What’s the difference between a cervical and dorsal vertebra?
June 6, 2021
In mammals — certainly the most-studied vertebrates — regional differentiation of the vertebral column is distinct and easy to spot. But things aren’t so simple with sauropods. We all know that the neck of any tetrapod is made up of cervical vertebrae, and that the trunk is made up of dorsal vertebrae (subdivided into thoracic and lumbar vertebrae in the case of mammals). But how do we tell whether a given verebra is a posterior cervical or an anterior dorsal?
Here two vertabrae: a dorsal vertebra (D3) and a cervical vertebra (C13) from CM 84, the holotype of Diplodocus carnegii, modified from Hatcher (1901: plates III and VII):
It’s easy to tell these apart, even when as here we have only lateral-view images: the dorsal vertebra is tall, its centrum is short, its neural spine is anteroposteriorly compressed and its parapophysis is up on the dorsal half of the centrum; but the cervical vertebra is relatively low, its centrum is elongated, its neural spine is roughly triangular and its parapophysis hangs down well below the centrum (and has a cervical rib fused to it and the diapophysis).
But things get trickier in the shoulder region because, in sauropods at least, the transition through the last few cervicals to the first few dorsals is gradual — the vertebrae become shorter, taller and broader — and tends to have no very obvious break point. In this respect, they differ from mammals, in which the regional differentiation of the spinal column is more abrupt. (Although even here, things may not be as simple as generally assumed: for example, Gunji and Endo (2016) argued that the 1st thoracic vertebra of the giraffe behaves functionally like an 8th cervical.)
So here are those two vertebrae in context: the sequence D3 D2 D1 C15 C14 C13 in CM 84, the holotype of Diplodocus carnegii, modified from Hatcher (1901: plates III and VII):
Given that the leftmost is obviously a dorsal and the rightmost obviously a cervical, where would you place the break-point?
The most usual definition seems to be that the first dorsal vertebra is the first one that has a free rib, i.e. one not fused to the vertebra: in the illustration above, you can see that the three cervicals on the right all have their cervical ribs fused to their diapophyses and parapophyses, and the three dorsals on the left do not. This definition of the cervical/dorsal distinction seems to be widely assumed, but it is rarely explicitly asserted. (Does anyone know of a paper that lays it out for sauropods, or for dinosaurs more generally?)
But wait!
Hatcher (1903:8) — the same dude — in his Haplocanthosaurus monograph, writes:
The First Dorsal (Plate I., Fig. 1). […] That the vertebra now under consideration was a dorsal is conclusively shown not by the presence of tubercular and capitular rib facets showing that it supported on either side a free rib, for there are in our collections of sauropods, skeletons of other dinosaurs fully adult but, with the posterior cervical, bearing free cervical ribs articulating by both tubercular and capitular facets as do the ribs of the dorsal region. The character in this vertebra distinguishing it as a dorsal is the broadly expanded external border of the anterior branch of the horizontal lamina [i.e. what we would now call the centroprezygapophyseal lamina]. This element has been this modified in this and the succeeding dorsal, no doubt, as is known to be the case in Diplodocus to give greater surface for the attachment of the powerful muscles necessary for the support of the scapula.
Hatcher’s illustrations show this feature, though they don’t make it particularly obvious: here are the last two cervicals and the first dorsal, modified from Hatcher (1903:plate I), with the facet in question highlighted in pink: right lateral view at the top, then anterior, and finally posterior view at the bottom. (The facet is only visible in lateral and anterior views):
Taken at face value, Hatcher’s words here seem to imply that he considers the torso to begin where the scapula first lies alongside the vertebral column. Yet if you go back to the Diplodocus transition earlier in this post, a similar scapular facet is not apparent in the vertebra that he designated D1, and seems to be present only in D2.
Is this scapular-orientation based definition a widespread usage? Can anyone point me to other papers that use it?
Wilson (2002:226) mentions a genetic definition of the cervical/dorsal distinction
Vertebral segment identity may be controlled by a single Hox gene. The cervicodorsal transition in many tetrapods, for instance, appears to be defined by the expression boundary of the Hoxc-6 gene.
But this of course is no use in the case of extinct animals such as sauropods.
