Peg-like teeth could not sink in to flesh. So animals with peg-like teeth cannot be meat-eaters. As a result, they have to be plant-eaters..

Most plant-eating animals do not digest their food in their mouth, at least, not to begin with. Think of cows. They much away, pulling grass into their mouths with both teeth and tongue. This goes straight down into their stomachs.

Plants are not concentrated forms of energy like meat. So a plant-eater (or a human vegetarian for that matter) must eat far more food than a meat eater to get the same amount of energy. We et around this by eating some concentrated forms of plant food, such as nuts..

Now back to the cow. The cows has a stomach with lots of compartments. It fills one of these compartments with grass, then stops eating grass for a while and regurgitates some of what it has just eaten. It crushes this with the back of its jaw bone. We call it chewing the cud. Once ground up, the grass slips back down and moves into another stomach for more processing..

Many birds have a slightly different way of eating. They do not regurgitate their food, but swallow little stones. As they move about, the stones and food roll about together in the bird's stomach and the food gets crushed..

We keep looking at modern animals for clues. Once we can see the range of ways modern animals eat, we can see if this matches up with anything we can see from the fossil dinosaurs. As it happens, some stones have been found in dinosaur stomachs, so this helps us to understand what they were for.