Monthly Archives: December 2023

The ribs of Brachiosaurus: stranger than you thought

As we’ve often observed, it’s a funny thing that incredibly well-known dinosaur specimens can sit around for decades, or for more than a century, before someone notices something fascinating about them. One lesson to learn from this is the importance of collections — their creation, maintenance and accessibility. Another is of course to always look at the fossils we see.

In another iteration of this old theme, today sees the publication of Taylor and Wedel (2023), a short paper on pneumatic features in the dosal ribs of our old friend FMNH PR 25107, the holotype of Brachiosaurus altithorax.

Taylor and Wedel 2023:Figure 2. Sauropod dinosaur Brachiosaurus altithorax Riggs, 1903, holotype FMNH PR 25107 from Dinosaur Quarry No. 13 near Grand Junction, Colorado, dating to the Kimmeridgian–Tithonian ages of the Late Jurassic, right dorsal rib “Rib A” in posterior view with proximal to the left. A1, the whole proximal half of the rib; a distal portion also exists, of similar length but without features relevant to this study; A2, close-up of the tuberculum, highlighting the complex network of support structures that show signs of speculative reconstruction. Circles highlight two possible sites of the “second tubercle” referred to by Riggs (1901: 549, 1903: 303, 1904: 239) based on Marsh’s illustration (1896: figs. 7, 8), reproduced here in Fig. 4; A3, close-up of the pneumatic foramen in the shaft of the rib, showing natural bone texture around the margin and no indication of breakage. Scale bars provide only a rough indication of the size of the elements: see the text for measurements.

Here’s the thing about this rib, which Riggs illustrated in two of his papers — the initial brief description of Brachiosaurus (Riggs 1903:figure 6) and the subsequent monographic osteology (Riggs 1904:plate LXXV:figure 5). It has a pneumatic condition that has not been documented in any other sauropod specimen.

The pneumatic opening is part way down the shaft — about 60 centimeters down from the tuberculum. But there are no other pneumatic features more proximally on the same face of the rib. (We don’t know what’s on the other face: it’s sitting in a plaster half-jacket, and flipping it would not be trivial.)

What does this mean? The seven-location schema of Wedel and Taylor (in revision) predicts that pneumatic features in costal elements would follow vascular foramina from the segmental and intercostal arteries. The segmental arteries pass behind the ribs on their circuit of the centrum, vacularizing the posterior aspect of the proximal portion of the ribs, the tubercula and capitula and region between them — and so providing channels for pneumatization in these regions. Meanwhile, intercostal arteries extend along and beyond the length of the rib shaft, providing opportunities for vascularization and subsequent pneumatization.

But while it’s pretty common to see pneumatization of the proximal portions of ribs, pneumatization of the shafts — likely by diverticula following the intercostal arteries — is hardly ever seen. In fact “Rib A” of the Brachiosaurus altithorax holotype provides the only documented occurrence, and that has only been recognized 120 years after the initial description.

All of this is more evidence for the opportunistic and random behaviour of pneumatizing diverticula. They always have the possibility of passing along the length of rib shafts and pneumatizin them — but either they rarely extend along the intercostal arteries, or if they do then they rarely excavate the bone that they are adjacent to. Why? We have no idea. It seems to be just the way the dice fall.

References

 

doi:10.59350/kyae8-p9k09

Neck-muscle size differentials in diplodocids

Let’s look again at Figure 7 of our recent paper on bifurcated cervical ribs in apatosaurines:

Figure 7. Schematic reconstructions of ventral neck musculature in two diplodocid sauropods. A, Apatosaurus louisae holotype CM 3018, cervicals 6 and 7 in left lateral view (reversed), modified from Gilmore (1936, plate 24). B, Diplodocus carnegie holotype CM 84, cervicals 6 and 7 in right lateral view, modified from Hatcher (1901, plate 3). C, mounted skeleton of Apatosaurus louisae in the Carnegie Museum of Natural History, skull and first seven and a half cervical vertebrae in right posterolateral view. Red lines represent the longus colli ventralis muscles, originating on the anterior aspect of one cervical rib and inserting on the shaft of a more anterior vertebra. Blue lines represent the flexor colli lateralis muscles, originating on the anterior aspect of the tuberculum of one vertebra and inserting on the dorsal part of the shaft of a more anterior vertebra. In Apatosaurus the attachment areas are all much larger: in particular, the insertion of the flexor colli lateralis is increased in size by the incipient bifurcation.

