Dinosauria refers in general to all the creatures known as dinosaurs.
While there are many schemes of classification, we have selected one of
the more popular systems.
In this system, there are two different orders of dinosaurs: Ornithischia
and Saurischia. The division is based on the differing shapes of their
pelvic bones. The saurischian pubis juts forward,
and its ischium points backward. The ornithischian
pubis and ischium both point backward. The difference is simple, and it
provides a basic classification that has continued to be used since the
There is something curious about the division, however, that can confuse
people new to the study of dinosaurs. Birds, as we have noted, are descended
from the predatory dinosaurs; predatory dinosaurs were all from the saurischian
order; and the word saurischian means “lizard-hipped.”
One would think that the birds would be descended from the ornithischians,
which means "bird hipped", but such is not the case. The reason
for the confusion is that although the division by pelvic structure is
valid, the resemblance of the ornithischian pelvis to that of modern birds
is only superficial.
As we have said, the famous carnivorous dinosaurs were from the saurischian
order; the largest herbivorous dinosaurs were also from this order. Saurischian
dinosaurs include two suborders: Theropoda and Sauropodomorpha.
The ornithischians were all herbivores, and they included some of the most
interesting-looking dinosaurs. Ornithischian dinosaurs include three suborders:
Ornithopoda, Marginocephalia and Thyreophora.
Paleontologists disagree on details of classification, and if you browse
through the books listed in the Books About Dinosaurs, you will notice
the differences. One reason for the disagreements is that many fossils
are incomplete, and the information to be learned from them can be interpreted
in more than one way.