Like the saurischians, the ornithischians were already represented by several
groups in
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.
