Reconstructing the ilium of Brontomerus

March 1, 2011

Let’s look a bit more closely at the holotype element of Brontomerus mcintoshi, which as we all remember is the juvenile left ilium OMNH 66430.  Much of what we’ve said about Brontomerus is based on the shape of that ilium, so it’s important to get right.  Several commentators have expressed skepticism about how we reconstructed, so I thought it would be worth taking the time to explain why we put it together we way we did.

First, let’s orient ourselves.  Here is the torso from the skeletal inventory that was Figure 1 of the paper (Taylor et al. 2011, natch).  In this version, I’ve highlighted the ilium in red.  We’re looking at the left side of the animal, so the main part of the bone is further forward than the hip socket, towards the animal’s head.

As you’ll see from the area that we left shaded grey, a chunk is missing from the middle of the ilium, where it was damaged in the field.  As the figure of the ilium in the paper shows clearly, what we actually saw in the OMNH collection was three chunks of bone: a big one consisting of the acetacular margin, pubic and ischiadic peduncles and most of the preacetabular blade; and two smaller fragments, each contributing part of the dorsal or posterior margin.

We spent a while in the OMNH collection playing with the three chunks to see how they best fit together.  In doing this with the actual bones, we were able to take account of their curvature in the third dimension, which our figure don’t show — although a dorsal-view photo gives some idea.

Anyway, we this is what we came up with:

(Sorry if that image is getting a bit overfamiliar, but it’s worth seeing again in the context of this post.)

You’ll remember from the Clearing the Air post that Jim Kirkland, who excavated the ilium, felt that we’d got the two smaller fragments in the wrong places relative to the main chunk, and also that a fourth fragment which we’d missed also belongs to the ilium.  He kindly sent a photo of how he’d reconstructed the ilium, and I used the arrangement of pieces in the photo as the basis for a “what if” alternative reconstruction.

So far, this is old news.  But what was maybe not quite clear in the post is how very similar the two reconstructions really are.  Let’s fix that: here they are side by side, with ours on the left and Jim’s on the right:

It seems pretty clear that even if Jim’s arrangement is correct (which Rich Cifelli  disputes), that doesn’t affect the reconstruction in any significant way.

But the real question is why we put in that dotted line — and why we put it where we did.  How do we know there wasn’t a normal-sized postacetabular lobe sticking out behind?  This is what Jamie Headden wanted to know in an email to me shortly after the paper come out.  With his kind permission, I reproduce the illustration that he prepared, showing (A) the reconstruction from the paper, and (B) how it might have been different:

The reason we rejected a reconstruction like the one in Jaime’s part B is explained (too) briefly in the paper (pp. 80-81):

The postacetabular lobe is reduced almost to the point of absence […]  The ischiadic peduncle is reduced to a very low ventral projection from almost the most posterior point of the ilium. The near absence of the ischiadic peduncle cannot be attributed to damage as the iliac articular surface is preserved. Immediately posterodorsal to this surface is a subtle notch between the peduncle and the very reduced postacetabular lobe. This notch and the areas either side of it are composed of finished bone, demonstrating that the great reduction of the postacetabular lobe, too, is a genuine osteological feature and not due to damage.

To my lasting annoyance, I didn’t take any posterior-view photos of the ilium back in 2007, so I can’t show you this finished bone as well as I’d like — this was back before I’d learned all my lessons on how to photograph bones.  But here is a close-up of the posterovental extremity of the ilium, again from Fig. 2, showing the notch: I have left the postacetacular lobe in colour, and desaturated the ischiadic peduncle — the notch is between them.

This next photograph of the ilium, again in lateral view, is lit rather differently from the one we used in the figure, so that you can see a distinct shadow lying along the valley between the ischiadic peduncle and what there is of the postacetabular blade.

Here’s one that shows the main chunk of the ilium in anteromedial view: from here, you can more easily see the the distinction between the ischial peduncle (which projects towards the camera) and the preserved, ventralmost, part of postacetacular blade, which is further back.

And one in posteroventral view: this is similar to our Fig. 2b, but from a slightly more posterior (and medial) perspective, so that you can more easily see the mediolaterally compressed posterior lobe sticking out behind the broader ischial peduncle at top right:

What all these photos unfortunately do not show is the finished nature of the bone on the posterior margin of the postacetacular blade — on that, you just have to take our word.

