If birds didn't originate from the Ornithischia, doesn't that imply that the bird-like hip evolved twice?

Question

This is something that's been bugging me for a few days now - the avian group is usually associated with the Theropoda, which is an off-shoot from the lizard-hipped Saurischia. The Ornithischia are always depicted as a widely separate branch, yet the earliest known dinosaurs are Theropods (Eoraptor, Herrerrasaurus, etc), so isn't is more likely that the bird-like hip evolved once and that the common ancestor of (for instance) birds and hadrosaurs is closer than the common ancestor of birds and sauropods as the diagrams usually suggest? Or am I missing something here and the likelihood of the hips evolving twice are much greater than they seem?

Just to address the 2 answers received so far:

It could have evolved twice, but is that necessarily the most likely answer? The traditional view is that birds evolved from Theropods which include all two legged carnivorous dinosaurs, and these are in the lizard hipped group along with Sauropods - even the ones with more bird-like hips are counted as lizard-hipped dinosaurs by definition. Given that our earliest dinosaurs are Theropods, that all known families of dinosaur originate from these common ancestors, and that there are theropods with both hip variations, isn't it much more likely that an early Theropod with bird-like hip could be the common ancestor for not only birds themselves, but the entirety of the bird-hipped dinosaurs (stegosaurus, hadrosaurs, triceratops, the lot)? Or is there an indication in the hips of the ornithischia that show signs of convergence like the difference between a dolphin chest cavity and an icthyosaur - 2 origins for the same shape?

Answer 1

Uh, technically, bird hips only evolved ONCE--with birds. Ornithischians are called this because their hip structure is SIMILAR to avian hips. Ornithischian pubes (the 'lower' most hip bone) is swung back to allow for more space in the gut, which was needed for digestion of plant material, since plants are much harder to break down when compared to flesh. (Sauropods skipped this altogether and just got BIG) Compare the pubes positioning in an early theropod, like Ceolophysis, to a more derived theropod, like Deinonychus. The pubes in the dromaeosaur, Deinonychus, is swung back more than the one in his more 'primitive' cousin. As of now, dromaeosaurs are considered the closest link to modern Aves, whether it is as the source of birds or as a cousin group.
Unfortunately, I cannot say that true bird hip structure and ornithischian hip structure is a case of convergent evolution, but these 2 groups use their hips in different ways--I suppose this is a case of similar-looking structures being used in mulitple ways. The similar body shapes of Cetacea (whales) and Icthys (fish) is real case of convergent evolution.

Answer 2

it could have evolved twice. or the diagram just could have been wrong. from what i can tell birds evolved from the bird hipped dinosaurs. therefore it only evolved once.

Answer 3

Why couldn't it have evolved twice?

Other things have evolved more than once, independently -- eyes, for example, evolved independently at least 6 times (if I recall correctly).