What dorsal processes on cervical ribs tell us about neck muscles and their functions

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. Wedel and Taylor (2023: fig. 3).

Here are some cervical ribs of sauropods that show a spectrum of morphologies, from a low dorsal process that makes an obtuse angle with the shaft of the rib in Dicraeosaurus (upper right), to one that makes a right angle in Brontosaurus (center), to a prominent spike of bone in Apatosaurus (bottom left), to a fully bifurcated cervical rib in another vertebra of Apatosaurus (bottom right) and in the turiasaur Moabosaurus (upper left).

Whether they manifest as low bumps or full-on bifurcations, dorsal processes on cervical ribs are odd-looking. But they make intuitive sense. We’ve known for a while now that the cervical ribs of sauropods — like those of birds — are ossified tendons. And from comparisons with crocs and birds, we expect that sauropod cervical ribs had two sets of muscles inserting on them: a lateral set, and a ventral set. They’re the green lines, especially C and E, converging on the cervical rib in this diagram from our 2013 PeerJ paper:

Simplified myology of that sauropod neck, in left lateral view, based primarily on homology with birds, modified from Wedel and Sanders (2002, figure 2). Dashed arrows indicate muscle passing medially behind bone. A, B. Muscles inserting on the epipophyses, shown in red. C, D, E. Muscles inserting on the cervical ribs, shown in green. F, G. Muscles inserting on the neural spine, shown in blue. H. Muscles inserting on the ansa costotransversaria (“cervical rib loop”), shown in brown. Specifically: A. M. longus colli dorsalis. B. M. cervicalis ascendens. C. M. flexor colli lateralis. D. M. flexor colli medialis. E. M. longus colli ventralis. In birds, this muscle originates from the processes carotici, which are absent in the vertebrae of sauropods. F. Mm. intercristales. G. Mm. interspinales. H. Mm. intertransversarii. Vertebrae modified from Gilmore (1936, plate 24). Taylor and Wedel (2013a: fig. 5).

I don’t think we’ve ever shown those muscles in crocs, but they’re there, as you can see in this half-dissected alligator neck:

(Hypaxial neck muscles in crocs aren’t that different from those of birds, just shorter and simpler. It’s in the epaxial neck muscles that theropods and birds diverge wildly from the primitive archosaurian plan. See Figure 11 and related discussion in Taylor and Wedel [2013a].)

If the two sets of muscles converged from different angles, their tendons might ossify separately, at least in part, and that could create the spectrum of dorsal processes and bifurcated cervical ribs shown up top. And that bifurcation would be more likely to manifest if the angle between the converging muscles was wider, as it almost certainly was in apatosaurs. When we were at the Carnegie Museum back in 2019, I doodled this comparison between Diplodocus carnegii (top) and Apatosaurus louisae (middle) and showed it to Mike:

He took one look at the drawing and said, “That’s basically the paper right there.” A cleaner version, using illustrations from Hatcher (1901) and Gilmore (1936) and flipped to face the other way, appears in our new paper as part of 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 (re-versed), modified from Gilmore (1936, plate 24). B, Diplodocus carnegii 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. Wedel and Taylor (2023: fig. 7).

If apatosaurs were the only dinosaurs with bifurcated cervical ribs, the conclusion would be almost tautological: giant cervical ribs meant that the neck muscles converged on the cervical rib shafts at wider angles, which would improve the chances of a visible bifurcation in the ossified tendon that is the cervical rib.

Head and neck of mounted Carnotaurus sastrei cast LACM 127704 in right ventrolateral view, showing incipiently bifurcated cervical ribs. Photograph by the authors. Wedel and Taylor (2023: fig. 4).

But the weird thing is, dorsal processes and bifurcated cervical ribs aren’t limited to apatosaurines. As the image up top shows, they’re also present in some dicraeosaurids and turiasaurs, neither of which have giant, low-hanging cervical ribs like those of apatosaurs. And in fact, dorsal processes and bifurcated cervical ribs aren’t even limited to sauropods — the ceratopsian Zhuchengceratops has them, as do several theropods, including Carnotaurus. So what’s going on here?

The serial positions of the cervical ribs with prominent dorsal processes is telling — in every example that we know of, whether sauropod, theropod, or (shudder) ornithischian, the dorsal processes are best-developed in the middle of the neck. That suggests that the divergent muscles were pulling on the cervical ribs hard enough to leave separately-ossifying tendons only at mid-neck, at some distance from both the head and the trunk.

Just a reminder of what Apatosaurus louisae MWC 1946 — same vert as in D of the figure at the top of the post — looks like in ventral view.

It seems these critters were doing some real work with their necks. Ceratopsians and theropods had big heads to hold up and maneuver. Apatosaurs didn’t have big heads, but they had big heavy necks — weirdly, apomorphically, expensively heavy necks — so whatever they were doing, it was probably something important.

References

• 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
• Wedel, Mathew J., and Michael P. Taylor. 2023. The biomechanical significance of bifurcated cervical ribs in apatosaurine sauropods. VAMP (Vertebrate Anatomy Morphology Palaeontology) 11:91-100. doi:10.18435/vamp29394