Enantiornithine systematics are highly provisional and notoriously difficult to study, due to the fact that enantiornithines tend to be extremely homoplastic, or very similar to each other in most of their skeletal features due to convergent evolution rather than common ancestry. What appears fairly certain by now is that there were subdivisions within enantiornithines possibly including some minor basal lineages in addition to the more advanced Euenantiornithes. 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 to suggest delimitations of enantiornithine clades.