I've been experimenting with a new design. In this model I want the crocodile to lunge forward opening and snapping its jaws shut then, as it pulls back more slowly I want the jaws to open and close a smaller amount three or four times. Nom nom nom.
Here's my first draft for a mechanism to move the jaw. There is a cam follower on the underside of the jaw which runs down the groove in the slider. The cam follower is pushed across at the extremes of travel so that it changes lanes in the reverse direction. You should be able to make out one long cam bump of the far side groove to open the jaws wide. On the near side are three short bumps for the nomming.
The cam follower is hinged so that it can move from groove to groove.
Something like this.
It is looking promising for a first draft!