Protoceratops is very abundant in Southern Mongolia, and where they occur in what look like dune deposits, you do find numerous examples. Now, the animals you’re finding there could be associated in death and have nothing to do with each other in life, or they may be groups that were overwhelmed at the same time and the same event, and working this out is part of what being a paleontologist is all about.
The behavior of past animals can be understood from a number of different ways. Besides looking at the geological contexts in which they’re found, you can also look at the geochemistry of the bones and that will tell you something about what they’re eating, and the other thing to do is just look at their anatomy. For example an animal with long slender legs is quite likely to be a fast runner. An animal with an enlarged part of the brain that can actually see under low light conditions might have been active in a polar latitude. These are the sorts of things you can infer from anatomy. So you don’t just have one avenue of approach to solving these problems about ‘What did the animal do?’, you’ve got various lines evidence could be mustered, and depending on the situation, you might be able to pull two or three of them together to get some idea of the living animal’s behavior.
Modern animals can tell us a lot about fossils and their behavior. If we didn’t have modern animals we would know a lot less than we do, and a prime example of this would be the fact that we can tell some dinosaur groups either the young just more or less left the nest upon hatching, or else they stayed in place. Because when you go to a modern birds nest and you look at the eggshells, if they’re all crunched and smashed up it means that the young stayed in place after they hatched and gradually, in their movements, crushed the egg shells to tiny little bits. Where as, if the animal hatches, and just leaves the nest immediately, you wind up with big eggshells. So, that’s something you get as an insight by knowing something about living animals, living analogs.