What’s up with the Brachiosaurus coracoid?

January 25, 2010

In my not-long-quite-so-recent-any-more paper on Brachiosaurus and Giraffatitan, I gave as one of the autapomorphies of Brachiosaurus proper that the glenoid articular surface of its coracoid is laterally deflected.  Although we’ve discussed this a little in comments on SV-POW!, it’s not yet made it into one of our actual articles.  I hestitated to feature it here since it’s so darned appendicular, but in the end I concluded that it was too interesting and potentially important to overlook.

So here it is!

Brachiosaurus altithorax holotype FMNH P25107, left coracoid in lateral, posterior and ventral views (oriented as though the scapular blade were horizontal). Modified and composed from photographs by Phil Mannion; used with permission.

The deflected surface is most apparent in the posterior view at the right of the fiigure, in which it appears deflected about 55 degrees from the horizontal.  That’s misleading, though — partly because the shape is more complex in three dimensions than can be easily visualised from these orthogonal shots, and partly because of course the coracoid was not held perfectly vertical in life.  In fact, the orientation of the coracoid in sauropods, and of the entire shoulder girdle, remains rather controversial.  It’s not an area I’ve got involved in so far, but this Mystery Coracoid Of Weirdness (hereafter MCOW) might just be my gateway into the wacky world of pectoral girdles.

The ventral view at the bottom of the figure is also informative: as you can see from that angle, the articular surface extends a long way laterally (i.e. towards the top of the figure  in this orientation).  Once you’ve got your eye in with those images, it’s easy to see the facet in the lateral-view photo, despite the less than ideal saturated lighting: it’s shaped like a raindrop falling towards bottom left.  (Well, not really: raindrops are actually vertically flattened spheroids rather then raindrop-shaped, but that’s not the point.)

Observations and interpretations on this oddity will be very welcome.

Finally, here is your regularly scheduled sauropod vertebra:

Brachiosauridae incertae sedis NHM R5937 "The Archbishop", cervical S. Top to bottom: left lateral; dorsal with anterior to right; posterior, right lateral and anterior. Images copyright the NHM since it's their specimen.

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14 Responses to “What’s up with the Brachiosaurus coracoid?”

  1. ScottE Says:

    Cool.

    (Incidentally, you could maybe say teardrop instead of raindrop.)

  2. Alessandro Chiarenza Says:

    Is that “raindrop” present in other specimens?

  3. Jamie Stearns Says:

    And this is why I lamented the fact that the scapulocoracoid illustrated for Ultrasauros was not described as the holotype of that genus. As Taylor (2009) pointed out, it lacks the weird deflected glenoid seen in Brachiosaurus, and thus the name could have persisted based on this difference.

    Still, the differences would suggest that the Dry Mesa brachiosaur is not Brachiosaurus and thus we have more than one brachiosaur running (ok, lumbering) around in the Morrison.

  4. Michael O. Erickson Says:

    In fact, the orientation of the coracoid in sauropods, and of the entire shoulder girdle, remains rather controversial.

    Some thoughts on this issue: A vertically-oriented scapulocorocoid is nearly universal among the extant Reptilia, with never more than a slight posterior inclination being present, and even that seems to be rare. Here fallow some examples to help demonstrate my point.

    Alligator:

    (Specimen of the fossil species A. prenasalis, showing the same orientation of the scapulocorocoid)

    Crocodylus:

    Chameleo:

    Chameleon Skeleton

    Iguana:

    Varanus:

    Monitor lizard skeleton

    To my knowledge, such application of shoulder girdle data from extant reptiles has been applied to sauropods only once, by Tornier in his otherwise-terrible (I’m referring to the sprawling pose – the horror, THE HORROR!) reconstruction of Diplodocus:

    One thing of interest is that if the shoulder girdle of a diplodocid (or any sauropod with forelimbs shorter than the hind) is positioned so that the scapulocorcoid is vertical or nearly so, this raises the anterior portion of the body to the point that the back is almost perfectly level, rather than sloping down. Wouldn’t this make more sense for a high-browsing animal? (Assuming Diplodocus and kin were high-browsing animals, as indicated by the raised posture of the neck; Taylor et al. 2009).

    So in conclusion, given how widespread this condition (a nearly-to-fully vertical scapulocorocoid) is among Recent reptiles, would it be reasonable to infer this position in sauropods? I don’t know if such a pose results in any major anatomical violations – does anyone with a greater knowledge of sauropod scapulocorcoids than myself wish to chime in?

