How long was the neck of Diplodocus?

May 19, 2011

In our recent paper on how the long necks of sauropods did not evolve primarily due to sexual selection (Taylor et al. 2011), one of the ideas we discussed is that sexual dimorphism between the necks of male and female sauropods would be an indicator of sexual selection.  And, rather despairingly, we wrote (page 4):

As Senter himself recognized, available samples of sauropod taxa are unfortunately not large enough to demonstrate bimodal distribution of morphological features within any sauropod species.

But I wonder if we realise just how true this is, and how blind we are flying?  How very far short we are of being able to do any kind of statistical analysis on sauropod necks.

How many complete necks of a given sauropod would we need in order to demonstrate a bimodal distribution of, say, length?  (That is, to show that the necks mostly fall into two separate buckets, a short-necked group and a long-necked group of which one is presumably male and the other female.)  I don’t know enough about stats, but this article at least suggests that you’d need thirty or so before you could be confident that you were seeing something statistically significant.

And how many sauropod species do we have thirty complete necks for?

Correct: none.

All right, then how many do we have ten complete necks for?

Five complete necks?

OK, how about just two necks?

ONE neck?

The answer is: not many species.  Off the top of my head, I think complete necks are known for Camarasaurus lentus (Gilmore 1925, one specimen), Mamenchisaurus hochuanensis (Young and Zhao 1972, one specimen), Shunosaurus lii (e.g. Zhang et al. 1984; probably multiple specimens but the paper is in Chinese so I don’t know for sure) Mamenchisaurus youngi (Ouyang and Ye 2002, one specimen, I think), and Spinophorosaurus nigerensis (Remes et al. 2009, one specimen).

No doubt I have missed some, but the point is that the total number of sauropods for which even one complete neck is known is a tiny, tiny proportion of all the sauropods that have been named.  I have listed five species here, and of those only one is known from more than a single complete neck.  And those multiple specimens have not been described (have they?)  So while in theory it might be possible to determine whether there is a bimodal distribution in the length of Shunosaurus lii necks, the data doesn’t exist to do this work.  (If there really are enough complete necks then someone ought to get out to China and measure those babies.)

So anyway.  We have very, very few complete sauropod necks.

Diplodocus carnegii

“But Mike!”, I hear you cry; “What about Diplodocus carnegii?  We’ve all seen its skeleton in a half-dozen different museums!”

Oh yes.  Here is its “complete” neck, from Hatcher (1901:plate 8):

Let’s, for now, ignore the fact that the scapula seems to articulate with the base of the neck rather than the torso.  We can all see that there are fifteen cervical vertebrae, right?

Right?

Well, let’s see what Uncle J. Bell had to say (Hatcher 1901:4):

[Diplodocus carnegii holotype CM 84] has been entirely freed from the matrix and is found to consist of the right femur and pelvis complete except for the left ilium, which is for the most part wanting, right scapula and coracoid, two sternals, eighteen ribs and forty-one vertebrae divided as follows: fourteen cervicals including the axis, eleven dorsals, four sacrals, and twelve caudals.  These vertebrae are for the most part fairly complete, though unfortunately the sacrals and anterior cervicals are more or less injured.  This series of forty-one vertebrae are believed to pertain to one individual and to form an unbroken series from the axis to the twelfth caudal, although as was shown in a previous paper, there is some evidence that there are perhaps one or more interruptions in the series and that one or more vertebrae are missing.  On the other hand, as will appear later, it is not entirely impossible that at least one vertebra of this supposed series pertains to a second individual belonging perhaps to a distinct genus.

Oh and there’s this, from page 10:

Unfortunately no diagram of the quarry was made, at the time of exhuming the remains, showing the relative position of each of the several vertebrae and other bones as they lay in the rock.  [Plate 1 is a map of the quarry as remembered by W. H. Reed.]

Hey!  That’s not what it said in the brochure!  So, as it turns out, our conclusion is: Diplodocus carnegii had fifteen cervicals, or more, or maybe less.

Giraffatitan brancai

“Well, then, Mike, how about that awesome mounted Giraffatitan skeleton in the Berlin museum?”

Well, the presacral vertebrae of that mount are not real bone, nor even casts, but they are very good sculptures based on real bones.  However, the real bones that they’re based on are those of two specimens — the lectotype SI and paralectotype SII.  The former includes cervicals 2-7, and we can be confident about that because C2 in sauropods is very distinctive, having a completely different anterior articular surface from all the other cervicals; and the latter includes cervicals 3-13 (although many of them are damaged).

But but but.  SI and SII were smushed up and mixed in together, with little articulation.  Any reconstruction — or even assignment of individual vertebrae to one specimen or the other — has to be considered provisional.  Take a look at this quarry map, from Heinrich (1999:fig. 16):

Yeesh, what a mess.  I’ve previously suggested (Taylor 2009:800-801) that the distinctively high-spined dorsal vertebra usually considered the fourth of SII may not actually belong to that specimen, or even that taxon — that it may belong to a more Archbishop-like animal (which may be what SI is).  Janensch (1950:33) says that things are not so bad for the cervical vertebrae, but still not good:

The vertebrae from the 3rd to 15th presacrals [of SII] lay in articulation in a consolidated lime sandstone lens; of them, the 3rd to 5th vertebrae are tolerably complete, the remaining 10 vertebrae were articulated with one another, with one interruption that arose when the 8th presacral vertebra rotated out of the series and was displaced.

