conway and I recently got to take to pieces.  At half a dozen different points in that process, I found myself thinking “Oh, we should have done X earlier on!”  So it’s not a tutorial founded on the idea that I know how this should be done; it’s about how I am only now realising how it should be done, off the back of my dumb mistakes.

Cervical vertebra 5 of two-week-old giraffe: left column, anterior; middle column, top to bottom, dorsal, left lateral, posterior, all with anterior to the left; right column, posterior

What you want is to get the maximum possible information out of your specimen.  At each stage of preparation, information is lost — a necessary evil, because of course at the same time new information becomes available.  So don’t miss anything early on.

The whole neck

If you’re lucky, you’ll get the complete, intact neck to work with.  (Ours was not quite intact, having been skinned, and lost an indeteminate amount of superficial muscle and ligament in the process.)  So before you start cutting, photograph the neck in dorsal, ventral, lateral, anterior and posterior aspects.

Next, you want to measure the neck:

  • total mass
  • total length, front of atlas to back of last centrum.
  • maximum flexion (i.e. downwards bend)
  • maximum extension (i.e. upwards bend)
  • maximum deflection (i.e. lateral bend)

These last three are hard to do, because “maximum” flexion, extension and deflection are not exact things.  You can always push or squeeze or bend a bit harder.  These are the unpleasantly messy aspects of working with animals rather than robots — most kinds of tissue are flexible and resilient.  You just have to do the best you can, and supplement your measurements with photographs of the neck bent in each direction.

Skinning

Now you’re ready to start taking that baby apart.  Get the skin off, then redo all your photos and redo all your measurements — yes, even total length, even though you “know” removing the skin can’t affect that.  Because you don’t know what you don’t know.  Does removing the skin affect the maximum range of movement?  How much of the neck’s total mass was due to the skin?  Weigh the skin as well: does its mass added to that of the deskinned neck add up to that of the intact neck?  If not, is the discrepancy due to blood loss?

Stripping muscle

Once the skin is off, you can start removing muscles.  Ideally, you want to identify each muscle as you go, and remove them one by one, so that you leave the major ligaments behind.  In practice this is harder than it sounds, because the muscles in real necks are, inconveniently, not clearly delineated and labelled like the ones in books.  Still, going slowly and carefully, it’s often possible to avoid cutting actual muscles but just the fascia between them, which allows you remove complete muscles.  Done well, this can leave in place not only the nuchal ligament running along the top of all the neural spines, but the shorter ventral ligaments joining adjacent vertebrae.

John (left) and Darren (right) removing muscle from the giraffe neck (in right lateral aspect), while keeping ligaments intact

As you’re doing this, you want to avoid damaging the intercentral joints and the zygapophyseal capsules, so far as possible.  You’ll probably find it easy to preserve the former, which are tough, but harder not to accidentally damage at least some of the latter.  You want to keep them intact as far as possible, so you can see how the react when you manipulate the neck.  (Do these manipulations gently, or you’ll tear those capsules.)

Now that the skin and muscles are both off — at least, so far as you can remove the muscles, which will not be completely — you can redo all your photos and redo all your measurements again.  Yes, all of them.  Because you just can’t tell what you’re going to be interested in later, and curse yourself for missing.

Stripping ligament

Go right ahead.  Remove the short ligaments, and do your best to get the nuchal ligament off all in one chunk — not quite as easy as it sounds, because it doesn’t just sit on top of the neural spines, but sort of encloses them.  Measure the nuchal ligament at rest, then stretch it out as far as you can and measure it extended.  Calculate how far it stretched as a proportion of the rest length.  Compare this with what you learned from Alexander (1989:64-65).  Hmm.  Interesting, no?

You can guess what’s coming now: redo all your photos and redo all your measurements yet again.  You should find that the total length is the same, but who knows what you might find about changing flexibility?  Also, your progressive sequence of mass measurements will tell you what proportion of the whole neck was skin, muscle, ligament, etc.

Separating the vertebrae

This sounds like it should be easy, but it’s not.  The zygapophyses will come apart very easily, but the centra will be held firmly together with very dense connective tissue which has be cut carefully away, piece by piece, with the blade of a scalpel worked between the condyle of one vertebra and the cotyle of the next.  (I’m writing here about a giraffe neck, but I’m confident the same will be true of other artiodactyls and maybe most mammals; bird necks are different.)

Once you’ve got the vertebrae separate, photograph each vertebra separately, from each of the cardinal directions. Also, measure each vertebra separately — especially for centrum length, but you may as well get all the major measurements.  These measurements will include the cartilage caps at the front and back of each centrum.  (This is the step that I most regret missing out.)

