Up in the Air

If you think that things are nailed down concerning the basics of birds - think again!

http://en.wikipedia.org/wiki/Enantiornithes

Phylogeny

"Enantiornithes is the sister group to Ornithurae or Ornithuromorpha depending on the taxonomic authority, and together they could form a clade called Ornithothoraces. Most phylogenetic studies have recovered Enantiornithes as a monophyletic group distinct from the modern birds and their closest relatives. The 2002 phylogenetic analysis by Clarke and Norell, though, reduced the number of enantiornithine autapomorphies to just four.^[2] This raises the possibility that the discovery of new fossils could unite Enantiornithes and the birds closer to living species into one clade. If this proves to be true, then Enantiornithes is a paraphyletic taxon and thus phylogenetically invalid. All enantiornithines would then be united in the next larger clade Ornithothoraces instead, and called "ornithothoracines". (see Apsaravis for more on the possible invalidation of Enantiornithes)

On the other hand, Confuciusornis might be closer to Enantiornithes than to living birds rather than about equally distinct from both, which in turn would render the Ornithothoraces meaningless too. In that case, the Pygostylia would apply, but that taxon, too, is ill-defined. Altogether, the radiation of the early truly avian lineages (as opposed to "dinobirds" like Archaeopteryx or Dalianraptor) presents a highly confusing picture at present, and while the apparent deep divergence between Enantiornithes and Neornithes seems real, the relationships of these two and other Cretaceous groups like Hesperornithes or Liaoningornithiformes is not well resolved."

"Enantiornithine systematics are also highly provisional. The version used here, although based on many sources, is only tentative, and in need of revision in light of abundant new fossil discoveries. What appears fairly certain by now^[20] is that there were subdivisions within Enantiornithes possibly including some minor basal lineages in addition to the more apomorphic Euenantiornithes. The latter may be a clade or an evolutionary grade (and hence are also of questionable validity). The details of the interrelationship of all these lineages, indeed the validity of most, is disputed, although the Avisauridae, for one example, seem likely to constitute a valid group. Phylogenetic taxonomists have hitherto been very reluctant, and justifiably so, to suggest delimitations of enantiornithine clades.^[21]"

^AND

^"A consensus of scientific analyses indicates that Enantiornithes is one of two major sister groups of derived birds. The other group is the Ornithurae, which includes all living birds as a subset. This means that Enantiornithines are a successful branch of bird evolution, but one that diversified entirely separately from the lineage leading to modern birds.^[1] This consensus has never been universally accepted and is being challenged by new studies, so that it is possible that enantiornithines may actually represent successive outgroups on the lineage leading to modern birds.^[2]"