How to fix animation distortion? I'm completely new to 3D modeling and animation, so please cut me some slack. I've downloaded and imported a free 3D model. Added a bone structure (armature) to it and now I'm trying to create a basic animation.

All is working perfectly fine so far, except that I seem to get some kind of distortion (not sure if this is correct naming for it) when animating the model. Here are 2 screenshots to show what I mean:

Original position:


Animated position:


How do I either prevent or fix this? Has it something to do with the bone affect range and is it a simple matter of adding more bones or is it not that easy?
Looks like a weight-painting issue.  Keep one of those selected bones in the screenshot selected, select the mesh and get into Weight Painting Mode.  You'll probably see a ring of green, yellow around the mouth/gums.