Enantiornithes
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The Enantiornithes, or "opposite birds", are an extinct group of flying birds, so named because their foot bones were fused differently from those of modern birds. Early discoveries were incorrectly referred to modern bird groups. They were first recognized as a distinct "subclass" by C.A. Walker in 1981, based on some partial remains from the Late Cretaceous of Argentina. In the 1990s, more complete enantiornithines were discovered, and it was more demonstrated that a few previously found birds (Iberomesornis, Cathayornis) have enantiornithine features.
Enantiornithines have been found in North America, South America, Europe, Asia, and Australia. Known fossils attributable to this group are exclusively Cretaceous, and it is believed that enantiornithines became extinct at the same time as their non-avian dinosaur relatives. One biogeographic study in the 1990s suggested that the distribution of enantiornithines implies a Middle Jurassic origin for the clade, but this theory has not been widely accepted by paleoornithologists. The earliest known enantiornithines are from the Early Cretaceous) of Spain (e.g. Noguerornis) and China (e.g. Eoenantiornis), and the latest from the Late Cretaceous of North and South America (e.g. Avisaurus).
Enantiornithines were more advanced than Archaeopteryx, but more primitive than all living birds (Neornithes). Over 30 species have been named, but it is likely that not all are valid. All but the most primitive enantiornithes belong to a clade called Euenantiornithes. Some had teeth, others were toothless. They have been found in both inland and marine sediments, suggesting that they were an ecologically diverse group. The smallest enantiornithines are described as sparrow-sized, but some were much larger, such as Enantiornis, which had an estimated wingspan of 1.2 meters (4 ft).
Although many frustratingly incomplete specimens of enatiornithines have held back attempts to understand the phylogeny of this group, some examples are spectacularly well-preserved with even their feathers in place. A few examples of embryonic enantiornithines have been found inside fossil eggs. Recently, palaeontologists in China found an enantiornithine fossil with flight feathers on its legs as well as its arms, linking it with the four-winged dinosaur Microraptor. This may suggest that some early birds had four wings, not two. However, the leg feathers of the enantiornithine differ from those of Microraptor in being shorter, and only extending down to the ankle rather than along the foot as in the four-winged dinosaur.
Taxonomy
Enantiornithine systematics generally follows one of 2 phylogenies. Both are presented below.Phylogeny after Chiappe (1992) Chiappe & Calvo (1994), Zhou & Hou (2002) and Chiappe & Walker (2002):
Subclass Enantiornithes
- Basal Enantiornithes and Enantiornithes incerta sedis
- Superorder Euenantiornithes
- * Basal Euenantiornithes
- * Euenantiornithes incerta sedis
- ** Family Cathyornithidae
- * Order Gobipterygiformes
- * Order Enantiornithiformes
- ** Family Avisauridae
Phylogeny after Kurochkin, 1996:
Subclass Enantiornithes
- Basal Enantiornithes and Enantiornithes incerta sedis
- Superorder Euenantiornithes
- * Basal Euenantiornithes and Euenantiornithes incerta sedis
- * Order Euornithiformes
- * Order Alexornithiformes
- ** Family Alexornithidae
- ** Family Enantiornithidae
References
- Walker, C. A. (1981): New subclass of birds from the Cretaceous of South America. Nature 292: 51-53. DOI:10.1038/292051a0 [HTML abstract]
- Chiappe, L. (1992): Enantiornithine(Aves) tarsometatarsi and the avian affinites of the Cretaceous Avisauridae. J. Vert. Paleontol. 12(3): 344-350.
- Chiappe, L. & Calvo, J. M. (1994): Neuquenornis volans, a new Late Cretaceous bird (Enantiornithes: Avisauridae) from Patagonia, Argentina. J. Vert. Paleontol. 14(2): 230-246.
- 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.
- 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.
- Zhou, Z. & Hou, L.-H. (2002): The Discovery and Study of Mesozoic Birds in China. In Chiappe, L. & Witmer, L. (eds.): Mesozoic Birds: Above the Heads of Dinosaurs: 160-183.
- Zhou, Z. & Zhang, F. (2005): Discovery of an ornithurine bird and its implication for Early Cretaceous avian radiation. PNAS 102(52): 18998-19002. DOI:10.1073/pnas.0507106102 [PDF fulltext] [Supporting Information]
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