The Enantiornithes ("opposite birds") clade was established in 1981, based on some scattered postcranial elements discovered in Argentina by paleontologist José Bonaparte and scientifically described by C.A. Walker. Walker described the namesake of the clade, Enantiornis (Late Cretaceous, Argentina), noting that the fusion of bones in its feet and ankles differed from other birds. In modern birds, the foot bones, or metatarsals, are fused together from the middle bone to the outward bones. The metatarsals of Enantiornis were fused from the direction of the ankle toward the toes—the opposite of other birds and hence the bird's name. Since the discovery of Enantiornis, a host of other similar birds have been discovered.

The Enantiornithes were the most diverse and geographically widespread of all Cretaceous birds. Anatomically, these birds were not as basal as Archaeopteryx but not as derived as modern birds. They shared such traits as a relatively long tail capped by a short, fused pygostyle; a bony but weakly developed breastbone; a Y-shaped wishbone; similar limb elements; and toothed jaws—although some later species were toothless.

Early discoveries of Enantiornithes consisted primarily of fragmentary specimens, and it was difficult to paint a complete picture of the clade with certainty. Neuquenornis (Late Cretaceous, Argentina) was described by paleontologists Luis Chiappe and Jorge Calvo in 1994 and was based upon a partial skeleton that included most of the traits of this group in one specimen.

Enantiornis was a taxon of large birds with a wingspan of about 4 feet (1.2 m), but most other members of the clade were about the size of a sparrow or thrush. These birds also adapted to many lifestyles and habitats, with different taxa finding home in the trees, on the ground, and perhaps in the water as waders or swimmers. Enantiornithes are currently known from more than 25 taxa hailing from every continent but Antarctica. They apparently all became extinct at the end of the Cretaceous Period, alongside the last non-avian dinosaurs.

Sinornis (Early Cretaceous, China), the "Chinese bird," was described by Paul Sereno and Chenggang Rao in 1992 and based primarily on a nearly complete skeleton. Living about 25 million years after Archaeopteryx, its shortened body, pygostyle, and folding wings made it closer to the body plan of a modern bird, but it still had teeth, clawed fingers, and a small breast bone.

Eoalulavis (Early Cretaceous, Spain), the "dawn alula bird," had many anatomical improvements for flight over Archaeopteryx and other basal birds. Most notably, it had the oldest known alula, a set of feathers on the front edge of the bird's wing that helped sustain lift as the bird flew at slow speeds.

Other noteworthy Enantiornithes for which there is convincing fossil evidence are Aberratiodontus (Early Cretaceous, China); Alexornis (Late Cretaceous, Mexico); Bolouchia (Early Cretaceous, China); Concornis (Early Cretaceous, Spain); Dapingfangornis (Early Cretaceous, China); Elsornis (Late Cretaceous, Mongolia); Eocathayornis (Early Cretaceous, China); Eoenantiornis (Early Cretaceous, China); Gobipteryx (Late Cretaceous, Mongolia); Halimor-nis (Late Cretaceous, Alabama); Iberomesornis (Early Cretaceous, Spain); Longipteryx (Early Cretaceous, China); Longirostravis (Early Cretaceous, China); Otogornis (Early Cretaceous, China); Protop-teryx (Early Cretaceous, China); and Vescornis (Early Cretaceous, China).

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