Had to find out what's an avialan - Wikipedia article on the subject
actually says that Archeopteryx is Jurassic in origin.
I don't understand what the two parts of this sentence have to do with each
other.
Anyway, *Archaeopteryx*, note the _ae_, is Late Jurassic in age (from near
the end of that epoch). Are you talking about the age of "*Epidexipteryx*"
(which is Late Jurassic or Early Cretaceous, probably near the middle of the
latter)?
From the discussion it rather looks as if bird ancestors were a huge
clade.
Ancestors aren't a clade -- the definition of "clade" and its synonym
"monophylum" is "an ancestor and all its descendants".