So what’s going on here? In 1964, United States Supreme Court Justice Potter Stewart, in describing his threshold test for obscenity, famously said “I shall not today attempt further to define the kinds of material I understand to be embraced within that shorthand description, and perhaps I could never succeed in intelligibly doing so. But I know it when I see it.” Is that all we have for the definition of what makes a vertebra cervicals as opposed to dorsal? We know it when we see it?
Help me out, folks! What should the test for cervical-vs-dorsal be?
References
- Gunji, Mego, and Hideki Endo. 2016. Functional cervicothoracic boundary modified by anatomical shifts in the neck of giraffes. Royal Society Open Science 3:150604. doi:10.1098/rsos.150604
- Hatcher, Jonathan B. 1901. Diplodocus (Marsh): its osteology, taxonomy and probable habits, with a restoration of the skeleton. Memoirs of the Carnegie Museum 1:1-63 and plates I-XIII.
- Hatcher, J. B. 1903b. Osteology of Haplocanthosaurus with description of a new species, and remarks on the probable habits of the Sauropoda and the age and origin of the Atlantosaurus beds; additional remarks on Diplodocus. Memoirs of the Carnegie Museum 2:1-75 and plates I-VI.
- Wilson, Jeffrey A. 2002. Sauropod dinosaur phylogeny: critique and cladistic analysis. Zoological Journal of the Linnean Society 136:217-276.
Sauropod Vertebra Picture of the Week 
June 6, 2021 at 4:47 pm
My understanding is that the “true” definition of a cervical vertebra is arguably that the last cervical vertebra does not articulate with the pectoral girdle whereas the first dorsal vertebra does, but that can be difficult to identify when dealing with incomplete or disarticulated material. As such, it can be more useful for comparison across taxa to identify a particular arbitrary point of division close to the “true” point that can consistently be identified from even isolated bones, hence the alternate definitions.
The definition I’ve learned to use in sauropods is based on the position of the parapophysis. On the last cervical vertebra, the parapophysis is still ventrally positioned on the centrum, below the pneumatic fossa, whereas on the first dorsal vertebra, the parapophysis has shifted somewhat anterodorsally. Going by that definition, it does look like the putative 16th presacral of D. carnegii might be a cervical.
I believe that in some non-dinosaur reptiles, cervical and dorsal vertebrae are distinguished by whether the vertebra bears both a diapophysis and parapophysis or only a single facet for rib articulation.
June 6, 2021 at 5:06 pm
Attempting to define a major difference wherein none is implied (at least by Nature), it behooves us to do what we always do ….. assign a new definition to vertebrae in the transitional phase even if this is only about one or two vertebrae. Call it a TDC (Transiting-Dorsal-Cervical). Problem solved (wherein again none was really implied).
June 6, 2021 at 7:09 pm
Maybe I’m more blind than I realize, but my first (and enduring) amateur visual impression, is that D1 should be C16 in the diplodocus sequence. But by the various thresholds people are describing, D1 does look technically correct (including Hatcher, which is why I question my eyesight – that facet looks plausibly there to me).
Real Q may be, where in heck DOES the scapula articulate? As you’ve explained elsewhere, mammals are quite limited in the number of cervical verts while reptiles are less so, and it is plausible to hand-wave and say, that by expanding that number, the border cases get spread out as (maybe) the different Hox domains overlap, so I also agree with Dale – if there’s a visually-obvious cervical that technically is a dorsal, that’s – the odd bone out. But, I assume I’d be inclined to ultimately defer to the shoulder girdle. If it articulated as far forward as labelled-D1, that’s an interesting transition case for sure.
June 7, 2021 at 2:41 am
In practical terms you are going to need a definition that people can apply; but nature doesn’t care.
In the evolution of creatures such as elasmosaurs; stegosaurs (Miragaia I’m looking at you) and sauropods vertebrae have been “cervicalised” and shifted from being dorsals to cervicals as necks lengthened and torsos shortened.
There must have been individuals that were intermediate in this process and had vertebrae that were neither totally one or the other.
If you take the attachment of cervical ribs I wonder if some were asymmetrical with fusion on one side only.
