Euenantiornithes - Discovery and Naming

Discovery and Naming

The first enantiornithines to be discovered were incorrectly referred to modern bird groups (Gobipteryx minuta). They were first recognized as a distinct lineage, or "subclass" by Cyril A. Walker, in 1981, based on some partial remains from the late Cretaceous period of what is now Argentina. Since the 1990s, more complete enantiornithines were discovered and it was demonstrated that a few previously described birds (e.g. Iberomesornis, Cathayornis, Sinornis) had enantiornithine features.

"Enantiornithes" means "opposite birds", from Ancient Greek einantios (ἐνάντιος) "opposite" + ornithes (όρνιθες) "birds" . The name was coined by Walker in his landmark paper which established the group. 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."

This refers to an anatomical feature – the articulation of the shoulder bones – which has a concave-convex socket joint that is the reverse of that of modern birds. Specifically, in enantiornithines, the facet where the scapula (shoulder blade) meets the coracoid (the primary bone of the shoulder girdle in vertebrates other than mammals) is a convex knob and the corresponding point on the shoulder blade is concave and dish-shaped. In modern birds, the way the joint articulates is reversed.

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:

"The birds are so named because, among many distinctive features, there is a unique formation of the triosseal canal and the metatarsals are fused proximally to distally, the opposite of that in modern birds"

Feduccia's point about the tarsometatarsus (the combined upper foot and ankle bone) is correct, but Walker did not use this reasoning in his original paper. Walker never described the fusion of the tarsometatarsus as opposite, but rather as "Only partial". Also, it is not certain that enantiornithines had triosseal canals, since no fossil preserves this feature.