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FillRevolutionary900 t1_iwldib4 wrote

Reply to comment by LtMM_ in Why are saurischians (lizard-hipped) and ornithiscians (bird-hipped) named as such when literally all birds (and the things they are closely related to) are saurischians and herbivorous dinosaurs that are anything but bird-like are considered bird-hipped? by [deleted]

Leg position (under or beside the body) is definitely not what distinguishes saurischians from ornithiscians. It's to do with pelvic structure and the position of the pubis bone.

And birds are saurischians. This is from Wikipedia:

All carnivorous dinosaurs (certain types of theropods) are traditionally classified as saurischians, as are all of the birds and one of the two primary lineages of herbivorous dinosaurs, the sauropodomorphs. At the end of the Cretaceous Period, all saurischians except the birds became extinct in the course of the Cretaceous–Paleogene extinction event. Birds, as direct descendants of one group of theropod dinosaurs, are a sub-clade of saurischian dinosaurs in phylogenetic classification.

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LtMM_ t1_iwlf6n3 wrote

My bad, but I think you've somewhat answered your own question here. In this case saurischians and ornithicians are being used as phylogenetic terms to represent groups of evolutionarily related animals based on a key morphological trait that was easily distinguishable by comparison to birds versus lizards. The fact that bird and bird-hipped dinosaurs have similar pelvic structure is likely a simple case of convergent evolution.

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