There are five known kinds of Ornithischian dinosaurs in the fossil record of prehistoric animals. They all have one thing in common, in spite of the fact that their outer appearance differs greatly from each other. Each of these dinosaurs had a hip structure that is similar to the hips of birds. It is also true that most of these types of dinosaurs walked on four legs instead of upright on just two legs.
However, the earliest known Ornithischian class of dinosaurs did walk on two legs. They were about 3 feet long and they ate vegetation. It is believed that this first type of dinosaur evolved into four other verities. They are the Stegosaurus, Ankylosaurus, Pachycephalosaurus, other Ornithopods and Ceratopsians.
The class of Ornithopods includes Hypsilophodon, Edmontosaurus and the Duck-billed dinosaur. The class of Ceratopsians includes the Triceratops. The Triceratops walked on four legs and had three horns on the top of its head.
For your information, the Iguandon also has a bird-like hip structure but it is considered to part of the Saurischian class of dinosaurs. It also was a bipedal animal.
On the other hand, one would expect that if a dinosaur's bone structure, or part of the structure, was similar to the bone structure of a bird some paleontologists might believe that those creatures could possibly evolve into a bird. To date, there is no evidence that an Ornithischian class of dinosaur did evolve into some kind of bird.
The same is true for the Saurischian class of dinosaurs. But this particular class of dinosaurs is not the subject this story.
However, if the DNA could be obtained from any of those long dead creatures, it might be possible to find a link between those dinosaurs and the birds that we see today. Then again, these dinosaurs were cold-blooded and, as we all know, birds are warm-blooded animals.
So too, what we are seeing is the fossilized remains of those dinosaurs. The soft parts of those creatures are gone forever and that includes the skin. Then again, some fossils do reveal the fact that some dinosaurs did have what appear to be feathers on some parts of their body.
Recently, some bones and other remains of a few dinosaurs were not fully fossilized and one scientist was able to rehydrate the soft tissue within some of those bones. Those experiments are still ongoing and I await further publications of that scientist's work.
In spite of all of the knowledge that has been added to the fossil record during the last 200 years there are still many mysteries to be solved about those animals that lived a very long time ago. So too, the time is long overdue for a reorganization of those animals that have been somehow classified in error.
For example, both two legged and four legged dinosaurs are classified as Ornithischian. Something just doesn't seen right. There were many shapes and sizes as well, none of which have more than thing in common with those other kinds of dinosaurs.