So might there have been other displaced cervicals, before and/or after the “8th”, that were not recovered?  And can we be confident that the anteriormost cervical of SII really is C3?  Why?  Because of the overlap with vertebrae of SI?  But we’re not even certain that SI is the same species as SII.  Maybe the anteriormost preserved cervical is really C4?  Maybe some of the “SII” cervicals really belong to SI?

So all in all, our conclusion is: Giraffatitan brancai had thirteen cervicals, or more, or maybe less.

What does it all mean?

Only this: we don’t know as much as we think we do.  We don’t know how many cervical vertebrae Diplodocus and Giraffatitan had, even.  We don’t have complete necks for either of these sauropods, nor for almost any others.  Even those we do have are in some cases badly crushed (e.g. Mamenchisaurus hochuanensis, which I must post about properly some time).  To summarise: we are woefully short of sauropod necks.

We need to get out of the habit of blithely asserting, “oh, Diplodocus had 15 cervicals and Giraffatitan only 13″.  Because we really don’t know this.  We think it’s true: these numbers are certainly the best guesses for the taxa in question.  But they are, in the end, only guesses.

References

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12 Responses to “How long was the neck of Diplodocus?”

  1. Heinrich Mallison Says:

    Mike, have you talked to Kirby Siber about this? The Sauriermuseum Aathal has a wonderful Diplodocus neck, but I know nothing about how it was found.

  2. Nathan Myers Says:

    Just think how few cervicals we would have if their necks were much shorter. Of course, we wouldn’t care so much, then.

  3. Dean Says:

    If only those blasted sauropods had interred their dead in coffins!

  4. Nima Says:

    This sucks, so basically everything we “know” about most sauropod necks is speculative. And Hatcher’s diagram of D. carnegii is so good at making it all flow together like one species.

    At least for Diplodocus, there are some pretty complete specimens that could solve the mystery – problem is they’re in the hands of those private fossil hunters who invented “Amphicoelias Brontodiplodocus”.

    How much of the HMN Tendaguru fossil material still remains to be unpacked? There could be some clues to Giraffatitan in there.

  5. Mike Taylor Says:

    Yes, a great deal of what we “know” is educated guesswork. Doesn’t mean it’s necessarily wrong, of course, but more caution is definitely warranted. It’s certainly true that the sheer artistry of skeletal reconstructions can give the impression of much more certainty than we really have: even when they (rightly) shade out the bones that aren’t known, they can still hardly convey information such as “this vertebra was found rotated out of line with the others” or “there may have been additional vertebrae between these two”, For that, you have to read the fine print.

    Yes, there are probably better and more complete, articulated Diplodocus skeletons out there; but if there are, they’ve not been described so they’re not really available to us.

    I think pretty much all the HMN’s Tendaguru sauropod material was unpacked long ago. It’s true that they still have plenty of unopened barrels of material from there, but IIRC it doesn’t contain any more sauropod bits. Maybe Heinrich can comment (he knows that museum much better than I do), or Daniela if she’s here.

  6. Bruce J. Mohn Says:

    I’m guessing the Gilmore paper is on the juvenile Camarasaurus at Carnegie? There is also an in-situ mount of a sub-adult Camarasaurus at the National Museum of Natural History, Smithsonian Institutes. I could be wrong, but the cervical series on that seemed to be reasonably complete.

  7. Heinrich Mallison Says:

    the Berlin Tendaguru sauropod stuff is all unpacked. The drums contain dryosaur material, and some boxes contain isolated verts. No mystery neck there :(

  8. Mark Robinson Says:

    Thanks, Mike – another misapprehension of mine busted. I remember as a kid being certain that Diplodocus was 87½ feet long and being shocked when a new (better) book “got it wrong”.

    Then I found out that some very famous mounted skeletons were assembled from two or more individuals, or even had some made-up bits! Later still, I was amazed that some moderately well-known dinosaurs were known from only (say) a humerus, a couple of vertebrae, and maybe a broken rib.

    Now I’m informed that the numbers of cervical vertebrae in two of the best known sauropod genera are educated guesses rather than established facts.

    What’s next – sauropods didn’t really live in swamps? Oh wait…

  9. Becca Says:

    Do any foetuses in egg exist for any of these guys? If they do would there be a way to scan (MRI?) them and count vertebrae? Eggs are handy for keeping all the bits together…

  10. Matt Wedel Says:

    There are embryos for a couple of titanosaurs. Unfortunately, at least the South American ones have no ossified vertebrae. The limbs and skull bits are there, but the verts must still have been cartilaginous at the time of death. Bummer. There’s something distinctly unholy about having the skull for a sauropod but no vertebrae.

  11. Nima Says:

    “But we’re not even certain that SI is the same species as SII.”

    As I now look at both more closely, this sentence rings ever more poignantly.

    Janensch’s papers show some juvenile Giraffatitan neck verts have proportions (and neural spine slopes) far more like SII than the more truncated SI. They look like little copies of SII. Comparing all the published “Giraffatitan” cervicals, it looks increasingly like SI is the odd man out.

    Reference: Janensch, 1950. The Axial Skeleton of Brachiosaurus brancai. p. 38, plate 12 (juvenile cervical C2, specimen HMN-be 1), and supplement VII, Table 1, plates 1-8 (plate 1 is apparently the same vertebra as HMN-be 1). All of these juvenile remains have very elongated centra, and thus resemble SII far more than SI. The gently sloping neural spine in that second cervical, though incomplete, looks very different from the shark-fin neural spine of SI.

    Also one of the cervicals attributed to SI may actually be from another individual far more like SII: see p. 41, plate 26


  12. [...] a bit o’ fair-and-balanced, remember that this neck may not be complete, and that some of the neural spines are [...]


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