 Psittacosaurus and Protoceratops are the most common dinosaurs in the different Mongolian sediments where they are found . Triceratops fossils are far and away the most common dinosaur remains found in the latest Cretaceous rocks in the western United States , making up as much as 5 / <unk> of the large dinosaur fauna in some areas . These facts indicate that some ceratopsians were the dominant herbivores in their environments . 