Articulate the vertebrae in “neutral pose” by keeping the centra in full contact and rotating each intercentral joint about its midpoint until the corresponding zygapophyses are maximally overlapped.  What does this pose look like?  How does it compare to the animal’s habitual pose in life?  (If possible, compare with the pose shown by an X-ray of the live animal, since necks lie.)

Articulate the vertebrae in positions of “maximal” flexion, extension and deflection by keeping the centra in full contact and rotating each intercentral joint about its midpoint until the corresponding zygapophyses are displaced to a degree of your choosing.  Try it with the zygs allowed to slide until they are 50% disarticulated, then with 75% disarticulation, then displacing until they are just past the point of contacting each other.  Photograph all these poses and measure their deflection.  Compare these variant poses with those obtained when the vertebrae were still joined together, and when the ligaments, muscles and skin were still in place.  What degree of zygapophyseal disarticulation best matches the whole-neck bending ability?  How does this vary along the neck?  How does that this compare with what you may have been led to expect in the literature.  Hmm.

Using your earlier photos of the whole neck’s bending profile, arrange the vertebrae in the exact same pose.  How much do the zygapophyses disarticulate in these poses?  As you rotate the joints about the articulation of their centra, do the zygs just slide neatly past each other, or do they move far apart from each other as the neck bends?  Interesting, yes?

Cleaning the vertebrae

Have you recorded all the information you need from the intact vertebrae with their cartilage in place?  If you’re sure, then …

Lightly simmer the vertebrae for an hour or so, then remove the excess flesh by hand and using a toothbrush.  Repeat as needed to get them clean.  If you can do this really carefully — I couldn’t — you may be able to keep the cartilage firm, and firmly articulated with the bone.  (Bugging the vertebrae is probably a better approach for this purpose, but I find it hard to be that patient.)

Once the vertebrae have dried out — and especially, once their cartilage is dry — re-measure each vertebra.  Does the drying of the cartilage affect the centrum length?

Simmer the vertebrae again and gently peel off the cartilage caps at the front and back of each centrum.  Re-measure the centra: how long are they now?  What proportion of each centrum’s length was cartilage?

Articulate all the centra in a straight line, and measure the total length.  How does this compare with the whole-neck length you started with?  [Crib-sheet answer for our baby giraffe: 41 cm vs. a whole-neck length of 51 cm.  Expect a closer match if you’re dealing with an adult animal,which will have proportionally less cartilage.]

Articulate the vertebrae in “neutral pose” as you did back when the individual vertebrae were complete.  How does the new “neutral pose” compare with the old one?  With habitual life posture?  Huh.  Makes you think, doesn’t it?

Nearly done …

Articulate the vertebrae in positions of “maximal” flexion, extension and deflection as you did before, and compare your results with those from when the vertebrae were complete with their cartilage caps.  Well!  Who’d have thought?

Now remember that the fossils we have of, say, sauropod cervicals are those of the dry bone only, with no cartilage.  Think about how different the “neutral pose” and range of movement would be if we had the intact vertebrae with their cartilage.

Dammit all, I’ve given the game away

As I wrote this article, I found myself giving away more and more of a paper I’ve been planning to write, in which I go through essentially this process with a couple of necks, ideally from very different clades, and write up the results.  Say, a giraffe, an ostrich and  a croc.  The extent to which the dry-bone postures and flexibility vary from those of the live animals would give us a reasonable starting point for thinking about how life postures and flexibility of sauropods might have varied from what we’d deduce from the dry bones alone.

Wouldn’t that be a great little paper?

Well, I might still write it when I find the time, but I won’t stand in the way of anyone else who wants to plough straight in and just get it done.  (You might mention me in the acknowledgements if you do.)


In a comment on the previous post, Dean asked: “What was the difference in length between the neck with its cartilage and the bones flush together?”

I’m glad you asked me that.  You’ll recall from last time that the fully fleshed neck — intact apart from the removal of the skin and maybe some superficial muscle — was 51 cm in length from the front of the atlas to the back of the centrum of the seventh cervical vertebra.  When I pose the cleaned and cartilage-free bones together, the total length of the series is only 41 cm — 10 cm shorter, coming in at just over 80% of the live length.  Don’t believe me?  Here are the photos!

I’m sure I need hardly say, but the top image is the neck as we got it, the second is the cleaned bones posed in more or less the arrangement they must have been in life (both of these taken from the previous post) and the bottom image the bones fully abutting.

So!  The neck of Wallace the baby giraffe was very nearly a quarter as long again as the bones alone suggest.  Does this mean that the neck of Giraffatitan was really 10.6 m long instead of 8.5 m?

It’s an exciting prospect, but I’m afraid the answer is no.  As I hinted last time, while it’s perfectly acceptable, indeed obligatory, to recognise the important role of cartilage in sauropod necks qualitatively, we can’t blindly apply the numbers from Wallace the baby giraffe to adult sauropods for two reasons: 1, Wallace is a baby; and 2, Wallace is a giraffe.

