Marginocephalia
Marginocephalia are named for a shelf that projects over the back of the skull. They include the pachycephalosaurians and ceratopsians.
Pachycephalosaurs are best known for their thick upper fronts to their skull. The oldest known is Stenopelix, from the early Cretaceous of Europe.
Ceratopsians, famous for Protoceratops, Triceratops and Styracosaurus illustrate the evolution of frilled and horned skulls. The frills evolved from the shelf common to all Marginocephalia. Ceratopsians are separated into basal ceratopsians, including the parrot-beaked Psittacosaurus, and neoceratopsians.
Diversity of ceratopsian skulls. A) Skeleton of Protoceratops. B) to I) Skulls. B) & C) Psittacosaurus side & top. D) & E) Protoceratops side & top. F) & G) Triceratops side & top. H) & I) Styracosaurus side (without lower jaw) & top.
The evolution of ceratopsid dinosaurs shares characteristics with the evolution of some mammal groups, both were "geologically brief" events precipitating the simultaneous evolution of large body size, derived feeding structures, and "varied hornlike organs."
The sequence of ceratopsian evolution in the Cretaceous is roughly from Psittacosaurus (121 -99 Ma) to Protoceratops (83 Ma) to (Triceratops 67 Ma and Styracosaurus 72 Ma). In side view the skull of Psittacosaurus bears very little resemblance to that of Styracosaurus but in top view a similar pentagonal arrangement can be seen.
Read more about this topic: Evolution Of Dinosaurs, Ornithischia