Britannica Sources used in this answer

Why are ornithischian dinosaurs called bird-hipped even though birds did not evolve from them?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

Illustration showing various dinosaur species drawn to scale and grouped by geologic era: Late Cretaceous, Early Cretaceous, Jurassic, and Triassic. Dinosaurs range from large species like Brachiosaurus and Tyrannosaurus to smaller ones like Velociraptor and Compsognathus, with a human silhouette included for size comparison. Measurement bars at the bottom indicate 1 meter and 3 feet for scale reference. — dinosaur A selection of dinosaurs grouped by the geologic interval in which they lived.

The term "bird-hipped" for ornithischian dinosaurs is based on a superficial anatomical resemblance rather than evolutionary descent.12

In 1887, paleontologist H.G. Seeley classified dinosaurs into two orders based on the configuration of their pelvis. In most reptiles, the pubic bone points down and forward. However, in ornithischians, a portion of the pubis points downward and backward, lying parallel to the ischium. This specific pelvic arrangement is also found in modern birds, leading to the name "bird-hipped".132

Despite this naming convention, birds did not evolve from ornithischians. Instead, birds are descendants of theropod dinosaurs, which belong to the saurischian ("lizard-hipped") lineage. The similarity in hip structure between ornithischians and birds is an example of convergent evolution or a superficial likeness, as the two groups are not closely related.2341