Enantiornithes

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iEnantiornithes
Fossil range: Early - Late Cretaceous
Cathayornis/Sinornis
Cathayornis/Sinornis
Conservation status
Extinct (fossil)
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Aves
Subclass: Enantiornithes
Walker, 1981
Orders

see text

The Enantiornithes, or 'opposite birds', are an extinct group of flying birds, so named because their scapula and coracoid had a concave-convex surface different from those of modern birds[citation needed]. 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 Period of what is now Argentina. In the 1990s, more complete enantiornithines were discovered and it was more demonstrated that a few previously found birds (Iberomesornis, Cathayornis/Sinornis) had 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; a Late Jurassic/Early Cretaceous origin is more easier to reconcile with the fossil record. The earliest known enantiornithines are from the Early Cretaceous) of Spain (e.g. Noguerornis, a basal genus) and China (e.g. Eoenantiornis, a more derived genus) and the latest from the Late Cretaceous of North and South America (e.g. Avisaurus). The widespread occurrence suggests that the Enantiornithes were able to cross oceans on their own power; they are the first bird lineage with a global distribution.

Enantiornithines were more advanced than Archaeopteryx, but more primitive than all living birds (Neornithes) - or rather, since they were not ancestors of modern birds, followed another evolutionary path. Over 35 species have been named, but it is likely that not all are valid. 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 Avisaurus, 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[citation needed], similar to the four-winged dinosaur Microraptor. 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.

[edit] Systematics

Enantiornithine systematics is very badly resolved. The version used here, although based on many sources, is inaccurate, as the groundwork was proposed at a time when the enantiornithine fossil record was about half of what it was 10 years later. What appears fairly certain by now (Chiappe, 2002) is that there were some minor basal lineages and the Euenantiornithes which in turn repeats this branching pattern but with several more derived lineages. The details of the interrelationship of all these lineages, indeed the validity of most, is disputed at best, although the Avisauridae for example seem likely to consititute a valid group. Phylogenetic taxonomy has hitherto been very reluctant, and justifiably so, to suggest delimitiations of enantiornithine clades (Sereno, 2005).

Phylogeny based on Chiappe (1992) Chiappe & Calvo (1994), Kurochkin (1996), Zhou & Hou (2002), Chiappe & Walker (2002) and Haaramo (2006):