June 7, 2021 at 9:18 am
In plesiosaur-land we have the concept of pectoral vertebrae (coined by Seeley) where the diapophysis is on both the centrum and neural arch. There may be something like 7 of these in basal taxa, whereas the numbers tend to decrease in more derived animals, down to 3 or possibly 2. Especially in basal taxa there is variation as to whether these are predominantly contributing to the neck or trunk depending on the rib morphology. Then again, not everybody uses the “pectoral” concept, and in those cases the 1st dorsal is usually considered to be where the diapophysis is wholly on the neural arch, and the “pectorals” are considered to be cervicals. Fusion of the centrum and neural arch means there is always some uncertainty in older animals.
June 7, 2021 at 9:32 am
Thanks, all for fascinating comments — just the kind of thing I was hoping for when I posted this.
John: “My understanding is that the “true” definition of a cervical vertebra is arguably that the last cervical vertebra does not articulate with the pectoral girdle” — but do you know where you got this understanding from?
Dale: unfortunately what you propose doesn’t help, because then you’re just left with the question of how to define which vertebrae are designated as transitional.
Brad: of course part of the problem with scap-based definitions is that the location and position of the scapula in sauropods is itself controversial!
Lee: of course you (and Dale) are right that we are trying to impose a distinction where nature doesn’t necessarily provide one — but that’s no reason for us not to do it. A significant part of science is labelling. See also: all zoological taxonomy.
Mark: excuse my ignorance of plesiosaur anatomy, but do you really mean diapophysis? Or are you thinking of the dorsal migration of the parapophysis?
June 7, 2021 at 6:16 pm
I’m not sure where I got that understanding from, to be honest. It might just be a vague impression picked up from how there are a lot of clade-specific definitions (such as rib fusion) but the pectoral girdle-based definition is relevant to everything that has a pectoral girdle and so is more widely discussed. Incidentally, at least some snakes apparently don’t have cervical vertebrae except for the atlas (Cohn & Tickle 1999; doi: 10.1038/20944).
Moore et al. (2020) (doi:10.1080/14772019.2020.1759706) briefly discuss conflict between rib-based and parapophysis-based definitions for the cervicodorsal transition in mamenchisaurids. It seems that it’s fairly common for the last fused rib to be on a vertebra anterior to the last vertebra with a cervical-type parapophysis position.
Tschopp and Mateus (2017) (doi:10.7717/peerj.3179) discuss the cervicodorsal transition of diplodocids at some length, and mention that Stannius (1846) defined the transition based on articulation of the ribs with the sternum, which they note as the most universally applicable definition, although apparently it’s not clear if it works for diplodocids.
How much can we trust rib fusion as a marker of the cervicodorsal transition, anyway, given that immature individuals don’t have fused ribs anyway? It seems to me that the distinction should be made based on something less likely to be influenced by ontogeny.
June 7, 2021 at 6:23 pm
Well, I have this exact problem. I’m working on my first sauropod, trying to identify what genus/species it is, and presumably it is a young Barosaurus (Emmanuel Tschopp pers. com.). However, the number of dorsals is still an ‘unambiguous’ autapomorphy for Barosaurus Lentus (9 DV’s) based on his analysis from ’15, thus it seems to be quite important to recognize this transition as it could make a difference in your analysis (phylogeny/morphology etc.).
I have chosen to follow the paragraph in the discussion of Tschopp & Mateus (2017) on this topic, which lists 6 different ways to potentially address this issue, and just score them all when possible. You have likely read it already, but I thought I will just mention it if you didn’t. The only issue is that they write this for part Diplodocidae only, but they still may be helpful if adjusted for other sauropods.
June 7, 2021 at 6:38 pm
Many thanks, John, this comment is gold!
tpvanderlinden, I’ve been gently worrying about this “unambiguous autapomorphy” of Barosaurus. First thing, note that “unambigous” here is a term from phylogenetic analysis and doesn’t mean anything like “the scoring is really clear” — it just means that when the character is optmised on a cladogram, there is only one most-parsimonious transition point. So you shouldn’t treat it as inviolable just because it “unambiguous”. But the real question is whether then 10th vertebra forward from the sacrum in Barosaurus (which McIntosh designated as the last cervical) is really so different from the homologous vertebra in Diplodocus (which Hatcher designated as the first dorsal). I guess I ought to at least go back and re-read McIntosh on this.
Oh, what cans of worms we do open.
