system
Given the following post, respond as if you're commenting on Reddit:
human
[Land Before Time] How does the Great Valley keep out carnivores?

Adults long necks like Littlefoot's grandparents got through so it can't be a size thing. Even if it were, it isn't just the T rex's being blocked out; raptors are blocked from coming in too and a small opening wouldn't stop them. So what stops carnivores from coming in?
gpt
Herbivores in a herd are actually capable of killing a carnivore pretty easily.  This is why carnivores usually kill the old or the sick herbivores who can't keep up with the herd, and they usually hunt as a pack.

If there's only one entrance to the valley, and the herbivores are smart enough to speak and reason, then it's pretty trivial to just set up a bunch of guards near the entrance and bludgeon any carnivores who try to enter.