I'm sorry if I'm answering to a question not made to me.
anyway ,philidor11 wrote:
<<Go far enough back in any group you choose to distinguish and the
differences from an ancestral group will be small, by definition.>>
because the whole list of differences between the two groups appear over
time, you also have to choose how many of those differences have to be
present to say that a specific animal is part of one group or the other.>>
ok, but if you consider Archaeopteryx to be part of Aves, then you've to consider the presence of the characters you've previously chosen (having observed them in an animal thought to be derived enough to be considered a bird, which is a purely arbitrary choice ),in Archie too, and you surely won't find many of them, because they would have been recognized in animals considered to be "different enough" from , say, a dromie which is, however very similar to Archie.
Then you would have to consider Archie, and all the animals more "birdy" than a dromie, but not "birdy" enough to be considered a true bird, as something between a bird and a dinosaur, but ....
<<The difference in approach being considered is: are you emphasizing the end
result (bird/dinosaur) or the beginning of the separation between the two
groups? Doesn't it seem most reasonable to emphasize the end result when
deciding when differences are large enough to distinguish a group?>>
no, i think it's more reasonable to consider the group as a whole; in this case you have to arbitrarily choose to consider one branch of the tree as a different group, which it is , based on few characters seen only in the components of this group, but _in ALL_ of them. Many derived featurs will be present in some taxa, but this isn't important(well, at least as long as you don't think they're enough to estabilish another group).
ths is only a matter of human perspective as it has been pointed out many times on the list.