But the point is this: we have the whole of the ischiadic peduncle and the ventralmost part of the postacetacular blade — we know that the posteriormost preserved part of the main chunk of ilium is not part of the peduncle (so that the postacetabular blade is missing), but that this really is the blade itself.  And because the bone is not broken, we know that the trajectory of the posterior margin of the postacetabular blade was directed dorsally from the posterior point of the peduncle.

I hope that’s clear.  What I really should have done, of course, was take my own good advice and get photos from every angle — and, ideally, pairs that would have allowed me to show the relevant features as anaglyphs.

Anyway, all this shows that the shape of the ilium really was pretty much as we reconstructed it — and, most, importantly, that the bizarre proportions we reported in Table 4 are correct: preacetabular blade, measured parallel to the longest axis of the ilium equal to 55% of total length; postacetabular blade equal to 0%.

Exactly how strange is this almost non-existent postacetabular blade?  In the paper we described it as “remarkable”, but it’s not completely unprecedented.  Lehman and Coulson (2002:fig. 8) showed the left ilia of six somphospondylians:

As you can see, the Euhelopus zdanskyi and Saltasaurus loricatus ilia both lack postacetabular blades (although Powell 1992:fig. 18 suggests that the posterior portion of the Saltasaurus ilium may be broken).  Where Brontomerus is unique is in the combination of this postacetabular reduction with the enormous preacetabular blade.

All clear?  Good.

“But wait!”, I hear you cry.  “That ilium is juvenile!  How do you know that its strange shape is not a juvenile feature?”

Stay tuned!  All will be revealed.

References

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13 Responses to “Reconstructing the ilium of Brontomerus

  1. Nathan Myers Says:

    New revelations of your gimp-fu continue to impress me. (But “orientate”? What are you, a colonist?)

    I, for one, would welcome an anatomical primer showing the role of the ilium in locomotion, and assumed muscular articulation. A link would suffice, except that I know you guys could do it better than anyone else has done. Extra appreciation for corresponding or integrated treatment of the pubis.


  2. Mike, awesome explanation. Pretty clear, then atn this point, and I appreciate you taking the time responding to my emailed questions in this highly graphic and fundamental way. At this point, you’ve convinced me that the postacetabular ala is as you illustrate in the paper. Thank you.

    One further question, though: Why didn’t you, in your young and hedonistic days a few years back on thsi, did you not sample any histological or surficial features? With the fractures in the bones, histology would have been barely invasive.

    Nonetheless, the response on the juvenile issue will be interesting, I hope. The proportions at least come VERY close to saltasaurine titanosaurs, but the shape still compares strongly with the opisthopubic titanosauriforms (“brachiosaurids” et al.).

  3. Paul Barrett Says:

    It’s a bit disingenuous to state that Jim Kirkland’s reconstruction would not have an effect on your conclusions: if Jim is correct, this leaves a much larger gap around the caudal margin of the ilium, which would leave ample room for a postacetabular process, which could be separated from the ischial peduncle by a notch. Leaving this aside, as I am willing to accept Rich Cifelli’s reasons for your reconstruction, although the posterior part of the ischial peduncle does seem to be finished, this finished region is very short, and I still think it’s plausible you could end up with a process extending caudally from the point of the break (i.e. from the posterodorsal corner of the preserved posteroventral part of the ilium). The overall shape of the ilium would look rather similar to that of Alamosaurus in this case (the type of Brontomerus already looks suspiciously similar to what a broken version of an Alamosaurus ilium might look like).

  4. Mike Taylor Says:

    Jaime, thanks for that, I am glad that we finally managed to explain the reasoning clearly! As for histology — why, specifically? I mean, that are any number of techniques we could have used, including CT-scanning and SEM, but we didn’t have a particular motivation to use any of them, and none of us has the time or money to blindly apply every possible technique to every fossil.

    Hi, Paul, thanks for chipping in. I’m a bit nonplussed by your comment “… would leave ample room for a postacetabular process, which could be separated from the ischial peduncle by a notch.” The whole of the second half of this post is about showing that we have the notch, and that we therefore know what the trajectory of the ventralmost part of the postacetabular margin was. Unless you’re suggesting a two-lobed postacetabular blade — which would be totally unprecedented — I’m not sure what you have in mind.

