What is Saurischia?

Birds and all dinosaurs that are closer to birds than they are to Ornithischia. Back in 1887, Harry Seeley split all dinosaurs into two main branches based on the design of their hips: Saurischia is the branch of "lizard-hipped" dinosaurs with a pubis bone pointing forwards away from the ischium, towards the creatures head (See below). The other, Ornithischia, is the branch of "bird-hipped" dinosaurs with a pubis bone pointing backwards at the same angle as the ischium, towards the creature's tail.

Strangely, birds evolved from saurischian ("lizard-hipped") dinosaurs rather than ornithischian ("bird-hipped") dinosaurs and, just to complicate matters further, some saurischians (the pot-bellied therizinosaurs and ostrich-mimicking ornithomimosaurs) had beaks, while others (therizinosauroids and dromaeosaurids) evolved bird-like hips independently of birds!

Saurischia contains the largest of two great lineages of herbivores (sauropods) and the fiercest meat-eating dinosaurs (theropods). It also contains a handful of basal critters, such as Eoraptor, who has been passed backwards and forwards between Theropoda and Sauropodomorpha since its discovery at Cancha de Bochas in 1991, but which may be straddling the crossroads as a common ancestor of both.

Saurischia is derived from the Greek "sauros" (lizard) and "ischion" (hip joint), because its members retained the ancestral hip anatomy also found in modern lizards.