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

• Gilmore, Charles W.  1925.  A nearly complete articulated skeleton of Camarasaurus, a saurischian dinosaur from the Dinosaur National Monument, Utah.  Memoirs of the Carnegie Museum 10:347-384.
• Hatcher, Jonathan B.  1901.  Diplodocus (Marsh): its osteology, taxonomy and probable habits, with a restoration of the skeleton. Memoirs of the Carnegie Museum 1:1-63 and plates I-XIII.
• Heinrich, Wolf-Dieter.  1999.  The taphonomy of dinosaurs from the Upper Jurassic of Tendaguru, Tanzania (East Africa), based on field sketches of the German Tendaguru expedition (1909-1913).  Mitteilungen aus dem Museum fur Naturkunde in Berlin, Geowissenschaftliche Reihe 2:25-61.
• Janensch, Werner.  1950.  Die Wirbelsaule von Brachiosaurus brancai.  Palaeontographica (Suppl. 7) 3:27-93.
• Ouyang Hui and Ye Yong.  2002.  The first mamenchisaurian skeleton with complete skull: Mamenchisaurus youngi.  111 pages + 20 plates. Sichuan Science and Technology Press, Chengdu.
• Remes, Kristian, Francisco Ortega, Ignacio Fierro, Ulrich Joger, Ralf Kosma, Jose Manuel Marin Ferrer, for the Project PALDES, for the Niger Project SNHM, Oumarou Amadou Ide, and Abdoulaye Maga.  2009.  A new basal sauropod dinosaur from the Middle Jurassic of Niger and the early evolution of Sauropoda.  PLoS ONE 4(9):e6924. doi:10.1371/journal.pone.0006924
• Taylor, Michael P.  2009.  A re-evaluation of Brachiosaurus altithorax Riggs 1903 (Dinosauria, Sauropoda) and its generic separation from Giraffatitan brancai (Janensch 1914).  Journal of Vertebrate Paleontology 29(3):787-806.
• Taylor, Michael P., David W. E. Hone, Mathew J. Wedel and Darren Naish. 2011. The long necks of sauropods did not evolve primarily through sexual selection. Journal of Zoology, Early View (Online Version of Record published before inclusion in an issue). doi: 10.1111/j.1469-7998.2011.00824.x
• Young, Chung-Chien, and X. Zhao.  1972.  [Chinese title.  Paper is a description of the type material of Mamenchisaurus hochuanensis]. Institute of Vertebrate Paleontology and Paleoanthropology Monograph Series I, 8:1-30.
• Zhang Y., Yang D. and Peng G.  1984.  New materials of Shunosaurus from Middle Jurassic of Dashanpu, Zigong, Sichuan, Journal of Chengdu College of Geology (suppl. 2):1-12.