Hesperornis was a Cretaceous bird that looked
a lot like a duck trying to be a penguin. 
But its really quite different from either
of its dopplegangers. 
Its legs are capable of an entirely different method of swimming, its wings are diminished and useless as an emu's arms, and it has lots of very distinct reptilian teeth in its beak. 
It looks familiar enough.  But its at least one case
against the old cliche' where just because it may look like a duck and walk like a duck, doesn't mean that
it actually is a duck.