LtMM_ t1_iwlcycq wrote on November 16, 2022 at 2:26 PM

The terms have to do with whether the legs are positioned underneath the body (like a bird), or splayed out to the side (like a lizard). I don't think you're correct saying birds are saurischians, apologies if wrong.

FillRevolutionary900 t1_iwldib4 wrote on November 16, 2022 at 2:30 PM

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.

LtMM_ t1_iwlf6n3 wrote on November 16, 2022 at 2:43 PM

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.