In this figure, the red muscles (longus colli ventralis) are primarily ventral muscles used to draw the neck downwards, while the blue muscles (flexor colli lateralis) are primarily lateral muscles used to move the neck from side to side. (I say “primarily” because anatomy is never that simple and orthogonal: everything does two or three things, and apparently simple movements are generally the result of many different muscles working together.)

In parts A and B of the figure, we showed relatively small ventral and lateral muscles in Diplodocus, and both of them larger by similar amounts in Apatosaurus. If anything, the difference in size is shown as greater in the ventral muscles.

I’m ashamed to say that I (for it was me) didn’t give that a ton of thought at the time: our point was just that the attachments areas for the muscles are bigger, so the muscles themselves were likely bigger.

But the distinctive feature that apatosaurs added here is the dorsal process that we think is the attachment point for the lateral muscles. So it would make more sense if it were those lateral muscles that were most enlarged by the change. So perhaps I should have drawn the top part of that figure like this:

Redrafted version of Wedel and Taylor 2013: Figure 7, parts A and B, emphasizing the relatively large lateral muscles (Flexor colli lateralis, in blue) in Apatosaurus compared with Diplodocus.

If that’s right, then … why? The obvious interpretation would be the the necks of apatosaurines were engineered for lateral motion more than for ventral motion, which suggests we might have misconstrued the primary combat mechanism when we formulated the BRONTOSMASH! hypothesis (Taylor et al. 2015).

So my new take is, tentatively, that apatosaurs may have been smashing their necks sideway into each other more than they were slamming them down on each other.

Let me be quite clear about this: I’m thinking out loud. I could easily, easily be wrong — and if anyone thinks I am and has reasons, I am actively keen to hear them.

References

 

doi:10.59350/kkmhx-gqk23

Brief thoughts on qualitative and quantitative projects

Back in the first post about our recent paper on bifurcated cervical ribs in apatosaurines, I noted:

I’m fond of this one because it’s pleasingly low-tech and traditional. We looked at some fossils, noticed some interesting features, thought about what they mean, wrote it up, illustrated it with specimen photos and diagrams, and called it done. There is certainly a time and place for phylogenetic analysis, geometric morphometrics, and all the other numerical methods that are increasingly common in vertebrate palaeontology, but I genuinely think it’s important that this kind of work doesn’t squeeze out the more foundational process of looking at, and thinking about, fossils.

This has been much on my mind of late, especially as the majority of talks at SVPCA 2023 and many new papers involve numerical methods. Sometimes I feel that Matt and I are in danger of being left behind by a new wave of palaeontology, and it’s definitely true that we could usefully apply (say) geometric morphometrics to our specimens if we had the time to learn how it’s done.

And yet, and yet …

Today I came across a Mastodon thread summarising a preprint (Ploner and Stafford 2021), “How analysis strategy affects analysis results”. Stafford’s summary says, in part:

A host of […] projects have confirmed the worrying conclusion that you can have defensible analyses which produce differing results. But Sebastian and I wanted to pursue further the issue of exactly how wide the spread of results is.

Sebastian computationally generated 1000s of different possible analysis models — permuting possible covariates and interactions — to get a size of the space of possible results. The question was this: how do human teams fill space? Does expertise in analysis mean outcomes cover a restricted, perhaps tiny, zone of the possible outcomes?

The multiverse of computationally generated analyses covered a smaller range than the spread discovered by human teams! Whatever human analysis teams were doing — choosing different possible statistical frameworks, model forms, data recoding, outlier exclusion, etc — it produced more widely varying output than randomly combining covariates in a single model form (mixed model logistic regression, since you asked)

And the punchline:

This result suggests […] that there is a hidden universe of data analysis choices which can both be a) legitimate and b) poorly recorded or recognised by researchers

The important part of this, to me, is Stafford’s in-passing point that the 29 teams whose results differed so widely all made legitimate and defensible data-analysis choices. They all did good work. But they all did different good work with the same dataset, and the outcome was that they all got different results.

