Archaeopteryx may be used as a model for a transition species, but it itself is not the transitional species, as true birds had already evolved by the time archaeopteryx was around. Therefore, it could not have been a transitional species.
Which birds are you saying evolved before Archaeopteryx? Do you have any species examples? There are a couple fossils commonly used as counterexamples to Archy, but they're misleading at best and are actually not classified as birds. From the TalkOrigins article I posted earlier:

Two species of dinosaur have recently been found in northeast China which possess feathers (Qiang et al. 1998). Protoarchaeopteryx robusta and Caudipteryx zoui show regiges, rectrices and plumulaceous feather inpressions. Further, they are not birds, lacking a reverted (backwards facing) big toe (see number 2 below) and a quadrratojugal squamosal contact, having a quadrojugal joined to the quatrate by a ligament and a reduced or absent process of the ishium. These and other characters group Protoarchaeopteryx and Caudipteryx with maniraptoran coelurosaurs rather than birds.

Systematics Note (from Padian 1998): Systematists define the names of organisms by their ancestry, in this case birds (Aves) consist of Archaeopteryx plus living birds and all the descendants of their most recent common ancester. Birds are diagnosed by unique features that only they possess and which are inherited from that common ancestor. Even if feathers are shared by a wider group than just birds, birds are still defined as Archaeopteryx and later relatives. Protoarchaeopteryx and Caudipteryx are not birds even though they have feathers because the suite of morphological characters they possess mark them as belong to the maniraptoran coelurosaur dinosarus.
In any case, the existence of similar traits in other populations doesn't necessitate that one evolved from the other (feathered theropods =/= birds, necessarily; they could be related, but phylogenetic similarity alone is an insufficient basis on which to determine which species evolved first, or whether they both evolved from a more distant common ancestor; you need more evidence to say for certain). It could easily be a case of convergent evolutions rather than divergent.