The PCs are all dinosaurs. You run around avoiding meteors and stomping things.
This could be played totally straight where the players are all dinosaurs doing dinosaur things. I spent a lot of time pretending I was a dinosaur as a kid, so there's got to be some fun there.
Or it could be more of a reverse Turok kind of deal, and the dinosaur-PCs eat Marines or Lost World explorers or scientists or whoever it would be entertaining for dinosaurs to eat.
Maybe Dinosaur Escape: Washington, D.C.