Hey, just one minute! Those big fat dinosaurs with long necks (the one that sneeze on that girl who "prefers to be called a hacker") has never had any teeth. They commonly eat the leaves and swallow rocks to mash the leaves down. In the movie those species ARE indeed shown with teeth.
Brachiosaurus's do have teeth. They are like short stubby pegs used for stripping leaves. That is one of the few things Spielberg got right in this scene.
OK. First of all, the "fat-necked" dinosaur was a brachiosaurus. These creatures DID have teeth! They actually had many teeth. They used these to strip branches of their leaves. Do you honestly think any creature could use it's gums to take leaves from trees? It IS true, however, that they swallowed stones to help break down food. There was no mistake here.