Saurischia

Harry Seeley established Saurischia in 1888, arguing that Richard Owen's "Dinosauria" actually comprised two distinct groups defined by their hip structure. The name "lizard-hipped" refers to the ancestral pelvic anatomy where the pubis bone points forward and downward. This name is somewhat misleading because the other major clade, Ornithischia, has hips that superficially resemble those of birds. However, birds are actually theropods and members of the Saurischia clade. They evolved their backward-pointing pubis independently through convergent evolution.

Saurischia includes two vastly different lineages. Theropoda retained a bipedal stance, sharp teeth, and grasping hands. This group produced massive predators like T. rex and Spinosaurus, as well as small feathered hunters like Velociraptor and the ancestors of modern birds. Sauropodomorpha took a different evolutionary path. Beginning as small bipedal animals in the Late Triassic, they eventually transitioned to obligate herbivory and four-legged movement. This led to the emergence of long-necked giants such as Brachiosaurus, Diplodocus, and Argentinosaurus.

Other skeletal features that define saurischians include elongated neck vertebrae with epipophyses, a relatively long second finger, and a large temporal opening behind the eye.

Since Saurischia includes everything from hummingbirds to titanosaurian sauropods, their behaviours were incredibly diverse. The clade encompassed apex predators, fish-eaters, and browsers. There is a general consensus that many saurischians laid eggs in clutches and provided some level of parental care, a behaviour documented in oviraptorids and inferred from their ornithischian relatives. Large theropods served as the primary predators in their ecosystems, while sauropods shared resources through niche partitioning based on their neck lengths and tooth shapes.

• Eoraptor lunensis (PVSJ 512) — one of the earliest known saurischians, Ischigualasto Formation, Argentina, ~231 mya. Sereno et al., 1993.
• Plateosaurus engelhardti Trossingen specimens — basal sauropodomorph, dozens of individuals, Stuttgart Natural History Museum.
• Coelophysis bauri Ghost Ranch quarry (AMNH FR 7223 et al.) — early theropod bonebed, American Museum of Natural History.

The 2017 Ornithoscelida proposal by Baron, Norman, and Barrett would dissolve Saurischia by moving theropods next to ornithischians and leaving sauropodomorphs (plus Herrerasaurus) as a separate lineage. Most subsequent re-analyses (Langer et al. 2017; Marsola et al. 2018) have recovered the traditional Saurischia and the hypothesis remains a minority position, but the early dinosaur tree is genuinely close to the resolution limit of available data.

A related debate concerns Herrerasaurus and the herrerasaurids: are they basal theropods, basal saurischians, or pre-saurischian dinosauromorphs? Different analyses give different answers.

The "saurischian" label rarely escapes textbook diagrams, but its members dominate popular culture: Jurassic Park's T. rex, Brachiosaurus, and Velociraptor are all saurischians. Birds at the feeder are too.

• dinosauria
• theropoda
• sauropodomorpha
• ornithischia

• Seeley, H. G. (1888). On the classification of the fossil animals commonly named Dinosauria. Proceedings of the Royal Society of London, 43, 165–171.
• Langer, M. C., et al. (2017). Untangling the dinosaur family tree. Nature, 551, E1–E3.
• Sereno, P. C. (1999). The evolution of dinosaurs. Science, 284, 2137–2147.
• Baron, M. G., Norman, D. B., & Barrett, P. M. (2017). A new hypothesis of dinosaur relationships and early dinosaur evolution. Nature, 543, 501–506.