>Not quite true of most ornithopods, from hypsilophodontians to hadrosaurs,
which had multijointed movable jaws and almost certainly some kind of cheeks
to keep the food from falling out the sides of the mouth.<
True for the movable jaws. However it doesn't necessarily mean they needed mammalian herbivore cheeks to do the job. Just shortening the lip-mouth line would help them keep their food inside their mouth. Recent herbivorous lizards are doing just fine in chewing and digesting their plant food and there is a lot of similarity between the jaws and teeth of say hypsilophodonts and iguanas.
On the other hand crocodilians are lipless (fish-eating specialization: therefore some pterosaurs and some extinct aquatic reptiles were lipless, too), while most of the other recent reptiles have scaly lips (so did most of the dinosaurs). Yes, they are differ from mammalian soft, movable lips, but they are true lips which cover the teeth when mouth is closed. Certainly, no lips are necessary for drinking water.
Beri's Dinosaur World