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 remain found in the latest Cretaceous rocks in the western United States , making up as much as 5 / 6ths of the large dinosaur fauna in some areas . These facts indicate that some ceratopsians was the dominant herbivores in their environments .