Saurischian Versus Ornithischian

The Saurischia order of dinosaurs is one of two broad groups of dinosaurs, Saurischia and Ornithischia. The Order Saurischia is characterized mainly by the pelvic structure of these dinosaurs, and includes such animals as Tyranasaurus rex, Allosaurus, and Diplodocus.

In 1888, Harry G. Seeley published a paper that divided the traditional Order Dinosauria into two groups. His research preparing for this paper led him to investigate several other classifications that were already in existence, and he was drawn to the four-ordered system of Othniel Charles Marsh, which divided Dinosauria into Sauropoda, Theropoda, Ornithopoda, and Stegosauria. However, Seeley wanted to divide dinosaurs based on a single feature. He discovered that he could neatly divide all dinosaurs into two groups based on their hip structure. The first, which Seeley designated Saurischia, meaning “lizard hips,” contained dinosaurs that had a distinctly lizard-like pelvis. The second group he called Ornithischia, meaning “bird hips,” because the dinosaurs in this order seemed to have a pelvis more closely related to that of birds. Seeley stated that two distinct orders of dinosaurs had independently arisen from the more primitive archosaurs. Saurischians and ornithischians most likely diverged from an unknown common ancestor by the end of the Triassic. By the Late Triassic, the two major saurischian clades, theropods and sauropodomorphs, also diverged.

In Saurischians, the three pelvic bones radiate out from the hip socket (acetabulum) as in reptiles, with the pubis pointing forward. The bones in the ornithischian pelvic structure, on the other hand, are parallel to the ischium. The pubis is rotated backwards, and a forward-pointing process gives the pelvis a four-pronged structure instead of the three-pronged structure of Saurischians. Although there are distinct differences between the two orders, both Saurischians and Ornithischians have a uniquely dinosaurian pelvis that distinguishes them from reptiles and birds. There is an opening in the acetabulum below a lip of bone against which the femur pressed. All dinosaurs also have an elongated pubis and an ischium that hung down between the legs. Other characteristics of Saurischia include the lack of a toothless predentary bone in the lower jaw and ossified (bone-like) tendons in their backs and tails, both features of ornithischian dinosaurs.

In the order Saurischia, there are two suborders. Sauropodomorpha contains the infraorders Prosauropoda and Sauropoda, and is characterized by the massive sauropod dinosaurs such as Argentinosaurus and Diplodocus. Theropoda contains the infraorders Herrerasauria, Carnosauria, Ceratosauria, Deinonychosauria, Ornithomimosauria, and Oviraptorosauria. All carnivores such as Allosaurus and Tyrannosaurus rex are theropods, and therefore saurischians.