Saurischia

Saurischia is one of the two main groups of dinosaurs. In 1888, Harry Seeley classified dinosaurs into two orders,Seeley, H.G. (1888). "On the classification of the fossil animals commonly named Dinosauria." Proceedings of the Royal Society of London, 43: 165-171. which are now generally considered unranked clades. Their hip structure was why they were put into these groups. Saurischians ('lizard-hipped') and the ornithischians ('bird-hipped') have differences in the ways bones in the hip are put together.

The saurischian pubic bones primarily pointed backward, while the ornithischian pubic bones pointed forward. Despite this pattern, the pubic bones of several saurschians, including herrerasaurids, therizinosauroids, dromaeosaurids, and birds, point forward. It's important consider than the etymology of names Saurischia and Ornithischia does not reflect the evolutionary relationships of these groups. Ornithischians were called 'bird-hipped' because their hips are similar to those of birds, but birds themselves evolved from 'lizard-hipped' saurischians.

Saurischia includes two main subgroups: Theropoda and Sauropodomorpha. Theropods are represented by bipedal, primarily carnivorous dinosaurs (such as Tyrannosaurus and Velociraptor). Birds are theropods and the only group of dinosaurs that has survive to the present day. Early sauropodomorphs were similar to early theropods because they shared a common ancestor. However, the most diverse group of sauropodomorphs, Sauropoda, was represented by long-necked herbivorous dinosaurs that often reached gigantic sizes (such as Diplodocus and Brachiosaurus).

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Ornithischia

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