Of course, Dinosauria is itself diverse. It contains several smaller groups, each marked by the evolution
of some new set of features. Each group retains the original ancestral features of dinosaurs, but has added
This most basic division of dinosaurs is into the two great groups:
Therefore, all dinosaurs are either saurischians or ornithischians. The two groups first appeared at the start
of the Late Triassic Period, and both survived until the very end of the Late Cretaceous. They are known from
all continents—nearly from pole to pole—and a broad range of environments.