Enantiornithes is an extinct group of primitive birds. They were the most abundant and diverse avialans of the Mesozoic. Almost all retained teeth and clawed hands, like other primitive birds. Enantiornithines are thought to have left no living descendants. See protobirds.
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 experiment in bird evolution, but one that diversified entirely separately from the lineage leading to modern birds. 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. See Apsaravis.
Most researchers place Enantiornithines in Aves, but those that use the more restricive definition of Aves put them lower in the Avialae. Enantiornithines were more advanced than Archaeopteryx or Confuciusornis, but in several respects more primitive than all living birds (Neornithes), perhaps following an intermediate evolutionary path. Due to the primitive features, some early studies placed Enantiornithes with Archaeopteryx in the clade Sauriurae, but only a few researchers still do.
Over 40 species of Enantiornithines have been named, but some names represent only single bones, so it is likely that not all are valid. They have been found in both inland and marine sediments, suggesting that they were an ecologically diverse group. Enantiornithine fossils appear to include waders, swimmers, fish-catchers, and hook-beaked raptors. The smallest are described as sparrow-sized, but some were much larger, such as Avisaurus which had an estimated wingspan of 1.2 meters (4 ft). Enantiornithine birds went extinct at the K-T boundary, along with hesperornithine birds and all other non-avian dinosaurs, and many other life forms.