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Tyrannosaurus rex saurischian pelvis and hind limbs (left side), taken at the
Oxford University Museum of Natural History.
Saurischians (from the Greek sauros (σαυρος) meaning 'lizard' and ischion (ισχιον) meaning 'hip joint') are one of the two orders/branches of dinosaurs. In 1888, Harry Seeley classified dinosaurs into two great orders, based on their hip structure. Saurischians ('lizard-hipped') are distinguished from the Ornithischians ('bird-hipped') by retaining the ancestral configuration of bones in the hip. All carnivorous dinosaurs (the theropods) are saurischians, as are one of the two great lineages of herbivorous dinosaurs, the sauropodomorphs. At the end of the Cretaceous Period, all non-avian Saurischians became extinct. This is referred to as the Cretaceous-Tertiary extinction event.
The Saurischians were differentiated from the Ornithischians in the Late Triassic Period. They had a three-pronged pelvic structure. The Ornithischians evolved a new hip structure, with the pubis rotating caudally, to become parallel with the ischium, often also with a forward-pointing process, giving a four-pronged structure. This hip structure is similar to that of birds, and so Ornithischians are termed 'bird-hipped' dinosaurs, while the Saurischians are 'lizard-hipped'. Ironically, the true bird-hip possessed by modern birds evolved from the lizard-hipped theropods in the Jurassic Period, an example of convergent evolution.
While Seeley's classification has stood the test of time, there is a minority theory, first popularized by Robert Bakker in The Dinosaur Heresies that separates the theropods into their own group and places the two great groups of herbivorous dinosaurs (the sauropodomorphs and ornithischians) together in a separate group named as the Phytodinosauria ('plant dinosaurs') (Bakker), or Ornithischiformes (Cooper).
[edit] Taxonomy
