Paleontologists in Mongolia have discovered a new dome-headed dinosaur species, and it’s notable for both its age and its condition. Not only was the fossil in exceptional condition, but it also turned out to be the oldest known pachycephalosaur fossil. The Z. rinpoche fossil is only about three feet long, indicating that it died when it was still a juvenile. Previously, most of what paleontologists knew about dome-headed dinosaurs had to do with their uniquely shaped heads. Next, take a look at the newly found dinosaur species with a large “sail” on its back.