Extinct terrestrial reptiles having birdlike pelvises armored dinosaurs (thyreophorans)

Usage examples of ornithischia.

In the Saurischia, this indentation is large and carries at least one fenestra, whereas in the Ornithischia the fossa as well as its fenestra are significantly reduced or missing.

Now we know that they fall into two long-separated orders: the Saurischia and the Ornithischia, no more closely related than, say, we are to bats.

Today, this view has been modified radically, and almost all researchers believe that both groups, Ornithischia and Saurischia, are closely related and that all dinosaurs form a socalled monophyletic group, originating from a common ancestor.

This tooth structure provides a clear proof that all members of the Ornithischia were herbivorous.

In the course of evolution, these defining features of the Saurischia and the Ornithischia underwent many modifications, and the state of these changes together with appearance of other, new anatomic features characterize different subgroups of the two main dinosaurian lines.

Much debate, however, dealt with mutual relationships among smaller groups within the Ornithischia, and especially the role played by the Ornithopoda in the development of the bird-hipped dinosaurs.

The basic differences between fabrosaurids and all other members of the Ornithischia is that the cheeks are missing, though they are present in all other reptiles, recent reptiles included.

Turning to the terrestrial realm, reptiles found largely in this group of habitats ate the squamates and the two groups of dinosaurs, the Ornithischia and Saurischia.