  5. Jamie Stearns Says:

    Problem is, though, that no extant reptile walks fully upright. An upright posture would almost certainly require a different position of the shoulder girdle than a sprawling one would. That’s why Tornier’s monstrosity is the only sauropod restoration with a vertical scapulocoracoid; it’s the only restoration of a sauropod in a sprawling posture.

  6. Bruce Schumacher Says:

    Agree with Jamie. A vertical shoulder blade would mean posterior inclination of the glenoid and thus posterior slope of the humerus as well, effectively lowering the front quarters rather than raising.
    Don’t see how this system could work for a large ‘graviportal’ animal.
    Interesting to see all the photos of modern reptilians with vertical scap-cors.

  7. Michael O. Erickson Says:

    Problem is, though, that no extant reptile walks fully upright. An upright posture would almost certainly require a different position of the shoulder girdle than a sprawling one would.

    Chameleons do not sprawl; their forelimbs work in a nearly parasagittal plane, yet the vertical scapulocorocoid persists. It seems to me that if a different orientation of the scapulocorocoid is essential for an upright limb, then chameleons should show some (at least mild) scapular reoreintation – but they don’t.

    My point is not that I necessarily beleive that sauropods possesed vertical scapulocorcoids (I’m neutral as of right now); rather, it is that I fail to see how the parasagittal forelimbs of sauropods would demand a scapulocorocoid oriention that strays significantly from the reptilian norm.

    That’s why Tornier’s monstrosity is the only sauropod restoration with a vertical scapulocoracoid; it’s the only restoration of a sauropod in a sprawling posture.

    Did some Googling, and it turns out that I was wrong – Tornier’s hideous creation isn’t the only sauropod reconstruction showing a vertical scapulocorcoid, after all:

    http://www.plosone.org/article/showImageLarge.action?uri=info%3Adoi%2F10.1371%2Fjournal.pone.0006924.g005

    (Skeletal reconstruction of Spinophorosaurus, from the PLoS ONE paper.)

  8. Mike Taylor Says:

    Alessandro Chiarenza asked: “Is that “raindrop” present in other specimens?”

    What other specimens? Sauropod workers labour under the chronic problem of lack of specimens: while invertebrate palaeontologists often have big enough samples of their taxa to do all kinds of statistical analysis, it’s not uncommon for large vertebrates to be known from a single specimen — and this is, effectively, the case with Brachiosaurus. My 2009 paper discusses the relatively small amount of referred material and concludes that none of it can be reliably inferred to be belong to Brachiosaurus, which remains known only from Riggs’s (1903) holotype, FMNH P25107. Sad but true.

  9. Nathan Myers Says:

    I can’t help with scapulocoracoid orientation, but “Orthogonal Shorts” would make a good band name.

    [Mike writes: thanks, Nathan, I fixed the amusing typo.]

  10. Mike Taylor Says:

    Thanks, Michael, for tracking down all those extant-reptile scapula photos — very illuminating. You are right that the scapula of sauropods has not often been reconstructed as vertical, and the Spinophorosaurus reconstruction you linked to shows why: as illustrated, the scapular glenoid surface is sustaining the entire ground-reaction force of the forelimb, and that of the coracoid is completely unused. Mechanically, this doesn’t make much sense of a big, heavy, graviportal animal like a sauropod. (Chameleons and suchlike can get away with it because they are so tiny; even crocs and monitors are rather negligible in the scheme of things, so mechanical issues are not so constraining for them.)

    That said, there has been a major study in recent times that advocates a vertical or near-vertical scapula in sauropods (Schwarz et al. 2007), which to my shame I have not yet read in the detail it deserves, so I can’t tell you how convincing I found it. I can tell you that it relies, in part, on the scapular orientation in extant reptiles.

    REFERENCE

    Schwarz, Daniela, Eberhard Frey and Christian A. Meyer. 2007. Novel reconstruction of the orientation of the pectoral girdle in sauropods. The Anatomical Record 290:32-47. doi:10.1002/ar.a.20405

  11. Michael O. Erickson Says:

    That’s very interesting, Mike! I had no clue that this vertical scapulocorocoid concept had ever been discussed in the technical literature (independent of the Spinophorosaurus skeletal reconstruction and Tornier’s sprawling leviathan). I’ll definately have to try to track down a copy of Schwarz et al. 2007.

  12. Andreas Johansson Says:

    Shouldn’t there be a “stinkin’ appendicular skeleton” tag here somewhere?

    [Mike writes: excellent point, thanks. Now fixed.]


  13. […] (This is of course the same vertebra that we last saw in a multi-view composite figure at the end of the Brachiosaurus coracoid post.) […]


  14. […] — in particular, with the Archbishop bones, as for example “Cervical S” in the Brachiosaurus coracoid […]


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