Neck-muscle size differentials in diplodocids

Monday, December 18, 2023 18:02

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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 Apatosaurus compared with Diplodocus. ” data-medium-file=”https://svpow.files.wordpress.com/2023/12/figure-7-parts-a-and-b-reconsidered.jpeg?w=300″ data-large-file=”https://svpow.files.wordpress.com/2023/12/figure-7-parts-a-and-b-reconsidered.jpeg?w=480″ class=”size-full wp-image-21607″ src=”https://svpow.files.wordpress.com/2023/12/figure-7-parts-a-and-b-reconsidered.jpeg?w=480&h=194″ alt=”" width=”480″ height=”194″ srcset=”https://svpow.files.wordpress.com/2023/12/figure-7-parts-a-and-b-reconsidered.jpeg?w=480&h=194 480w, https://svpow.files.wordpress.com/2023/12/figure-7-parts-a-and-b-reconsidered.jpeg?w=960&h=388 960w, https://svpow.files.wordpress.com/2023/12/figure-7-parts-a-and-b-reconsidered.jpeg?w=150&h=61 150w, https://svpow.files.wordpress.com/2023/12/figure-7-parts-a-and-b-reconsidered.jpeg?w=300&h=121 300w, https://svpow.files.wordpress.com/2023/12/figure-7-parts-a-and-b-reconsidered.jpeg?w=768&h=310 768w” sizes=”(max-width: 480px) 100vw, 480px” />

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.

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