are based on only a fragment of a single bone. O'Connor and Dyke argued that while these specimens can help expand knowledge of the time span or geographic range of the Enantiornithes and it is important to describe them, naming such specimens is "unjustifiable". Relationships Enantiornithes is the sister group to Euornithes, and together they form a clade called Ornithothoraces (though see above). 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 Enantiornithes autapomorphies to just four. Enantiornithes systematics are highly provisional and notoriously difficult to study, due to the fact that Enantiornithes 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 Enantiornithes 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. One such delineation named the Euenantiornithes, was defined by Chiappe (2002) as comprising all species closer to Sinornis than to Iberomesornis. Because Iberomesornis is often found to be the most primitive or basal enantiornithine, Euenantiornithes may be an extremely inclusive group, made up of all Enantiornithes except for Iberomesornis itself. Despite being in accordance with phylogenetic nomenclature, this definition of Euenantiornithes was severely criticized by some researchers, such as Paul Sereno, who called it "a ill-defined clade [...] a good example of a poor choice in a phylogenetic definition". The cladogram below was found by an analysis by Wang et al. in 2015, updated from a previous data set created by Jingmai O'Connor. The cladogram below is from Wang et al., 2022, and includes most named taxa and recovers several previously-named clades. Letters on branches indicate the positions of "wildcard" taxa, those which have been recovered in multiple disparate positions. Key to letters: b = Boluochia c = Cathayornis e = Enantiophoenix f = Houornis h = Longipteryx i = Parabohaiornis j = Pterygornis l = Vorona m = Yuanjiawaornis n = Yungavolucris List of genera Incertae sedis Enantiornithes taxonomy is