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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".