The first of these reasons is part of why I am keen to do this all over again with an adult giraffe when I get the opportunity; but there’s not much we can do about the second.  One might think that a more closely related extant animal such as an ostrich might have a neck that is more homologous with those of sauropods; and that’s true, but my feeling is that the giraffe is more analogous.  That is, although the birds share more recent common ancestry with sauropods, giraffes’ more similar size seem to have encouraged them to evolve cervicals that are in some ways more similar to those of sauropods, most notably in the possession of ball-and-socket intervertebral joints rather than the saddle-shaped joints that are ubiquitous in birds.

How big a deal is Wallace’s juvenile status?  Well, take a look at his fifth cervical vertebra in posterior view:

If this bone were found in 150 million years by competent palaeontologists, in a world where there were no extant artiodactyls to compare with, what would they make of it?  Most of the articular area of the centrum is very obviously damaged, exposing the internal spongy texture of cancellous bone — presumably the bone surface was attached more firmly to the cartilaginous posterior end of the element than to the inner part of the bone, so it came away with the cartilage during simmering.  So it would be obvious to our future palaeontologists that the articular surface was missing, and that the complete vertebra would have been somewhat longer — but it would be hard to judge by how much.

But the state of this bone is particularly interesting because the middle part of the centrum does have a preserved bone surface.  It would be easy to extrapolate that out across the whole area of the posterior end of the centrum, and assume that this was the maximum posterior extent of the element’s functional length in life — an assumption that we know, having taking the neck apart ourselves, would be completely wrong.

Are we making similar incorrect assumptions with our sauropod vertebrae?

An even more interesting case is the postzygapophyses.  The posterodorsal surface of the left postzyg is slightly damaged, but the bone of the right postzyg has a nice, perfectly preserved surface.  But I can tell you that the functional articular surface of the postzyg was totally different from this: different size, different shape, different position, different orientation.  If we tried to calculate range of movement from these zygapophyseal facets, the results we got would be literally meaningless.

The good news is, there’s a clue that would prevent us from making this mistake — a really nice, obvious one.  The texture of the bone on the postzyg is irregularly crenellated in a way that strongly indicates a cartilaginous extension: it’s the same texture you see on the ends of the long-bones of (even mature) birds if you peel off the cartilage caps.  (It’s also what you see, at a much bigger scale, on the ends of the sauropod long-bones.)

But while the presence of this texture indicates the presence of cartilage, I don’t know whether the converse is true.  In the absence of such a texture, can we assume the absence of cartilage?  I just don’t know.  Anyone?

Back in early Februrary, Darren and I got an email out of the blue from biomechanics wizard and all all-round good guy John Hutchinson, saying that he’d obtained the neck of a baby giraffe — two weeks old at the time of death — and that if we wanted it, it was ours.

Of course, the timing wasn’t great for me — Brontomerus day was coming up fast, and the final publicity arrangements were buzzing around like crazy, so it wasn’t possible to go and fetch the neck right then.  But John had an even better proposition: that he could keep the neck frozen, and we could come to the Royal Veterinary College and dissect it on site.  As soon as I’d established with Darren that I’d get to be the one to keep the bones when the dissection was done, we enthusiastically agreed, and booked a date with John.  [The photo here shows a baby giraffe, not the one that we had — note that the neck is proportionally much shorter than in an adult.]

And so it was that on Wednesday 9th March, I drove up from Ruardean to Potter’s Bar and picked up Darren and pterosaur-jockey John Conway from the railway station.  From there, we found our way to the RVC campus easily enough, with only the statutory minimum number of times getting lost (once).

The bad news was that the neck had already been skinned before it made its way to the RVC.  We don’t know why, by whom, or when, and more importantly we don’t know how much of the other soft tissue was removed in the process — for example, the trachea and oesophagus were gone — along with, we assume, the recurrent laryngeal nerve that Matt had asked us to look out for — and we wonder whether our nuchal ligament was complete.  (That is the long ligament that runs along the top of the neck and helps to prevent it from sagging.)

But anyway, here is our baby, in left lateral view, as it came out of its plastic sack, measuring a healthy 51 cm in length.

Like so many specimens, at this point it really looks like an undifferentiated blob of gloop.  There are a couple of things to look for, though.

On the left of the picture, you’ll see that the terminal 10% or so is well separated from the rest, ahead of of portion of exposed bone.  That bone is the anterior margin of the axis (i.e. the second cervical vertebra).  The atlas (first cervical) is still encased in soft tissue at this point, but could be moved around fairly freely, including twisting.

On the right, and you’ll probably need to click through to see this, is a strange metal pin, stuck right into the back of C7.  This was firmly embedded and we never figured out what it was, or what it was doing there.  As you’ll see in the photos below, I’ve allowed it to stay in place, even in the final prepared vertebrae.  If anyone knows what it is, do tell!

