Ornithischian skulls also had small, reduced openings (antorbital fenestrae) between the eye socket and the nares (nostrils), unlike saurischians.

Ornithischians outnumbered the Saurischians (the lizard-hipped dinosaurs, theropods and sauropods). The Ornithischians were herbivores (plant-eaters).

Major divisions of the Ornithischian dinosaurs:

• Ornithopods (meaning "Bird feet"): Plant-eaters, Two-legged walk or two-legged and four-legged walk, No body armor, Lived during the early Jurassic to Cretaceous periods
  • Heterodontosaurids - small, early dinosaurs with three kinds of teeth
  • Hypsilophodontids - small, fast-running dinosaurs with self-sharpening teeth and a small beak
  • Dryosaurids - small, fast-running dinosaurs with a long, stiff tail
  • Thescelosaurids - late, fast-running dinosaurs
  • Camptosaurids - beaked dinosaurs with hoof-like claws
  • Iguanodontids - beaked dinosaurs with hoof-like claws and a spike on each thumb
  • Hadrosaurids - large duck-billed dinosaurs with no head crest
  • Lambeosaurids - large duck-billed dinosaurs with a head crest
• Thyreophorans (meaning "shield bearers"): Plant-eaters, Four-legged walk, Armored plates on the body, Lived during the early Jurassic to Cretaceous periods
  • Ankylosaurids - armored dinosaurs
  • Stegosaurids - plated and spiked dinosaurs
• Marginocephalians (meaning "fringed heads"): Plant-eaters, Horns or thick skulls, Lived during the Cretaceous period
  • Ceratopsians - horned, beaked dinosaurs
  • Pachycephalosaurs - dome-headed dinosaurs with huge bony skulls