When I showed this thread to Matt, his comment was:

That is interesting. And a bit worrying, since a lot of the “big science” to come in future decades will be big analyses of big datasets.

I’m glad to be poking around weird anatomy instead.

I think there is wisdom in that. I’m nervous about the idea that if we did (for example) apply geometric morphometrics to a set of cervical ribs, we might get significantly different results depending on what landmarks we chose, or on other factors.

Whereas the kind of largely descriptive work we did in the recent paper has a different quality. What we wrote has no computer-generated veneer of objectivity: it’s just what we saw and what we thought about it. Our interpretations could be wrong, but that’s fine: they’re written down so they’re refutable. Heck, even our descriptions could be wrong — we might have misinterpreted structures. But that’s OK, too: other people can look at the fossils, reach their own conclusions, and argue their case about why what we wrote is wrong.

But you can’t really argue against the results of a finite element analysis or what have you. All you can do is run another finite element analysis, get different results from the team that did the first one, and say “huh, the computer spat out a different result this this”.

I would find that unsatisfying.

Again, please note: I am not saying that numerical method are without value! I’m not even necessarly saying they are less valuable than we assume (though I do think we should treat the outputs of any given numerical analysis with a bit more scepticism). I’m just saying I’m glad I don’t have to do much of that kind of work.

References

  • Ploner, Sebastian, and Tom Stafford. How analysis strategy affects analysis results: assessing results space and structure of Silberzahn et el. (2018) through model specification. PsyArXiv, 8 Dec. 2023. doi: 10.31234/osf.io/b2hm7
  • Silberzahn, R., E. L. Uhlmann, D. P. Martin, P. Anselmi, F. Aust, E. Awtrey, Š. Bahník, F. Bai, C. Bannard, E. Bonnier, R. Carlsson, F. Cheung, G. Christensen, R. Clay, M. A. Craig, A. Dalla Rosa, L. Dam, M. H. Evans, I. Flores Cervantes, N. Fong, M. Gamez-Djokic, A. Glenz, S. Gordon-McKeon, T. J. Heaton, K. Hederos, M. Heene, A. J. Hofelich Mohr, F. Högden, K. Hui, M. Johannesson, J. Kalodimos, E. Kaszubowski, D. M. Kennedy, R. Lei, T. A. Lindsay, S. Liverani, C. R. Madan, D. Molden, E. Molleman, R. D. Morey, L. B. Mulder, B. R. Nijstad, N. G. Pope, B. Pope, J. M. Prenoveau, F. Rink, E. Robusto, H. Roderique, A. Sandberg, E. Schlüter, F. D. Schönbrodt, M. F. Sherman, S. A. Sommer, K. Sotak, S. Spain, C. Spörlein, T. Stafford, L. Stefanutti, S. Tauber, J. Ullrich, M. Vianello, E.-J. Wagenmakers, M. Witkowiak, S. Yoon, and B. A. Nosek. 2018. Many analysts, one data set: Making transparent how variations in analytic choices affect results. Advances in Methods and Practices in Psychological Science 1(3):337–356. doi: 10.1177/2515245917747646

 

doi:10.59350/tpx31-qby94

How sauropods increased the size of ventral neck muscles

Last time I promised you exciting news about sauropod neck-muscle mass. Let none say that I do not fulfil covenents. And, as usual, when talking about sauropod neck muscle mass, I’m going to start by talking about bird legs. Look at this flamingo:

Ridiculous, right? Those legs are like matchsticks. How can they possibly work. Where are the muscles?

And the answer of course is that they’re on this ostrich:

Check out those huge drumsticks!

Birds make it easier to move their legs by lightening them: shifting the muscles proximally and operating the legs via tendons. (I assume that if we could see the behind the feathers of the flamingo, we’d see a similar, though smaller, “drumstick” at the top of the tibiotarsus.) Lighter legs are easier to shift back and forth, and help to make the ostrich such a superb long-distance runner.

