While there are too many famous dinosaur species to keep track of — and potentially more types of dinosaurs to be discovered — the paleontologist Harry Govier Seeley split dinosaurs into two major dinosaur groups: the order Saurischia ("lizard-hipped" dinosaurs) and the order Ornithischia ("bird-hipped" dinosaurs).

Both orders probably had a common ancestor that lived sometime during the Middle Triassic.

Structural Differences Between Dinosaurs

As in all land animals, there were three bones in each side of the pelvis. The left and right ilia (singular: ilium) firmly gripped the spine in the sacrum. The left and right pubes (singular: pubis) extended down beneath the ilia. The left and right ischia (singular: ischium) extended down and back beneath the ilia and behind the pubes.

In so

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