Neck-muscle size differentials in diplodocids
December 18, 2023
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
Sauropod Vertebra Picture of the Week 
December 18, 2023 at 10:48 pm
this is presumably totally coincidental (and not neck muscles), but I think constrictor snakes primarily use their lateral muscles for constriction? As well as for some kinds of movement, and for climbing. I don’t know why I was reminded of that, but there it is. If anyone knows what muscles snakes fighting use against each other, that might be more relevant. I guess it’s also primarily lateral muscles as well, though.
December 18, 2023 at 11:25 pm
Possibly important: virtually all apatosaurine cervical ribs are wide (laterally expanded), but only some of them are deep (ventrally expanded). The Tokyo and Field Museum apatosaurs have cervical ribs that are wide without being deep (f’rinstance).
Related thought: if you’ve already evolved wide cervical ribs, and you want to increase the surface area for lateral muscle attachment still further, maybe ventral is the only way left open. Can’t go dorsal because that volume is already occupied by epaxial muscles. Maybe.
Further related thought: they’d already forked their epaxial muscle attachments by evolving bifurcated neural spines. Which could also help with side-swinging.
Sideways neck-bashing makes more sense than ventral neck-bashing. Although I hadn’t thought about it in those terms until just now.
December 19, 2023 at 3:29 am
My first thought is that since gravity is going to bend the neck ventrally anyway, you wouldn’t need much larger ventral muscles no matter how massive your neck became. Then maybe the larger lateral muscles are simply needed because the neck is more massive in apatosaurines, so needs more force to move sideways. Similarly, I note the epipophyses on top of the postzygapophyses are more massive in Apatosaurus in your figure, since these are where the muscles attach that bend the neck up.
December 20, 2023 at 1:06 am
This is some interesting stuff, and it makes a lot of sense. Do you think the reduced anterior process/development of the ventrolateral process in Apatosaurus louisae would be part of the same trend, then? Like, if you’ve already made the neck wider and deeper to accomodate an expanded flexor colli lateralis, maybe the next step is to shift the longus colli ventralis as far ventrally as it will go to give even more room for the flexor colli lateralis. Or something like that.
December 20, 2023 at 5:37 am
John, that’s definitely something to consider. Also, I have thoughts on what happened to the anterior processes in A. louisae cervical ribs, but they’re not fully-formed yet (the thoughts, not the processes); more on that in time.
Mickey, solid point about the epipophyses!
December 20, 2023 at 6:13 pm
May I humbly suggest feeding rather than fighting? (or at least as an addendum to it) Back in the DinoMorph days Kent Stevens and Co found that Apatosaurus had more rotational motion possible in the neck (around the long axis of the spine). Rearing up just to eat some snippets at head height in e.g. Diplodocus isn’t so great unless there’s a lack of other food, but an apatosaur could rear up and sit there feeding in an enlarged envelope around the tree cylinder. Given how posteriorly the CoG was they could probably slowly saunter bipedally to the next tree and do the same thing. Being able to both eat low-lying food and efficiently browse on an expanded upper feeding envelope would have made them versatile in several different environments.
December 20, 2023 at 6:18 pm
I don’t buy it, Scott. If those lateral-motion muscles were hypertrophied, as they appear to be, then they must have been doing something different with them from what other diplodocids were doing. And lateral movement of the neck calls for slow, careful movements, not forceful ones.
December 20, 2023 at 6:26 pm
I do think they are doing something different – Diplodocus and Barosaurus don’t show adaptations for swivelling around trees to extend their feeding envelopes. And those necks are large and fairly heavy even with air sacs, so better leverage and larger muscles for twisting and holding the necks in place check out to me. Forceful does not automatically equal fast.
December 20, 2023 at 10:31 pm
Diplodocus and Barosaurus don’t show adaptations for swivelling around trees to extend their feeding envelopes
I’m not so sure about that for Barosaurus — it has much wider cervicals and in particular much wider and flatter zyg surfaces than Diplodocus, or any other diplodocid for that matter.
I agree that feeding is almost certainly involved somehow. I’m just not convinced that it’s a strong enough driver for apatosaurines to have probably doubled the skeletal mass of their necks with those giant cervical ribs. Usually when something bizarrely expensive like that shows up, it’s for mating. But I admit that’s hard to demonstrate convincingly, or test.