I took a bunch of photos and measurements before we ploughed in, but I am ashamed to say that I failed to get many, many of the images and numbers that I should have.  Even allowing for the fact that the specimen was not intact when we got it, we and particularly I fumbled the ball badly.  So much so that I will shortly publish a tutorial on How To Dissect A Neck which will be based primarily on what we failed to do.

I suppose it’s true that we only ever learn from mistakes.  The trick is to learn from other people’s, rather than going through the frustrating and expensive process of making your own.  Oh well.  Next time, for sure.

Here we have John (left) and Darren (right) hard at work teasing away the long epaxial muscles from their fascia.

It was only after that process was complete that we thought to do one of the things we should have done up front — test the range of motion.  We put the necks into poses of maximal extension, flexion and lateral deflection.  Contrary to what I would have expected, the last of these was significantly more impressive than the other two, and is shown here.  You can easily make out the separate extents of vertebrae 2, 3, 4 and 5, and from those see where 1, 6 and 7 are.

(Those are the long epaxial muscles in the background.)

We continued removing muscle and fascia until we had the vertebrae as close to naked as we could manage without risking damage to them, while retaining the integrity of the intervetebral joints — both intercentral and zygapophyseal articulations.  One of the big surprises to us was how very flexible and fragile the latter were compared with the former.  The membrane that contains the zygapophyseal joint is very thin and would contribute almost no mechanical strength of its own.  By contrast, the adjacent centra were bonded very firmly together by extremely tough tissue.  There was no trace of a separate cartilage disc between any pair of centra, just this very dense but flexible material which had to be slowly cut away with scalpels before the vertebrae could be be separated.

The exception to this was the atlas-axis joint, which surprised all three of us in how completely different it was to all the others.  There was no connective tissue at all between the front of the axis and the back of the atlas — the two bones (or rather their cartilaginous surfaces) were free to move against each other without let or hindrance, as shown here (right anterolateral view with anterior towards the bottom of the picture):

And yet the axis was very firmly attached to the axis: although we couldn’t see any attachment, it wouldn’t come away — not even when a great deal of force was applied.  The connection turned out to be between the ventral face of the odontoid process and the dorsal surface of the ventral portion of the atlas.  (If you’re not familiar with anterior cervicals, this should become clearer later on when I show you the individual bones.)  Suffice it for now to say that the atlas is basically ring-shaped, and that the odontoid process is a chunk of the axis that sticks out the front of that bone and sits within the O of the atlas.

Before we separated the vertebrae, though, we prepared the nuchal ligament out from its surrounding muscle.  Here it is, with John and Darren holding its posterior portion up above the vertebrae: you can see that it’s in the form of a sheet rather than, as often envisaged, a cylinder.  (It does extend further anteriorly than shown here, but its much less extensive over C2 than it is more posteriorly.)

We did the best we could at detaching this ligament intact so that we could measure how compliant it is.  It was difficult to remove without damaging, and much more irregular in shape than we’d expected, so that the anteriormost portion had almost no strength and broke as soon as we exerted any force on it.

We were initially able to remove a portion that measured 45 cm at rest (from a total neck length of 51 cm, remember), but once the thin anterior end had broken off, we were left with 32 cm.  We were able, by application of a significant force courtesy of Darren, to extend this to 42 cm but no further.  That’s a strain of (42-32)/32 = 0.3125, which is a lot less than I’d been expecting.  Alexander (1989:64-65) wrote (in the passage that was my first ever encounter with nuchal ligaments):

I am going to suggest that these necks [i.e. those of sauropods] were supported in the same way as the necks of horses, cattle, and their relatives.  These animals have a thick ligament called the ligamentum nuchae running along the backs of their necks (figure 5.5).  Unlike most other ligaments it consists mainly of the protein elastin, which has properties very like rubber.  It can be stretched to double its initial length without breaking […]  In experiments with deer carcasses, my colleagues and I found that the ligament was 1.4 times its slack length when the head was raised to the position of figure 5.5 [i.e. a typical alert posture], and almost twice its slack length when it was lowered to the position of figure 5.5b [grazing posture].  Notice that the ligament was stretched even when the head was high: I doubt whether a deer can get into a position that allows the ligament to shorten to the point of going slack.  If you cut the ligament in a dissection the cut ends spring apart, as if you had cut a stretched rubber band.

