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

• Klein, N., Christian, A., & Sander, P. M. (2012). Histology shows that elongated neck ribs in sauropod dinosaurs are ossified tendons. Biology letters, 8(6), 1032-1035.
• Taylor, Michael P., and Mathew J. Wedel. 2013. Why sauropods had long necks; and why giraffes have short necks. PeerJ 1:e36. 41 pages, 11 figures, 3 tables. doi:10.7717/peerj.36