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.
- Lehman, Thomas M. and Alan B. Coulson. 2002. A juvenile specimen of the sauropod dinosaur Alamosaurus sanjuanensis from the Upper Cretaceous of Big Bend National Park, Texas. Journal of Paleontology 76(1):156-172.
- Powell, Jaime E. 1992. Osteología de Saltasaurus loricatus (Sauropoda-Titanosauridae) del Cretácico Superior del Noroeste Argentino. pp. 165-230 in: J. L. Sanz and A. D. Buscalioni (eds), Los Dinosaurios y su Entorno Biotico. Actas del Segundo Curso de Paleontologia en Cuenca. Instituto Juan de Valdés, Ayuntamiento de Cuenca. 397 pages.
- Taylor, Michael P., Mathew J. Wedel and Richard L. Cifelli. 2011. A new sauropod dinosaur from the Lower Cretaceous Cedar Mountain Formation, Utah, USA. Acta Palaeontologica Polonica 56(1):75-98. doi: 10.4202/app.2010.0073