So the least stretched life position of the ligament, according to Alexander, is significantly more extended than the most stretching we could achieve.  What does this mean?  I see four possibilities:

  • Alexander was talking a pile of poo.  I don’t believe this for a moment, and mention it only for completeness.
  • I am talking a pile of poo.  I can see why you’d think so, but I know it ain’t so (and Darren and John can verify it).
  • The composition of the nuchal ligament changes through ontogeny, becoming more elastic as the animal gets older: we had a baby, and Alexander had adults.  I don’t think this is very likely either — I can’t see any reason why juveniles would need less elastic ligaments than adults.
  • The composition of the giraffe nuchal ligament is different from that of the deer.

Since I already eliminated the first three options, it won’t come as a great surprise to find that I favour the last one.  And this has some interesting implications if it’s true.  (Darn, darn, we should have saved a chunk of the ligament and found a way to get it analysed for composition.)  If that nuchal ligament of giraffes is largely collagen rather than elastin, then it suggests the possibility of something similar for sauropods, and that would be interesting because the tensile strength of collagen is much greater than that of elastin.

Does anyone know if anyone’s done any work on this?

Well, anyway.

I drew the long straw, and got to bring the remains of the neck home to prepare out as bones.  I simmered gently, then removed the cooked flesh, and was astonished to find how much there was, removed from vertebrae that we thought we’d cleaned pretty well at dissection time:

The disappointing part of this is that such large parts of the vertebrae turned out to be cartilage (partially ossified, I guess) and so came away during the simmering: huge chunks at the front and back of each centrum, like a full centimeter at each end, and all of the zygapophyseal articular surfaces.  I wish I could have kept them intact … and of course a different preparation method probably would have done.  More stupid still, I neglected to get photos of the individual vertebrae before simmering, which would at least have enabled me to show you before-and-after comparisons.  Sorry.

Anyway, having peeled off the soft-tissue including cartilage, I re-simmered, re-picked, then bathed in dilute hydrogen peroxide for two days, and dried out the vertebrae in the sun.  This is the result — C1-C7 in order, in left lateral view:

Note that the odontoid process of the axis is a separate bone from the rest of the axis — you can see it on the left, between atlas and axis.  There was a big chunk of sculpted cartilage joining it to the rest of the atlas, and that’s all gone now, so I am not sure how I am going to join it up — maybe layer on layer of PVA representing the cartilage?

Oh, and notice that the metal pin is still in C7.

In the picture about, I have laid the vertebrae out in such a way that the total neck length (front of C1 to C7) is 51 cm, the same as it was in life.  Notice how this leaves huge gaps between the central: for example, as here between C5 and C6:

Needless to say, anyone trying to reconstruct the living animal from the bones alone — from fossils, say — would get a hopelessly wrong neck if they didn’t take the missing cartilage into effect.  As we’ve noted before, the same is true of sauropod necks.

But just how informative is a juvenile neck?  No doubt, the cartilaginous portions of these vertebrae were proportionally much larger than they would be in an adult, so we do need to be careful about casually extrapolating the huge gaps between ossified centra in the images above into our interpretation of sauropods.  For sure, I now need to go through this process with the neck of an adult giraffe — and if anyone happens to acquire one, I would love the opportunity to dissect it, please contact me if this comes up!

But maybe it’s not quite so misleading as it looks — for two reasons.  First, nearly all the sauropod specimens we have are from subadults, as shown by lack of fusion between scapula and coracoid in, for example, the Giraffatitan paralectotype HMN SII.  So it may be that their vertebrae were also not fully ossified.  And second, sauropods are more closely related to birds than to mammals, and in my limited experience bird necks seem to have a larger cartilaginous component than those of mammals.

Well.  Draw your own conclusions.  But keep ’em qualitative for now.

Next time, I’ll be presenting a tutorial on how to dissect a neck.  But it will be based on what we should have done rather than what we actually did.

Onward!

February 22, 2011

Sinclair brontosaur, Hennessey, Oklahoma. This one has cement shoes because some ne’er-do-wells ganked the old one in the middle of the night a couple of years ago.

This is so unspeakably cool. Today in PLoS Biology (yay, free reprints for everybody!), Wilson et al. (2010) describe a new snake, Sanajeh indicus, based on multiple specimens from multiple sauropod nests where they were apparently eating baby sauropods! This is sweet for loads of reasons. There aren’t that many well-documented cases of predation in the fossil record in the first place. Predation on dinosaurs by non-dinosaurs is especially cool–you may remember the announcement of Repenomamus by Hu et al. (2005), a giant (for its time and clade) badger-sized mammal from China that was found with a gut full of baby Psittacosaurus. And as Wilson et al. note, this is only the second secure association of sauropod bones with eggs; the other is the Auca Mahuevo site in Patagonia that produced the first definitive sauropod eggs and embryos. If we learn half as much about sauropod biology from these Indian nests as we have from the Patagonian ones, it’s going to be an exciting decade.

Fossils of the new snake (left), sauropod egg (upper right), and sauropod hatchling (lower right), Wilson et al. 2010, fig 1.

