There's also another factor to put into play. To pull off the bite, Sharptooth jumped right on the back of Mama Longneck. I'm sure that Sharptooth landing right on her back caused a few broken bones, maybe even other internal injuries. For all we know, there could've been internal bleeding. :blink:
The wound was indeed originally inflicted right near the joint between the neck and the spine. The animators just had trouble keeping the bite showing there.

: Anyways, this made it and so that the neck of Mama Longneck was hanging by a thread. It was only a matter of time before the neck detatched from the spine.
In fact, the animators cut out alot of realisim with the bite itself. Under normal circumstances, a bite that huge would cause alot of bleeding. So there's the account of blood loss that we don't see due to the animators trying to cut it out.
Man, when you look at it like this, the creators kinda made Sharptooth look like a savage killer. I'm just glad he's dead now. -_-