Enantiornithes
The Enantiornithes, also known as enantiornithines or enantiornitheans in literature, are a group of extinct avialans ("birds" in the broad sense), the most abundant and diverse group known from the Mesozoic era.[3][4][5] Almost all retained teeth and clawed fingers on each wing, but otherwise looked much like modern birds externally. Over eighty species of Enantiornithes have been named, but some names represent only single bones, so it is likely that not all are valid. The Enantiornithes became extinct at the Cretaceous–Paleogene boundary, along with Hesperornithes and all other non-avian dinosaurs.
The first Enantiornithes to be discovered were incorrectly referred to modern bird groups. For example, the first known species of Enantiornithes, Gobipteryx minuta, was originally considered a paleognath related to ostriches and tinamou.[6] The Enantiornithes were first recognized as a distinct lineage, or "subclass" of birds, by Cyril A. Walker in 1981. Walker made this discovery based on some partial remains from the late Cretaceous period of what is now Argentina, which he assigned to a new genus, Enantiornis, giving the entire group its name. Since the 1990s, many more complete specimens of Enantiornithes have been discovered, and it was determined that a few previously described "birds" (e.g. Iberomesornis, Cathayornis, and Sinornis) were also Enantiornithes.
The name "Enantiornithes" means "opposite birds", from Ancient Greek enantios (ἐνάντιος) "opposite" + ornithes (ὄρνιθες) "birds" . The name was coined by Cyril Alexander Walker in his landmark paper which established the group.[7] In his paper, Walker explained what he meant by "opposite":
Perhaps the most fundamental and characteristic difference between the Enantiornithes and all other birds is in the nature of the articulation between the scapula [...] and the coracoid, where the 'normal' condition is completely reversed.[7]
This refers to an anatomical feature – the articulation of the shoulder bones – which has a concave-convex socket joint between the scapula (shoulder blade) and coracoid (the primary bone of the shoulder girdle in vertebrates other than mammals) that is the reverse of that of modern birds. Specifically, in the Enantiornithes, the scapula is concave and dish-shaped at this joint, and the coracoid is convex.[3]: 249–50 In modern birds, the coracoscapular joint has a concave coracoid and convex scapula.[8][9]
Walker was not clear on his reasons for giving this name in the etymology section of his paper, and this ambiguity led to some confusion among later researchers. For example, Alan Feduccia stated in 1996:
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