Mine’s the jawbone. The jaw has a loose degree of freedom but is also anchored at both ends giving it a lot of sturdiness.
It’s also representative of what I’ll term a meso-skeletal structure… We do not have exoskeletons, but the ribs and skull don’t function as rods hugged by meat. The ribs and skull serve as protective shells (the way an exoskeleton does), coated for the mostpart with a basic regenerative layer of skin. As they are neither exoskeleton nor stricly endoskeleton, I say mesoskeleton.
The jaw serves as a mesoskeletal joint and is the closest thing we have to a lobster’s hinges. And I have seen someone online describe the anatomy of xenomorphs as mesoskeletal, so we know it’s the kind of thing that a tough higher-species is made of.
Edit: Oh shit, this page didn’t used to come up on Google results for the term, but now I know that it exists for-realsies: https://thebigdictionary.com/mesoskeleton/