The best bit, though, is the window this gives us into Mesozoic ecosystems. Dinosaurs made lots of offspring, and sauropods seem to have been particularly R-selected. With loads of multiton animals producing zillions of defenseless babies for most of the Mesozoic, it would be weird if other critters, dinosaurian and otherwise, didn’t take advantage of that seasonally abundant food source. It’s great to get some direct evidence.

This is like a swamp full of radioactive awesome. Go roll around in it and let it mutate you.

References

Addendum (from Mike)

Let’s not miss the opportunity to reproduce this classic, uh, life restoration, executed pre-emptively by William Stout decades before this fossil was even found!  It’s from his 1981 book The Dinosaurs: a fantastic new view of a lost era.

Madtsoia crushes a young Laplatasaurus. By William Stout.

    Othniel Charles Marsh, who was always careful to base all of his hundreds of new taxa on the best, most diagnostic material available (Alert: Sarcasm detected!), named Pleurocoelus nanus based on a handful of junenile sauropod vertebrae centra from the Arundel clays of Maryland (Marsh 1888). Here’s the dorsal. As you can see, it is loaded with unique features like big pneumatic fossae, which at the time were only known in all other sauropods (we have since found some with less pneumaticity in the dorsals, or none at all), and the absence of a neural arch, which is shared with any sufficiently immature vertebrate.

    Here’s a cervical, which was not figured by Marsh (1888). These views are after Lull (1911:pl. 15), as modified by Wedel (2003:fig. 10); pfs stands for pneumatic fossa.

    And a sacral, again from Marsh (1888).

    To be fair, the criteria for “diagnosably distinct” in the 1880s were different than they are now. Wilson and Upchurch (2003) addressed this in their revision of Titanosaurus: as we find and describe more fossil taxa, characters that originally diagnosed small taxonomic groups (like species and genera) are often found to be more broadly distributed. For example, the original diagnosis of Titanosaurus ended up applying to almost everybody in the clade Titanosauria. It is conceivable that in the future we will discover an entire clade of xenoposeidonids with identical weird dorsals and all of their diagnostic characters elsewhere in the skeleton, and the longish list of weird characters that diagnose Xenoposeidon will turn out to be present in all xenoposeidonids. There’s not much we can do about this, other than to keep working, revisit old diagnoses from time to time and see if they need updating, and generally be nice about it.

    I am cool with not being nice about Pleurocoelus, though, because of what happened later. But that’s a story for another post.

    Note: In 2005 Carpenter and Tidwell sunk Pleurocoelus into Astrodon, which is totally cool by me, and which makes Astrodon the correct name for the poorly-known Arundel titanosauriform, just like Apatosaurus is the correct name for the Morrison diplodocine that is built like a brick outhouse. But in this series I am Telling a Tale about the Days of Yore, past tense, pre-2005, so I’m using Pleurocoelus.

    References

    Sauropod Nano

    March 5, 2008

    Every once in a while it’s good to remember that no matter how big you end up, everybody starts out small.

    baby-cam-vert-500.jpg

    Jack McIntosh came through the OMNH a few years ago and identified all of our sauropod material. There are babies of both Camarasaurus and Apatosaurus from this quarry. I have no idea if this centrum belongs to Camarasaurus or Apatosaurus–juvenile centra are not horribly diagnostic–but if Jack says it’s Cam, I’m prepared to believe him. Stuff that I used to agonize over a few years ago–like how to tell busted anterior cervicals of Brachiosaurus and Barosaurus apart–I can make out almost reflexively now. So it does not strike me as improbable that after looking at sauropod vertebrae very carefully for better than half a century, which Jack has done, this might scream Camarasaurus at an intellectual frequency to which I am currently deaf.

    It would be awesome to do morphometrics on a zillion or so sauropod bones and see whether Jack or the computer was better at making identifications. Sort of like Kasparov versus Deep Blue, but over something really important.

    I’d bet on Jack. Bigtime.

    I was going to write about mystery cervicals of the Cloverly Formation, but that requires knowing something about juvenile vertebrae and Pleurocoelus, so I decided to write about Pleurocoelus, but that still requires knowing something about juvenile vertebrae. So I’m writing this tutorial to lay the groundwork for more goodness to come.

