The cylindrical zygapophyses of Trionyx spinifera

December 7, 2022

I was googling around some photos, confirming to myself that turtles don’t have cervical ribs, when I stumbled across this monstrosity (and when I use that word I mean it as a compliment):

Softshell turtle Trionyx spinifera, cervicodorsal transition in ventral view, anterior to right. Copyright © Mike Dodd, used by kind permission. Original at https://www.amanita-photolibrary.co.uk/animals/trionyx_spinifera_1496.htm

The specimen is from the collection amassed by Caroline Ponds, formerly a reader in Zoology at Oxford, who picked up most of her specimens as roadkill in Milton Keynes. She has donated this collection to WildCRU (Wildlife Conservation Research Unit) just outside Oxford, just 90 minutes away from me.

The hot news here is of course the zygapophseal articulation between what I am interpreting as the last cervical and the first dorsal. Let’s take a closer look:

As you can see the prezygapophyses of the first dorsal are cylindrical, wrapping smoothly around from a fairly traditional anterodorsal-facing aspect through anterior, anteroventral, ventral, and even posteroventrally-facing. There is no hint of inclination towards the midline as in sane prezygapophyses.

And, providing perfect mates to those prezygs, the postzygapophyses of the last cervical wrap around producing a negative cylinder that encloses the positive one.

This leaves me with questions. Lots of them. For example:

  1. Did I even identify the vertebrae right, or is that “first dorsal” really the last cervical, based on its not carrying a rib? It looks like it’s trying to bear a rib, but not quite carrying it off. (For now I will assume my identification is correct.)
  2. What is the centrum articulation like here? Sadly, it’s obscured in the photo. My guess would be positive cylinder on the front of the dorsal, and a small contact point on the back of the cervical — but it really is just a guess.
  3. Is this unique to Trionyx spinifera, or do all cryptodiran turtles do this to some extent?
  4. If this condition is common among cryptodires, are there  species that take it to an even greater extreme?
  5. What do pleurodire turtles do here?
  6. Why haven’t I spent more of my like looking at the cervicodorsal transitions of turtles?

7 Responses to “The cylindrical zygapophyses of Trionyx spinifera

  1. llewelly Says:

    Wow. That’s very interesting. I don’t know anything, but I couldn’t resist a brief bit of googling. And so I stumbled across some anatomical drawings of cervicals from Apalone spinefera and Chelodina longicollis (both also cryptodirans) , which to my uneducated eye seem to show a similar, but less extreme version of the same morphology.

    They’re on pages 166 to 168 of this chapter 7 Cervical Anatomy and Function in Turtles from Herrel et al 2008 biology of turtles. Anthony Herrel kindly put a pdf on his website, it appears:

    Click to access Herrel%20et%20al%202008%20Biology%20of%20turtles.pdf

  2. Mike Taylor Says:

    Interesting Llewelly. Figure 7.6’s C8 and D1 (Apalone spinifera) seem to have something going on that resembles what we’re seeing here, but I’m not seeing it in Chelodina longicollis.

    The gold is on page 169: “Joint D1-C8 (cervico-dorsal joint). […] In Apalone, this joint lacks a central articulation.”

    I mean to say, what?

  3. Zin Says:

    Just a heads up, Apalone spinifera and Trionyx spinifera are the same species, Apalone be9jg current generic combination. And Chelodina longicollis is a pleurodiran, not a cryptodiran.

  4. Matt Wedel Says:

    In Apalone, this joint lacks a central articulation.

    I just threw up a little in my brain.

  5. llewelly Says:

    My apologies for getting Chelodina in the wrong group; that must explain why it doesn’t have the morphology of Trionyx/Apalone . Thank you, Zin, for pointing that out.

  6. Jura Says:

    Yeah, turtle necks are some of the craziest things. Romer 1956 (Osteology of the Reptiles) covers turtle cervical articulations in good detail. Cryptodires and pleurodires both have biconvex joints at 2–3 locations along the cervical column. In most cryptodirans, the joints are seen in the fourth and eight cervical vertebrae. Centra are often biconvex at this location (in trionychids, there is a convexity in the rostral end of the eight cervical, but not the caudal end).

    Zygapophyses get even weirder. Cryptodiran zygapophyses splay away from each other as seen in your photo, whereas pleurodiran zygapophyses often fuse together into a single structure. Romer comments specifically about trioncyhid zygapophyseal articulation (pg 244):

    “Zygapophysial [sic] specializations for vertical retraction are highly developed in all Cryptodira—perhaps most extreme in the case of the eight vertebrae of tryonychids, in which the ventral surfaces of the eighth cervical and the first dorsal are in contact in extreme retraction.”

  7. Mike Taylor Says:

    All right, this is getting really freaky. Somewhere in my woodshed of decomposing animals is a tortoise. I gotta get that baby out and play with its neck.


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