Like the saurischians, the ornithischians were already represented by several
the Early Jurassic. Their diversity increased similarly by the end of the Jurassic and
reached their maximum at the end of the Early Cretaceous and in the Late Cretaceous.
All ornithischians were herbivorous and many of them were secondarily quadrupedals.
A unique structural peculiarity of all ornithischians is a predental bone in the frontal part
of the lower jaw. The toothless predental bone was covered by horny plate, forming a
beak-like structure. The ornithischians are divided into several main groups:
the ankylosaurians, pachycephalosaurians, ornithopods, and ceratopsids.