The main groups of ornithischians are ankylosaurians, ornithopods, ceratopsians, and pachycephalosaurians. Pachycephalosaurs were bipedal ornithischians characterized by their thickened skulls. Numerous vertebrate groups are from this formation, including fishes, crurotarsans, ornithischians, saurischians, pterosaurs, and turtles. Like other ornithischians, "Ankylosaurus" was herbivorous. However, not all ornithischians were strictly herbivorous. As in most quadrupedal ornithischians, its neck is moderately short. Either would be exceptional evidence of carnivory among ornithischians. Neornithischia ("new ornithischians") is a clade of the dinosaur order Ornithischia. Like all ornithischians, "Kentrosaurus" was a herbivore. Heterodontosaurids like "Abrictosaurus" were small, early ornithischians, named for their markedly heterodont dentition. It was one of the earliest ornithischians. Herbivory among the ornithischians and sauropodomorphs was never reversed. There is strong evidence that some ornithischians lived in herds. "Shantungosaurus giganteus" is one of the largest known ornithischians. The of "Ouranosaurus" are unique among ornithischians.