Certainly people can eat animals that eat the plants that exist. Grains didn't exist much during the time of Spirits of The Ice Forest, but then again, neither did humans for that matter. If you are going to have a Tolkeinesque style fantasy set back in the days of the dinosaurs, you are going to have to import a lot of things, otherwise you just won't have people and therefore no castles, no magic, no elves, dwarves, trolls, orcs, fairies or dragons. The world of Mystara was set back in the days of the dinosaurs as well, but they replaced most of the dinosaurs with modern and mythological beasts, and just left a few places like the Island of Dread for Dinosaurs to stomp around in.