I would mount acceleration motion sensor on the jaw. It can give you complete complex motion that jaw (or other parts of body) can produce. You need 6 axes of motion detected (3 linear and 3 rotational) to fully define motion. Now this might be overkill for your application and you might need only one inear sensor for your case. Your description says very little about purpose of measurement, range, resolution and accuracy and options, even if you restrict yourself to acceleration sensors only, are plenty. As skin is not rigidly attached to that bone, depending on speeds involved and mass of the sensors, some error might occur due to small ammount of skin streching. Again, this might be irelevant in your case.
There are plenty of semiconductor manufacturers that make MEMS based acceleration sensors, Analog, Motorola,... Be more precise to get more precise answer.
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