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Re: correct this "definition"

* Three or more vertebrae fixed to the hip bones

In general, yes; but either reversed in a few Late Triassic forms or independantly evolved in Ornithischia, Sauropodomorpha, Theropoda.

This, in turn, depends on the definition of "sacral vertebra", as pointed out in Sereno's Historical Biology paper. "Fixed to the hip bones" doesn't begin to cover the observed variation.

* a hole rather than a socket in the hip for the leg bone


Not that it matters, but this is reversed in ankylosaurs and partially reversed in lots of birds.

but aren't the "ruling reptiles" the archosaurs?

Yes, except it's more like "ruling lizards".