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Enantiornithes (Walker, 1981)
Enantiornithes is a major infraclass of Aves that is without question the most spectacular and most speciose avian assemblage from the Mesozoic. The discovery of the bones later attributed to Enantiornithes by Cyril Walker of the British Museum (Natural History), took place during field work under the supervision of enterprising South American paleontology Jose Bonaparte near Salta from 1974 to 1976. The adapative radiation of Enantiornithes, apparently a major function of the early evolution of the class Aves, appears to have occurred extremely rapidly and by the Upper Cretaceous Enantiornithes were the most diverse and abundant of all land birds (Feduccia 1996, 1999, Hou et al. 1996). Sereno defined Enantiornithes as a stem-taxon consisting of all birds closer to Sinornis santensis than to Neornithes (1998). This definition has been supported by multiple independent analyses, both cladistic (e.g., Chiappe 2002) and non-cladistic (e.g., Kurochkin 1996). The enantiornithine assemblage is united by a bizarre suite of apomorphic characters unknown outside of this clade, including:
- Proximo-distal fusion of the tarsometatarsus, the exact opposite of that seen in Ornithurae.
- Triosseal canal completed by a broad scapular process
- Significantly expanded hypocleidium
- Coracoid possesses a distinct triangular depression in dorsal aspect
The intra-taxon relationships of Enantiornithes remain contentious, but it is generally accepted that Iberomesornis romerali, the famed Las Hoyas bird, is the most basal known enantiornithines, while the avisaurids represent the most derived. The author, provisionally, follows Hou et al. (1996) and Kurochkin (1996) for a phylogenetic map of Enantiornithes. Nevertheless despite repeated attempts to clarify the internal topology of the infraclass, there remains a vast polytomy at the base of Enantiornithes, and the only real consensus which can be acheived is that Enantiornithes cleaves into two distinct lineages, one consisting of all forms closer to Iberomesornis than Sinornis and thus presumably basal amongst enantiornithines, and the other closer to Sinornis than Iberomesornis, this latter clade has been named Euenantiornithes by Chiappe & Walker (2002). The monophyly of Euenantiornithes is robustly supported, but the internal relationships of this grouping, as is the case with Enantiornithes in general, remain intractable.
- Chiappe, L. 2002. Basal bird phylogeny: problems and solutions. In: Chiappe, L. & Witmer, L. (eds.), Mesozoic Birds: Above the Heads of Dinosaurs, 448-472.
- Chiappe, L. & Walker, C. 2002. Skeletal morphology and systematics of the Cretaceous Euenantiornithes (Ornithothoraces: Enantiornithes). In: Chiappe, L. & Witmer, L. (eds.), Mesozoic Birds: Above the Heads of Dinosaurs, 240-267.
- Feduccia, A. 1996. The Origin and Evolution of Birds, First Edition. Yale University Press, New Haven.
- Feduccia, A. 1999. The Origin and Evolution of Birds, Second Edition. Yale University Press, New Haven.
- Hou, L., Martin, L. D., Zhou, Z., & Feduccia, A. 1996. Early adaptive radiation of birds: evidence from fossils from northeastern China. Science 274: 1164-1167.
- Kurochkin, E. 1996. A new enantiornithid of the Mongolian Late Cretaceous, and a general appraisal of the Infraclass Enantiornithes (Aves). Paleontological Institute, Moscow, 60 pp.
- Sereno, P. 1998. A rationale for phylogenetic definitions, with application to the higher-level taxonomy of Dinosauria. Neues Jahrbuch fur Geologie und Palaontologie 210: 41-83.