June 8, 2021 at 9:11 am
I really do mean diapophysis as both rib facets are on the cervical centrum in plesiosaurs, when they are present. They tend to fuse in the more derived members of several lineages, so most plesiosaurs from the late Jurassic onwards have a single rib facet on the cervicals (and all plesiosaurs have single rib articulations on the dorsals, which I suppose you would call diapophyses). Some derived pliosaurids and rhomaleosaurids have a mix of single and double cervical rib facets, often on the same centrum.
June 8, 2021 at 9:19 am
How magnificently weird plesiosaurs are! Thanks, Mark.
June 14, 2021 at 2:53 pm
Mark, plesiosaurs are a bit far from my field of expertise, but what you are describing is familiar to me.
Just to be clear : I will distinguish here the facets for the articulation with the ribs from the processes that bear these facets, to avoid confusion, as suggested by Gomes de Souza (2018).
If diapophyseal and parapophyseal facets are fused, you should name the resulting structure a synapophyseal facet. Idem for the corresponding processes. This is clearly stated by Hoffstetter and Gasc (1969) in a monograph on the vertebrae and ribs of extant non-avian reptiles. Anatomists working on squamates usually follow this nomenclature… but it doesn’t seem that common for other taxa, unfortunately.
The synapophysis was previously also known as the paradiapophysis but the latter name caused much confusion and was abandoned for the former.
What remains unclear, at the moment, is the fact that both the diapophyseal and parapophyseal processes may be fused while ending distally with two distinct facets. In which case there would be a synapophyseal process but both parapophyseal and diapophyseal facets.
This is frequently the case in the lumbar, sacral, and anterior caudal series where the presence of an anteroposterior foramen (in youngs) or an anterior and a posterior notches for the segmental artery separate the diapophyseal and the parapophyseal facets on the vertebra and the tubercular and the capitular facets on the ribs. At least in early amniotes. I don’t know if these are separated in the same way or separated at all in dinosaurs, but the convergence of the facets occurs at least in the sacrals and caudals of sauropodomorphs, from my experience.
Gomes de Souza, L. 2018. Comments on the serial homology and homologues of vertebral lateral projections in Crocodylia (Eusuchia). The Anatomical Record 301:1203–1215.
Hoffstetter, R., and J.-P. Gasc. 1969. Vertebrae and ribs of modern reptiles; pp. 201–310 in C. Gans, A. d’A. Bellairs, and T. S. Parsons (eds.), Morphology A., . Biology of the Reptilia vol. 1. Academic Press, London & New York.
June 16, 2021 at 1:42 pm
Hello, Mike: I had trouble the other day to stay logged in since my comments was… let’s say a bit long. Since it did not appear, I thought there had been an issue with it, before WordPress told me that it had already been posted. Nothing appeared since then – since several days, in fact.
Do you think it has been considered as spam (by Akismet or WordPress)? Or is my comment somewhere in limbo, waiting for moderation? Or should I just repost it, which would annoy me a lot though I considered it pretty relevent to the discussion?
Do tell, please! :-D
Jocelyn Falconnet
June 16, 2021 at 2:35 pm
Hi, dimetroblog. This was a very rare example of a non-spam comment being misclassified as spam. Thanks for drawing my attention to it: I have waded into the spam-bucket and rescued your comment.
June 16, 2021 at 2:50 pm
Hi too ! Thank you very much, Mike, and sorry if you had to read dozens of physiology-related spams for that ! :-D
June 16, 2021 at 3:00 pm
Don’t worry, yours was easy to find :-)
October 17, 2021 at 3:14 pm
This is an even more fraught issue in snakes.
My view is that when cervical and thoracic regions are defined with reference to any particular morphological attribute of single vertebrae, the boundary/transition (1) may or may not agree with that based on any other criterion, (2) may be individually variable within a species, or between the two sides of a single individual, (3) may change during ontogeny.
‘Cervical’ is an abstraction, not a discrete set.
October 17, 2021 at 5:32 pm
I increasingly think you’re right, John, not just about the specifics regarding snakes, but on the fluffiness even in principle of the distinction between cervical and dorsal.
January 28, 2022 at 7:53 pm
[…] I started to put together a new manuscript section on that issue, and posted my initial thoughts as What’s the difference between a cervical and dorsal vertebra?. This post, too, generated some useful feedback that made its way into the version of the section […]