SUBCLASS Enantiornithes

  • Basal Enantiornithes and Enantiornithes incerta sedis
    • "Liaoxiornis" (Yixian Early Cretaceous of Liaoning, China) - a nomen dubium
    • Enantiornithes gen. et sp. indet. CAGS−IG−02−0901 (Xiagou Early Cretaceous of Mazongshan, China)
    • Enantiornithes gen. et sp. indet. CAGS-IG-04-CM-023 (Xiagou Early Cretaceous of Changma, China)
    • Enantiornithes gen. et sp. indet. MCSNM V3882a (Late Cretaceous of Ouadi al Gabour, Lebanon)
    • Enantiornithes gen. et sp. indet. MZ unnumbered (Adamantina Late Cretaceous of Presidente Prudente, Brazil) - enantiornithiform?
    • Dapingfangornis (Jiufotang Early Cretaceous of Liaoning, China)
    • Elsornis (Late Cretaceous of Gobi Desert, Mongolia)
    • Order Iberomesornithiformes (disputed)
      • Family Iberomesornithidae
        • Iberomesornis (Early Cretaceous)
        • Noguerornis (Early Cretaceous of El Montsec, Spain)
  • Superorder Euenantiornithes
    • Basal Euenantiornithes
    • Concornis (Calizas de La Huergina Early Cretaceous of Las Hoyas, Spain)
    • Eoalulavis (Middle Cretaceous of Las Hoyas, Spain)
    • Euenantiornithes incerta sedis
    • Aberratiodontus (Jiufotang Early Cretaceous of Shangheshou, China)
    • Boluochia (Loufotang Early Cretaceous of Liaoning, China) - cathayornithid?
    • Cuspirostrisornis (Jiufotang Early Cretaceous of Liaoning, China)
    • Enantiornithes gen. et sp. indet. CAGS−IG−04−CM−007 (Xiagou Early Cretaceous of Mazongshan, China)
    • Eoenantiornis (Yixian Early Cretaceous of Liaoning, China)
    • Largirostrornis (Jiufotang Early Cretaceous of Liaoning, China)
    • Longchengornis (Jiufotang Early Cretaceous of Liaoning, China)
    • Longirostravis (Yixian Early Cretaceous of Yixian County, China)
    • Hebeiornis (Yixian Early Cretaceous of Senjitu, China) - possibly a nomen nudum; if valid, includes Vescornis
    • Enantiornithes gen. et sp. indet. RBCM.EH2005.003.0002 (Northumberland Late Cretaceous of Hornby Island, Canada)
    • Gurilynia (Nemegt Late Cretaceous of S Mongolia) - enantiornithiform?
    • Halimornis (Mooreville Chalk Late Cretaceous of Greene County, USA)
    • Lectavis (Late Cretaceous) - enantiornithiform?
    • Lenesornis (Bissekty Late Cretaceous of Kyzyl Kum, Uzbekistan)
    • Yungavolucris (Late Cretaceous) - enantiornithiform (avisaurid)?
    • Order "Cathayornithiformes" (disputed)
      • Family "Cathyornithidae"
        • Sinornis/Cathayornis (Jiufotang Early Cretaceous of Liaoning, China) - which name is correct is disputed
        • Eocathayornis
    • Order Alexornithiformes (disputed)
      • Family Alexornithidae
        • Alexornis (Late Cretaceous)
        • Kizylkumavis (Bissekty Late Cretaceous of Kyzyl Kum, Uzbekistan)
        • Sazavis (Bissekty Late Cretaceous of Kyzyl Kum, Uzbekistan)
    • Order Gobipterygiformes (disputed)
    • Order Enantiornithiformes
      • Family Enantiornithidae (disputed)
      • Family Avisauridae
        • Avisaurus (Late Cretaceous)
        • Neuquenornis (Late Cretaceous of Patagonia, Argentina)
        • Soroavisaurus (Lecho Late Cretaceous of El Brete, Argentina)

Sometimes included in the Enantiornithes are the following taxa:

  • Longipteryx (Early Cretaceous) - euenantiornithine (own order)?
  • Protopteryx (Early Cretaceous of China)
  • Nanantius (Early and possibly Late Cretaceous) - enantiornithiform?
  • Abavornis (Late Cretaceous)
  • Horezmavis (Late Cretaceous of Kyzyl Kum, Uzbekistan) - gobipterygiform?
  • Gargantuavis (Late Cretaceous of S France)
  • Incolornis (Late Cretaceous)
  • Patagopteryx (Barro de la Carpa Late Cretaceous of Sierra Barrosa, Argentina)
  • Family Zhyraornithidae - enantiornithiform?
  • Catenoleimus
  • Explorornis.

The validity of the supposed Early Cretaceous euenantiornithine Jibeinia is disputed. The Late Cretaceous taxon "Cerebavis" is based on an endocranial cast and while this is not diagnostic, it is different from modern birds and as far as can be told from Ornithurae in general.

[edit] References

  • Chiappe, L. (1992): Enantiornithine (Aves) tarsometatarsi and the avian affinites of the Cretaceous Avisauridae. Journal of Vertebrate Paleontology 12(3): 344-350.
  • Chiappe, L. & Calvo, J. M. (1994): Neuquenornis volans, a new Late Cretaceous bird (Enantiornithes: Avisauridae) from Patagonia, Argentina. Journal of Vertebrate Paleontology 14(2): 230-246. HTML abstract
  • Chiappe, L. & Walker, C. A. (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.
  • 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.
  • Walker, C. A. (1981): New subclass of birds from the Cretaceous of South America. Nature 292: 51-53. DOI:10.1038/292051a0 (HTML abstract)
  • 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.
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