Do bifurcated ribs imply single-segment neck muscles? Sometimes

Monday, May 13, 2024 6:06

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Here at SV-POW!, we love bifurcated cervical ribs. Those of Turiasaurus are one of the autapomorphies proposed by Royo-Torres et al. (2006:figure 1K). Their diagnosis of the new genus included “accessory process projecting caudodorsally from the dorsal margin of the shafts of proximal cervical ribs”. Here is the best example of such a rib in Turiasaurus, attached to its vertebra. (It’s a shame the black backdrop doesn’t extend beneath the rib, but you can make it out easily enough nevertheless.)

Cervical vertebra of the holotype individual of Turiasaurus riodevensis, in left lateral view. Photograph by Andrew Moore, reproduced with permission, and I am delighted to get this out there!

This is a very similar but not identical photo to that used in Royo-Torres et al’s (2006:figure 1K) illustration; but because that paper was unfortunately published in Science instead of in a scientific journal, the illustration is microscopic and the description perfunctory. There are no further illustrations of the material in the supplementary document.

Aaanyway. We mentioned but did not illustrate this rib in our recent paper (Wedel and Taylor 2023), and we wrote of it (on page 93) that:

Royo-Torres et al. (2006) described and illustrated bifurcated cervical ribs in Turiasaurus, and Britt et al. (2017) described and illustrated bifurcated cervical ribs in Moabosaurus (Fig. 3A). Turiasaurus and Moabosaurus are both members of the clade Turiasauria, but bifurcated ribs are absent in Mierasaurus, which Royo-Torres et al. (2017) recovered as the sister taxon of Moabosaurus within Turiasauria. This implies either a single origin of bifurcated cervical ribs in Turiasauria, with a reversal in Mierasaurus, or parallel origins of bifurcated cervical ribs in Turiasaurus and Moabosaurus. [emphasis added]

I think we missed the most obvious explanation here: that while the potential to develop bifurcated cervical ribs is phylogenetically determined, the actual development of bifurcation in any given rib is highly variable between individuals and indeed between vertebrae of a single individual. Given that we showed this individual variability pretty clear for apatosaurines — within the Apatosaurus louisae holotype CM 3018, for example — it’s a bit dumb that we failed to apply the same observation to the variable appearance of bifurcated cervical ribs in turiasaurs.

Anyway, that’s not why we’re here. We’re here to look again at how different the bifurcation is between Turiasaurus and some apatosaurines. Here’s a composite, based on the photo above and our old friend MWC 1946 (also appearing as Wedel and Taylor 2023:figure 3d).

Top: Turiasaurus riodevensis, left rib of unspecified cervical vertebra with unknown specimen number, left lateral view. Bottom: Apatosaurinae indet MWC 1946, right cervical rib in right lateral view, reversed.

(Why don’t we know the specimen number of the Turiasaurus cervical? Again, because the “description” of this important and unusually complete sauropod was published in Science, which is far too important to waste space on trivia like specimen numbers. All we’re told is that the holotype elements have specimen numbers in the range CPT-1195 to CPT-1210. Once, more Science is the opposite of science. Digression ends.)

Back in November last year, Matt asked the question: Single-segment neck muscles in diplodocids?. He tentativly concluded yes, based on the posterodorsal trajectory of the upper prong in apatosaurine cervical ribs, which would have anchored short flexor colli lateralis muscles attaching to the cervical rib loop of the immediate successor vertebra. (We know where on the vertebra these muscles originate in birds, and the upward-and-backward orientation od the prong points to where that site is on the very next vertebra.)

Looking at the composite image above suggests that the same was not true in Turiasaurus. Here, the upper and lower prongs of the bifurcated rib are close to parallel, implying that flexor colli lateralis muscles inserting on the upper prongs can only have originated some segments further back.

Now if we return to the Freak Gallery in our recent paper:

Wedel and Taylor 2023:Figure 3. Bifurcated and incipiently bifurcated cervical ribs of sauropods. A, Moabosaurus utahensis holotype individual, left cervical rib BYU 14063 (not right as stated by Britt et al. 2017), probably associated with C5, in medial view. B, Dicraeosaurus hansemanni holotype MB.R.2379, right cervical rib 8 in lateral view. Modified from Janensch (1929, fig. 21). C, Brontosaurus parvus CM 555, right cervical rib 7 in lateral view. D, Apatosaurus louisae MWC 1946, cervical vertebra in right lateral view. E, Apatasaurus louisae MWC 5659, cervical vertebra in left lateral view (reversed). All photographs by the authors.

We can see that the rib of Moabasaurus has near-parallel prongs like those of its relative Turiasaurus — implying that it, too, likely had multi-segment flexor colli lateralis muscles. But by contrast the Dicraeosaurus rib, which we described as “incipiently bifurcated”, has a more or less dorsally projecting flange which seems likely to have anchored a single-segment muscle as in apatosaurines.

To my tentative conclusion is that bifurcated ribs in diplodocids (such as apatosaurines and Dicraeosaurus) do indeed imply single-segment neck muscles (or at least single-segment flexor colli lateralis); whereas the bifurcated ribs of turiasaurians imply multi-segment muscles.

Admittedly this is a biggish conclusion to hang on such scant evidence as the apparent angle of divergence in the handful of bifurcated cervical ribs that we’ve seen. But I think it at least stands as the hypothesis best supported by presently available evidence, and it’s there to be corroborated or contradicted by further observations.

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