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Palaeontography & Anatomy
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juvenile P. engelhardti by theropod1, visual art

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Are you from Carnivora
This may be quite a really, really stupid question, but I've been wondering about it.

Let's say a cat or bear grappled onto the neck of an equal-sized predatory theropod (of what clade (carnosaurs, tyrannosaurids, megalosaurids, etc.) you decide). Would the latter's cervical flexibility and cranial flexibility from the atlas be sufficient for it to turn its head/neck to the side and bite down on the carnivoran's forelimb (theropod necks were particularly flexible, right?)? Would some with powerful necks have the power and flexibility to free themselves through vigorous shaking?

I'm so sorry. Spending >4 years debating on animal vs. animal forums can make you think a little hard about this stuff.
First of all, depends on the theropod in question, they aren't that similar across the ranks in that regard. Some had more flexible necks than others. And of course it depends on what angle the opponent would be relative to the theropod.

So rather than giving you an overgeneralized answer to that question, here’s a very helpful visualisation from Snively et al. 2013:…

Shown (3) is the maximum lateral range of motion of the head and neck of Allosaurus.
A couple of things to note: Of course Allosaurus is considered one of the large theropods with a rather high degree of cervical flexibility. I don’t know much about Megalosaurs in this regard. A derived tyrannosauroid would almost certainly be less flexible, but of course it would also have a shorter neck to make up for that, which would make grappling it even more difficult.
As you see the individual joints, including the craniocervical articulation, aren’t all that flexible, but if you add up the angles over several you end up able to flex that neck quite a bit rather quickly. So it’s very important to take into consideration where that hypothetical grappling animal has its hold on the neck, and how much of its length is still free to move. That’s of course also relevant because grappling more anteriorly would give the grappler much better leverage and make escaling its grasp much more difficult.
So perhaps theropod flexibility was sufficient for what you describe, perhaps not, depending on where the attacker is standing compared to the theropod and what theropod we are talking about.

As for shaking vigorously, there are certainly theropods I could see doing that, though of course the neck is a vulnerable region and doing so would be risky. But again, depending on the attacker I suppose it could work in some cases.
"As you see the individual joints, including the craniocervical articulation, aren’t all that flexible"

Yeah, I find that puzzling. I read that the large articular surfaces of theropod zygapophyses promoted neck flexibility; I imagined it would be by making the individual joints flexible. The opisthocoelus morphology of carnosaur cervicals should also have helped with this. Lastly, Snively's reconstruction doesn't look like the cranium is flexed all that much from the atlas, but theropods had more or less spherical occipital condyles, allowing for great mobility at the joint.

Did these things just really not help that much to make the individual joints flexible in absolute terms?
I didn’t mean to say Allosaurus’ neck wasn’t flexible. For a cervical skeleton, it’s in all likelyhood very flexible. But individually, intervertebral joints or the craniocervical joint still don’t have huge ranges of movements.
They never do, probably those in Allosaurus already have a comparatively large range of movement (as you correctly point out, they are in fact adapted for that), but individual vertebrae simply don’t move all that much. It’s the summation of many small rotations that ends up making the neck flexible.
What I meant to say by that: it really matters how much neck there is to make that turn you were referring to, because it can’t just bend 90° in a single spot.

Simply put, the measure of what’s flexible for an intervertebral joint isn’t the same as that for a jaw joint, or knee joint etc., because they don’t have to be and vertebrae are loaded in a very different way from jaw or limb bones. They need those support structures, like zygapophyses, cervical ribs and neural spines to provide adequate support to the spine without excessive muscular effort despite its mostly horizontal posture. Allosaurus does have a comparatively flexible neck. Part of the reason why is that its skull likely wasn’t very heavy compared to some other theropod skulls, meaning it needed less rigid support. But still individual vertebral joints will generally not be as flexible as some other joints in the body, and this is why.
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Hey theropod.

You might be aware of what I asked you on WoA* (i.e. seeing if my argument on theropod agility on Carnivora was sound). If I was asking for too much, then I apologize.

I just have one question for you now regarding the topic. Since theropod legs were pretty much at the center of mass, theropods would have produced little torque compared to quadrupedal animals. Did they have a way of solving this problem?

Thanks. :) (Smile) 

*Or if you're not, well, you could still check out hippo vs. Carnotaurus on Carnivora to at least see what I'm talking about.