Cursorial mammals do something similar, though perhaps not to the same extent as birds: look how all the muscle mass in this pronghorn’s legs is concentrated at the top.

(This seems to be less true of short-distance speed-runners like the cheetah, where the sheer amount of muscle mass may be more important.)

That’s all very well, Mike, but what has it got to do with sauropod necks?

Well, cursorial animals need to shift muscle mass proximally to reduce the energy required to keep moving their legs back and forth. And in the same way, sauropods needed to shift the muscles of their necks proximally to reduce the sheer weight of the neck. With sauropod necks, it’s not so much a matter of being able to move the neck around quickly (although shifting muscle proximally will help with that, too): it’s about being able to keep the darned thing up at all.

Just as the proximally located leg muscles of birds operate their legs by tendons, so proximally located neck muscles in sauropods — whether proximally within the neck, or even moved right back onto the torso — would have operated the neck by tendons. We know from histological studies including Klein et al. (2012) that the long cervical ribs found in many sauropods are ossified tendons, and a full decade ago we argued in Taylor and Wedel (2013) that this ossification occurred to avoid the energy wastage involved in stretching tendons — the same reason the the tendons in the distal limb segments of birds also ossify.

So far, so good: we’ve discussed all this before. The question is this: what were diplodocids doing? We’ve argued, at least to our own satisfaction, that shifting muscles proximally and ossifying the tendons is a good thing for sauropod necks, yet diplodocids (and other sauropods with short cervical ribs) were evidently not doing this. Why not?

One important question is, what exactly were they not doing? Were they still shifting the muscles proximally, but not ossifying the tendons? Or were they not shifting the muscles proximally at all? We’re arguing, at least tentatively, that the evidence of bifurcated cervical ribs suggests the flexor colli lateralis muscles were single-segment. But I don’t think it follows that the longus collus ventralis muscles were necessarily also single-segment. It’s possible that they were still multi-segment muscles, but that the tendons remained unossified in diplodocids for some reason. But if so, what reason?

Suppose for a moment that in diplodocids the ventral muscles, as well as the lateral ones, were single-segment. If apatosaurine necks being used in combat, as we think, and there was an evolutionary advantage to increasing the ventral muscle mass, then they would not have had the option of larger muscles more proximally, operated via long ossified tendons. Their only option would have been to make those single-segment muscles larger — which could be the origin of the gigantic cervical ribs in apatosaurines.

Or perhaps the important movements in apatosaurine neck combat were lateral movements. In this case it might make sense for the neck to become deeper just to provide enough space for large (i.e. dorsoventrally deep) lateral muscles.

Finally, one more thought: all of this is to do with ventral and lateral musculature, but what about dorsal muscles (longus colli dorsalis, intercristales and interspinales)? One would expect these to be much larger and more significant, given the problem of holding up a multi-ton neck at all, let alone moving it around. Yet we see no osteological evidence of special morphology here, beyond relatively small epipophyses.

We discuss this in our 2013 paper, starting at the bottom of page 26 — see the section “Asymmetric elongation of cervical ribs and epipophyses”. In fact, since the relevant part of this section is short, I’ll just quote it here:

Why did sauropod necks not evolve this way [with posteriorly elongated epipophyses]? In fact, there are several likely reasons.

  • First, positioning and moving the neck for feeding would have required fine control, and precise movements requires short levers.

  • Second, although bone is much stiffer than tendon, it is actually not as strong in tension, so that an ossified tendon is more likely to break under load.

  • Third, muscles expand transversely when contracted lengthways. For epaxial muscles in sauropods necks, this expansion would strongly bend ossified epipophyseal tendons, subjecting them to greater stress than simple longitudinal tension. (The same effect would also have caused some bending of cervical ribs, but the lower stresses in ventral musculature would have reduced the effect.)

Truthfully, I have never found this section 100% persuasive. The reasons we give for not elongating the epipophyses make sense so far as they go, but they don’t do much to explain why we see absolutely no obvious muscle-attachment modifications in the dorsal parts of sauropod vertebrae.

Or maybe we do, but we’re just not recognizing them?

What are we failing to see?

References

 

doi:10.59350/7ppge-gpg13