Yanornis

''Yanornis'' () is an extinct genus of fish-eating Early Cretaceous birds. Two species have been described, both from Liaoning province, China: ''Yanornis martini'', based on several fossils found in the 120-million-year-old Jiufotang Formation at Chaoyang, and ''Yanornis guozhangi'', from the 124-million-year-old Yixian Formation.

Description

''Y. martini'' was the size of a chicken,Holtz, Thomas R. Jr. (2011) ''Dinosaurs: The Most Complete, Up-to-Date Encyclopedia for Dinosaur Lovers of All Ages,'
Winter 2010 Appendix.
/ref> had a long skull with about 10 teeth in the upper jaw and 20 teeth in the lower jaw, and was both able to fly and walk well, having a well-developed U-shaped furcula (wishbone).

The absence of the prefrontal bone and the non- diapsid skull allows ''Yanornis'' to be classified as an ornithuromorph, a member of a group of stem-birds which also includes the common ancestor of living birds. Similarly, its scapula and coracoid had evolved the basic shape and layout as in modern birds, enabling ''Yanornis'' to lift its wings far above its back for an efficient upstroke. It was probably a more efficient flyer compared to Enantiornithes (which have the modern condition in a less well-developed form), and especially compared to '' Confuciusornis'' and ''Archaeopteryx'', which were only marginally able to perform upstrokes. To allow for the necessarily large flight muscles, the sternum of ''Yanornis'' was longer than it was wide, again representing an essentially modern condition.

Diet

Several fossil specimens of ''Y. martini'' preserve the remains of fish in the stomach and crop, suggesting that these birds were primarily fish-eaters. Their fish-eating and associated adaptations show convergent evolution with the unrelated enantiornithine '' Longipteryx''.

One specimen preserved large amounts of supposed gastroliths ("stomach stones") in the gizzard region. Large numbers of small gastroliths are usually associated with species that need to grind tough plant material, such as seeds, after swallowing them. This led some scientists to suggest that ''Yanornis'' was capable of "diet switching", perhaps seasonally, between fish and seeds. However, later studies cast doubt on the diet-switching hypothesis. Further study of the specimen found that the supposed stomach stones were not massed around a single region corresponding with the gizzard, as in other fossils with such stones, including specimens of '' Archaeorhynchus'' and '' Hongshanornis''. Rather, the stones were spread throughout the body cavity in a front-to-back arrangement. This has been suggested to correspond more closely with the intestines, and may represent impacted sand. In modern birds, sand is often swallowed accidentally during feeding (particularly when feeding on dead fish), and, due to some obstruction, may eventually become impacted in the intestines, leading to death.

Classification

In a 2006 study of early bird relationships, it was found that ''Yanornis'', '' Yixianornis'', and '' Songlingornis'' formed a monophyletic group; since ''Songlingornis'' was the first of these birds to be described, the family containing this group is Songlingornithidae. The order Yanornithiformes has been erected to mark their distinctness from other early Ornithurae such as '' Gansus'', but might be called Songlingornithiformes; especially if the present taxon is indeed a junior synonym of ''Songlingornis'' as sometimes proposed.

The cladogram below follows O’Connor ''et al.'', 2013 phylogenetic analysis. The clade names are positioned based on their definitions (contra O’Connor ''et al.'' (2013)).

Name and synonyms

The genus name ''Yanornis'' is derived from the Ancient Chinese Yan dynasties, whose capital was at Chaoyang, and Ancient Greek ''ornis'', "bird". The species ''Y. martini'' was named for avian paleontologist Larry Martin.

''Yanornis'' gained notoriety when the front half of a fossil bird was combined with the tail of a ''Microraptor'' to make the paleontological forgery " Archaeoraptor". Upon discovering this, the bird half was described as ''Archaeovolans repatriatus'', which was later found to be a junior synonym of ''Yanornis''.

Some studies have found that the bird species ''Aberratiodontus wui'' is in fact a poorly preserved specimen of ''Yanornis martini'', or at least a close relative, an opinion which has been supported by subsequent reviews of enantiornithine taxonomy.

References

{{Taxonbar, from=Q133517

Euornithes
Early Cretaceous dinosaurs of Asia
Jiufotang fauna
Yixian fauna
Fossil taxa described in 2001
Taxa named by Zhou Zhonghe