The cylindrical zygapophyses of Trionyx spinifera

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):

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?