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These were the ornithischians, which later included Stegosaurus and Triceratops; and the saurischians, which gave rise to huge, long-necked species such as Brachiosaurus, and theropods such as Tyrannosaurus rex and birds.

Modifications from that basic setup - perhaps kept conservative in ornithischian dinosaurs and highly modified with air sac systems in saurischians - allowed for the respiratory diversity that paleontologists are now assessing and studying.

Birds, which do show a similar hip anatomy, are actually saurischian dinosaurs that independently evolved a convergent hip shape.

A complete skeleton of a Late Triassic saurischian and the early evolution of dinosaurs.

The theropods and sauropods make up the saurischian side of the family tree, sharing a closer common ancestor with each other than the ornithischian dinosaurs that make up the other half of the famous family.