Of the two orders that divide Dinosauria, Saurischia claims the largest land animals, the fiercest carnivores, the oldest known dinosaurs, and the ancestral lineage of the birds. The Order Saurischia was established by Harry G. Seeley in lectures presented in 1887 but was not officially published until 1888 when he divided Owen's Dinosauria into two groups based on the form of the pelvis, the braincase, the vertebrae and the armor. The Order Saurischia is characterized mainly by the pelvic structure of these dinosaurs, and includes such animals as Tyranasaurus rex, Allosaurus, and Diplodocus.
The Physical Characteristics of Saurischian Dinosaurs
In the three-pronged pelvic structure of saurischians, the three pelvic bones radiate out from the hip socket (acetabulum) as in reptiles, with the pubis pointing forward. Although saurischians were named for their lizard-like hips, they have a uniquely dinosaurian pelvis that distinguishes them from other reptiles. There is an opening in the acetabulum below a lip of bone against which the femur pressed. They also have an elongated pubis and an ischium that hung down between the legs. Saurischian skulls also have large, pronounced openings (antorbital fenestrae) between the eye socket and the nares (nostrils). Saurischian dinosaurs lacked a toothless predentary bone in the lower jaw and ossified (bone-like) tendons in their backs and tails, both features of other dinosaurs.
The Lineage of the Saurischians
The oldest known dinosaurs (Eoraptor and Herrerasaurus) are saurischians, and date from the mid- to late-Triassic period, about 230 million years ago. Two distinct orders of dinosaurs, the saurischians and the ornithischians, had independently arisen from the more primitive archosaurs. They most likely diverged from an unknown archosaurian common ancestor by the end of the Triassic. By the Late Triassic, the two major saurischian divisions, theropods and sauropodomorphs, also diverged.
The Suborders of Saurischia
In the Order Saurischia, there are two suborders: Sauropodomorpha and Theropoda. Sauropodomorpha contains the infraorders Prosauropoda and Sauropoda, and is characterized by the massive four-legged herbivore sauropod dinosaurs such as Argentinosaurus and Diplodocus. Theropoda contains the infraorders Herrerasauria, Carnosauria, Ceratosauria, Deinonychosauria, Ornithomimosauria, and Oviraptorosauria. All bipedal carnivores such as Allosaurus and Tyrannosaurus rex are theropods.
Saurischia and the Evolution of Birds
Oddly enough, birds evolved from the “lizard-hipped” saurischians and not the “bird-hipped” ornithischians. The “bird-like” pelvis, with a pubis pointing toward the back of the animal, occurred in two independent instances, once in the ornithischians and once in the theropod lineage leading to birds. The ornithischian line died out, but the theropods evolved into today’s birds.