    Sutures

    Vertebrae do not spring forth fully formed, like Athena from the mind of Zeus. They grow from bits, and the bits come together at different times in development. The bits themselves start out as anlagen–bone precursors–made of cartilage, and these anlagen start to ossify–turn into bone–at one or more ossification centers. From those centers, the bone grows outward and replaces the cartilage like some kind of science fiction blob monster taking over its host. As the bone replaces the cartilage, the contacts between different bony elements are sometimes left behind as sutures, like the sutures in the bones of your skull. Once the replacement of cartilage by bone is complete, most of the new bone growth happens at the suture margins. This is easy to demonstrate experimentally: if you cut out the suture and glue together the bones on either side, the combined element will not grow to the normal length. Premature suture closure can be a big problem, if the bones that are now fused (say, skull bones) can’t grow fast enough to keep up with whatever is inside (say, a brain). And many of the sutures between skull bones stay open even into old age, at least in humans and other mammals (birds are another story), because sutures also serve another purpose, which is to help the skull respond to mechanical stress without cracking like an egg. However, the eventual fate of most sutures is to close as the bones on either side finally fuse. Some old folks do eventually fuse up most or all of the sutures in their skulls.

    If you look across the whole skeleton, the closures of different sutures–in the vertebrae, in the long bones, in the limb girdles, in the skull–are spread out through time. This is nice if you want to determine the age at which something–or more often, someone–died (forensic anthropologists get a lot of mileage out of this). But it can also be a pain, because it means that some things are hardly ever found intact. Sauropod skulls are particularly prone to exploding. There are complete, articulated sauropod skeletons for which the skull is either scattered all over the place or simply gone.

    How Vertebrae Form

    Vertebrae form as several distinct pieces. The centrum starts from paired ossification centers on either side of the cartilaginous notochord (see Tutorials 1 and 2 if you need to brush up on vertebral anatomy). The neural arches also start out as left-and-right paired elements, each of which forms one half of the arch over the spinal cord. The left and right components of the centrum fuse very early in development, and the left and right halves of the neural arch come together later. All of these events can and do fail on occasion. The anlagen may form asymmetrically or not at all, parts may not ossify, and left-right halves can fail to fuse. The best known pathology associated with vertebral development in humans is spina bifida, in which the two halves of the arch and spine fail to unite. The growing spinal cord can stick out through the hole and cause all kinds of problems.

    I’ll not dwell long on the embryology of vertebrae; there are whole textbooks full of that stuff if you’re curious and some good websites, too. In sauropods, in all of the embryos that have been discovered to date the vertebrae are not yet ossified, so there’s nothing to talk about.

    Usually in juvenile dinosaurs you find the centrum as a single unit and the neural arch and spine as another (the exception is the atlas, the first cervical vertebra right behind the head, which is so weird that it will have to be dealt with in a separate post). The centrum and neural arch complex come together at the neurocentral sutures, a pair of zipper-like tracts of rough bone (and, in life, cartilage) on either side of the neural canal. These sutures stay open for a long time, usually until the dinos are around half-grown.

    cm-555-c4-sutures-500.jpg

    Here are a couple of cervical centra from a juvenile Apatosaurus (in this and other photos in this post, click on each picture to see the unlabeled version). They are facing left, and the giant depressions in the sides are the pneumatic cavities. The centra (C3 and C4 if you’re curious) are propped up on their oversized parapophyses, which are typical for Apatosaurus.

    cm-555-c6-with-spine-anterior-500.jpg

    Here’s C6 from the same specimen, in anterior view. For this shot I put a thin sheet of foam over the centrum so that I could put the neural arch in anatomical position and see how the whole vert (minus cervical ribs, which form separately and fuse even later) would have looked.

    cm-555-c6-with-spine-lateral-500.jpg

    Same vert in left lateral view. Compare to the first picture above to see how the rough patches on top of the centra match the shape of the corresponding patches on the neural arch.

    cm-555-cervicals-7-8-500.jpg

    Here are C7 and C8 in right lateral view. The neural arches and centra are preserved together, but the sutures are still plainly visible. Had this individual lived longer, the neural arches and centra would have grown together, and the sutures would have been gradually erased by bone remodeling.

    Stages of Neurocentral Suture Fusion

    Because neurocentral sutures close over time, they can tell us something about how mature an individual dinosaur was when it died. Each vertebra goes through several stages as the sutures close. It’s a continuous process and you could divide it into any number of stages depending on how picky you want to be, but for this case I’m going to use four:

    1. completely unfused, with the neural arch and centrum as separate pieces that come apart after death
    2. partly fused, in which the neural arch and centrum are starting to grow together but the suture is still clearly visible on the surface
    3. mostly fused, with the neural arch and spine co-ossified, but with a suture still visible as a small line or scar on the surface
    4. fully fused, with no visible trace of the suture, which has been obliterated by bone remodeling

    Forensic anthropologists usually divide vertebrae into three bins based on neurocentral suture closure: unfused (1 above), fusing (2 and 3), and fused and obliterated (4). The problem is that you can’t really tell 2 and 3 apart based on external examination; a vertebra with a visible suture might be pretty well fused or it might be held together by only a handful of tiny bars of bone. X-rays or histological sectioning can solve the problem, but usually it’s not warranted; those tests cost time and money and the human skeleton has many other and better indicators of age.