    Yes, in lateral view the Brontomerus ilium looks less unlike the Alamosaurus illustration of Lehman and Colson (2002:fig. 8) than any other sauropod ilium I’ve seen (though of course it lacks the distinctive titanosaurian flaring that would be visible in other aspects). Unfortunately, I’ve not been able to find a better Alamosaurus ilium figure than L&C’s, despite having gone through more than a dozen relevant papers. Most promising is Plate X, part 3a, from Lawson’s (1972) unpublished MSc thesis, but unfortunately my copy only has the even-numbered pages so that plate is missing. Do you have a copy?

    (And note, of course, that the latest Cretaceous age of Alamosaurus separates it from Brontomerus by about 40 million years.)


  5. […] in response to Paul Barrett’s comment on a subsequent article, here is a superimposition of the ilium of Alamosaurus on that of […]

  6. Mike Taylor Says:

    Inspired by Paul’s comment on the similarity between the ilia of Brontomerus and Alamosaurus, I added a fourth composite to the previous post, showing these two ilia superimposed.

    Turns out, they’re not so similar as we thought.


  7. Mike,

    Close photography of the material, under a simple microscope, can reveal surface texture. It was, after all, part of your finding of “finished bone texture” (if this is a proxy at all for the external fundamental system, at least). This observation can be photographed without any additional funds by separating a basic lab scope from its base, plopping it to relevant surfaces, and applying a camera to the viewfinder.

    Note also that my argument on shape was based primarily on the lower, larger portion of the ilium, and had nothing to do with the lose fragments. Moving those fragments about, and placing space around their margins (as they seem to not actually be easily connected to one another and you admitted earlier you can easily switch their places, primarily influenced by their thinness) allows the dorsal margin to elongate, and distorts (and somewhat shortens) the intervening space with the large portion of the ilium. Not that this matters, as it doesn’t influence the features of the ilium that are apparently characteristic.

  8. Mike Taylor Says:

    Yes, we should have got better photographs! I didn’t realise that was all you meant by “sampling histological features”.

    But, no, by speaking of “finished bone”, we certainly don’t mean to claim that there is an EFS — the ilium is from juvenile, after all, and in fact very few “adult” dinosaurs have EFSs because such a small proportion of our specimens are fully grown — e.g. HMN SII and FMNH P25107 both have unfused scapulocoracoids.

    The point about the ilium shape — and I am kicking myself now for not having illustrated it — is that the main chunk tells us both the posteriorly directed trajectory of the dorsal border of the preacetabular blade and the dorsally directed trajectory of the posterior border of the postacetacular blade. And between them they pretty much constrain the shape of the missing part of the margin, which is always a more or less smooth curve in sauropods. So what we effectively did was fit the other two fragments TO that curve rather than relying on the to TELL us the curve.

  9. Paul Barrett Says:

    Hi Mike,

    I’m not sure that the notch you have preserved is the same as the notch that separates the ischial peduncle from the postacetabular process. It looks like more of a groove on the peduncle…

    Cheers, Paul

  10. Mike Taylor Says:

    Paul, take a closer look at the last two photos: from these anteromedial and posteroventral you can see that there is a distinction between the transversely broad ischiadic peduncle and the mediolaterally compressed postacetacular blade. So all of the former, and part of the latter, is preserved.

    Augusto, the point with Brontomerus is not just that its preacetacular blade is absolutely large, but that is is large compared with related animals. As you say, most birds have elongate ilia, but their whole body structure is dramatically different from that of sauropods. But Brontomerus, so far as we know, built pretty much like a vanilla sauropod — except for this one aberration.

    And don’t waste your sympathy on the “unfortunate theropod”. He had it coming.

  11. Dean Says:

    Speaking of Alamosaurus, check out the Paleo king’s most recent blog. A new super-Sized vert has been discovered!!!

  12. kattato Garu Says:

    Hi Mike – re. “Brontomerus, so far as we know, [was] built pretty much like a vanilla sauropod — except for this one aberration”. If your inferences about the pelvic girdle and shoulders are correct, isn’t there a fairly significant possibility that the appendicular skeleton might have been significantly different from ‘vanilla sauropods’? And if so – well, the neck and head might well have been pretty weird too. There’s just no way of knowing until you find more bones. Good luck!

  13. Mike Taylor Says:

    Kattato, yes, it’s possible (as we mention in the paper) that, for example, the legs of Brontomerus were unusually long for a sauropod. But that would still be a variation on the sauropod body shape. It’s not the kind of difference that can be meaningfully compared to the radical anatomical differences between sauropods and birds.


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