Single-segment neck muscles in diplodocids?

Thursday, November 30, 2023 16:42

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I don’t remember now when I first noticed bifurcated cervical ribs in apatosaurines. I imagine 2016 at the latest, because on our Sauropocalypse that year Mike and I saw examples at both BYU and Dinosaur Journey. I remember that we specifically set out to document them in CM 3018 on our Carnegie visit in 2019 (story here). (I’m using the term ‘bifurcated’ very loosely here, for any cervical rib that looks like it was getting pulled on in two directions.)

We also noted this cool example in the juvenile brontosaur CM 555. This is the right cervical rib of C7 in lateral view, as it appears in Figure 3 of our new paper. Anterior is to the right, and the dorsal process is the bit making the almost-too-good-to-be-true right angle with the shaft of the rib, near the posterior (left) end of the element.

And here is the same element in a dorso-medial oblique view, in a photo that did not make it into the new paper. Anterior is now to the left, and the capitulum is the fat cylinder of bone sticking out toward us at the lower left. The dorsal process is not perfectly vertical, but sort of ‘leans’ out laterally, which is why it no longer makes a right angle in this corrupted-by-perspective close-up view — it’s pointing not just dorsally, but also laterally, away from the camera.

(This is yet another example of the tyranny of 2d images, and a useful remind that you can’t trust photos of complex bones.)

Anyway, as we discussed last time, that 2019 visit and a bunch of other specimens led us to the new paper that described bifurcated cervical ribs in various apatosaurs but also speculated on the reason for the. Here, again, is Figure 7, which captures the heart of the paper:

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.

Until very recently, I assumed that the longus colli ventralis muscles (red in the image above) spanned more than one pair of vertebrae, and probably the same for flexor colli lateralis (blue in the image above), because that’s how they tend to behave in birds. We only drew them running between adjacent vertebrae to save space, and to make the contrast between Apatosaurus and Diplodocus as stark as possible.

But it’s not impossible that one or both of them were single-segment muscles in apatosaurines. The dorsal processes diverge so sharply away from the cervical shafts that any path longer than one segment is going to be a poor fit for the observed morphology of the cervical ribs — and remember, at this point we have ample evidence that sauropod cervical ribs are ossified tendons, so presumably we can infer the direction of muscle pull from the the angles of the cervical rib bits.

These short muscles would help explain the apomorphically short cervical ribs in apatosaurines and other diplodocoids: the cervical ribs are ossified tendons, and if the muscles they’re embedded in are only one segment long, then the ribs can’t be longer than that.

Tune in next time for exciting news about sauropod neck-muscle mass! Same sauropod-vertebra time, same sauropod-vertebra channel.

Source: https://svpow.com/2023/11/30/single-segment-neck-muscles-in-diplodocids/