    For paleontologists the problem is even worse, because we can’t tell 1 apart from 2 or 3. A vertebra with a visible suture line might not be fused at all; the centrum and arch might just be preserved in full articulation. The c7 and C8 shown above could be in 1, 2, or 3; without cutting them up it’s probably impossible to say. I doubt that even the current generation of medical CT scanners could resolve the sutures–which are convoluted in all three dimensions and probably packed with dense matrix–well enough. If there is no trace of a suture we say that it is closed or fused, and if the suture is visible (or if the arch and spine are preserved as separate pieces, as in C6 above) we say that it is open or unfused.

    Actually there is a step 3.5 in the list above, a partly obliterated suture, in which the suture line is visible along part of its length but obliterated elsewhere. These don’t turn up very often–that is, every vertebra goes through a stage like that, but it is evidently pretty brief compared to the other stages because you don’t often come across vertebrae in this condition. Brochu (1996) showed a couple in croc vertebrae, but I’ve never seen one in a sauropod.

    There is a final complication, which is that fusion of the neurocentral sutures usually proceeds along the vertebral column like a wave. In some tetrapods in starts in the neck and goes to the tail; in some it goes from tail to neck; in some it starts in the middle and goes in both directions; and in some fusion starts in more than one place. A little work has been done on this in sauropods, but I’ll save that for another post. If you’d like to read up on it in the meantime, see Brochu (1996) and Irmis (2007).

    The Point (at last!)

    The upshot of all of this is that if you find a sauropod centrum with no arch, or vice versa, you can be sure that the animal was not fully mature. Centra are pretty close to being cylindrical, which is a good shape for surviving the ravages of taphonomy. Neural spines are not, and they fragment pretty easily. There are a lot more juvenile sauropod centra with no arches, both in the ground and in museums, than there are arches with no centra, although I have seen a couple of the latter so they do exist.

    Whew! If you made it this far, thanks for sticking around for the long anatomy slog. It’s all groundwork for talking about baby sauropod bits, so your diligence will be rewarded. Stay tuned, true believers.

    ———

    All the vertebrae shown in this post are cervicals of CM 555, from the Carnegie Museum in Pittsburgh.

    References

    • Brochu, C.A. 1996. Closure of neurocentral sutures during crocodilian ontogeny: implications for maturity assessment in fossil archosaurs. Journal of Vertebrate Paleontology 16:49-62.
    • Irmis, R.B. 2007. Axial skeleton ontogeny in the Parasuchia (Archosauria: Pseudosuchia) and its implications for ontogenetic determination in archosaurs. Journal of Vertebrate Paleontology 27:350-361.

    apato-growth-series-500.jpg

    My favorite room in the world is the big bone room at BYU’s Earth Science Museum. It is the only place on the planet that has good material of all six of the best-known Morrison sauropods: Apatosaurus, Barosaurus, Brachiosaurus, Camarasaurus, Diplodocus, and Haplocanthosaurus. So if you are looking at, say, a middle cervical of Apatosaurus and you think, “Hmm, I wonder how this looks in X,” where X is one of the other five genera listed above, you can just go look. It’s phenomenal.

    The big vert here is a posterior cervical of Apatosaurus. Those big loops on the side are formed by the diapophyses and parapophyses (sticking out from the vertebra) and the capitula and tubercula of the cervical ribs. Capitula are rib heads, and they articulate with the parapophyses, which are the lower of the two sets of rib articulations on the vertebral centrum. Tubercula are rib tubercles, and they articulate with the diapophyses, which are at the ends of the massive transverse processes sticking out sideways from the neural arch. Here’s a labeled version, just in case the verbal description made no sense. I know Mike covered this stuff back in Tutorial 2, but Apatosaurus is frankly pretty freaky in the cervical rib department.

    apato-growth-labeled-500.png

    You’ll notice that the neural spine is split down the middle, which is the case in many diplodocoids but not all of them. Bifurcated neural spines are also found in Camarasaurus, some titanosaurs, and to a lesser extent in some mamenchisaurs, so the character definitely evolved more than once. More about bifid neural spines another day…

    At last, to the point. In front of the big vert you can see a smaller one, about the size of a fist. That’s a vertebral centrum from the same part of the neck from a much smaller individual of Apatosaurus, probably somewhere between horse- and elephant-size. And that’s not all–in front of the scale bar, wrapped up in plastic, is a centrum from a wee little baby Apatosaurus about the size of poodle. Why is the vert in plastic? Because it was going off to the micro-CT scanner at the University of Utah, which is in a cleanroom, so all specimens have to be hermetically sealed. I didn’t think to shoot this little growth series lineup until the vert was already bagged, and I haven’t been back since to set it up again.

    Alas, the baby Apatosaurus vert and the CTs of its internal structure will also have to wait for another day. We’re such teases…