I know that. I was making a different point about the persistence in the parent stock of the genetic raw material which was amenable to the mutation.
But why didn't T. rex inherit mammary glands from anybody?
I assume this rather silly question is one which stumped g3k and the other creationists. In the evolution model, I suppose it's because the mutation for such a feature hadn't yet appeared. In the "simultaneous creation of all species" model (to which I don't subscribe) such a mutation would most definitely have appeared at the same time as everything else, yet it would have been present only in those creatures upon whom it had been bestowed, and thus it would be "locked in" to them, as it were. In a "gradual creation of all species" model (my favorite) you get pretty much the same thinking. Mammary glands exist where they've been created. To the evolutionist, take the last sentence and substitute "when" for "where." Same result.
You have trouble staying in evo mode for long, but I think you're there. The new features can only inherit up the tree. There's no jumping branches or going down.
The ancestors of mammals, Synapsida, branch off that Amniote tree I linked earliest of all. The lineage they left behind does a bunch of branchings thereafter, giving rise eventually to the diapsids, which include dinos and their bird descendants.
T. rex having mammaries would be a case of inheriting from a fifteenth cousin, or more like the future descendants of a fifteenth cousin. Unless T. rexes invented their own which were lost when T. rex died out. An unlikely happening with no evidence.