Yanornis

Yanornis (simplified Chinese: 燕鸟; traditional Chinese: 燕鳥; pinyin: Yān niǎo; lit.: 'Yan bird') 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,[1] and Yanornis guozhangi, from the 124-million-year-old Yixian Formation.[2]

Yanornis Temporal range: Early Cretaceous, 125–120 Ma PreꞒ Ꞓ O S D C P T J K Pg N ↓
Fossil specimen of Y. martini
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Clade:	Dinosauria
Clade:	Saurischia
Clade:	Theropoda
Family:	†Songlingornithidae
Genus:	†Yanornis Zhou & Zhang, 2001
Type species
†Yanornis martini Zhou & Zhang, 2001
Species
†Y. martini Zhou & Zhang, 2001 †Y. guozhangi Wang et al., 2013
Synonyms
Archaeovolans repatriatus Czerkas & Xu, 2002 Aberratiodontus wui Gong, Hou & Wang, 2004

Description

Fossil specimen of Y. martini

Y. martini was the size of a chicken,[3] 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.[4] To allow for the necessarily large flight muscles, the sternum of Yanornis was longer than it was wide, again representing an essentially modern condition.[5]

Diet

Specimen of Y. martini with a whole fish in its crop, and macerated fish remains in its stomach

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.[6]

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.[7] 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.[8]

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.[9] 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.[5]

Reconstructed alimentary canal superimposed over a specimen of Y. martini

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)).[10]

Euornithes

Archaeorhynchus spathula     Jianchangornis microdonta     Ornithuromorpha   Chaoyangia beishanensis     Schizooura lii     Vorona berivotrensis     Zhongjianornis yangi       Patagopteryx deferrariisi     Hongshanornithidae   Hongshanornis longicresta     Longicrusavis houi       Songlingornithidae   Yixianornis grabaui       Hollanda luceria     Songlingornis linghensis     Yanornis martini           Gansus yumenensis     Ambiortiformes   Ambiortus dementjevi     Apsaravis ukhaana       Ornithurae

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.[6]

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.[1]

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,[11][12] an opinion which has been supported by subsequent reviews of enantiornithine taxonomy.[13]

References

1. Zhou, Z.; Clarke, J.A.; Zhang, F. (2002). "Archaeoraptor 's better half". Nature. 420: 285. doi:10.1038/420285a. PMID 12447431.
2. Wang, Ji; Teng; Jin (2013). "A new species of Yanornis (Aves: Ornithurae) from the Lower Cretaceous strata of Yixian, Liaoning Province". Geological Bulletin of China. 32 (4): 601–606.
3. Holtz, Thomas R. Jr. (2011) Dinosaurs: The Most Complete, Up-to-Date Encyclopedia for Dinosaur Lovers of All Ages, Winter 2010 Appendix.
4. Senter, Phil (2006). "Scapular orientation in theropods and basal birds, and the origin of flapping flight" (PDF). Acta Palaeontologica Polonica. 51 (2): 305–313.
5. Gong, Enpu; Hou, Lianhai; Wang, Lixia (2004). "Enantiornithine Bird with Diapsidian Skull and Its Dental Development in the Early Cretaceous in Liaoning, China". Acta Geologica Sinica. 78 (1): 1–7.
6. Zhou Z. and Zhang F. (2001). "Two new ornithurine birds from the Early Cretaceous of western Liaoning, China." Chinese Science Bulletin, 46 (15), 1258-1264. PDF
7. Zhou, Zhonghe; Clarke, Julia A.; Zhang, Fucheng; Wings, O. (2004). "Gastroliths in Yanornis: an indication of the earliest radical diet-switching and gizzard plasticity in the lineage leading to living birds?" (PDF). Naturwissenschaften. 91: 571–574. doi:10.1007/s00114-004-0567-z.
8. Zheng, X.; O'Connor, J. K.; Huchzermeyer, F.; Wang, X.; Wang, Y.; Zhang, X.; Zhou, Z. (2014). "New Specimens of Yanornis Indicate a Piscivorous Diet and Modern Alimentary Canal". PLOS One. 9 (4): e95036. doi:10.1371/journal.pone.0095036. PMC 3986254. PMID 24733485.
9. Clarke, Julia A.; Zhou, Zhonghe; Zhang, Fucheng (2006). "Insight into the evolution of avian flight from a new clade of Early Cretaceous ornithurines from China and the morphology of Yixianornis grabaui". Journal of Anatomy. 208 (3): 287–308. doi:10.1111/j.1469-7580.2006.00534.x. PMC 2100246. PMID 16533313.
10. O’Connor, J. K.; Zhang, Y.; Chiappe, L. M.; Meng, Q.; Quanguo, L.; Di, L. (2013). "A new enantiornithine from the Yixian Formation with the first recognized avian enamel specialization". Journal of Vertebrate Paleontology. 33: 1–12. doi:10.1080/02724634.2012.719176.
11. Cau, A.; Arduini, P. (2008). "Enantiophoenix electrophyla gen. et sp. nov. (Aves, Enantiornithes) from the Upper Cretaceous (Cenomanian) of Lebanon and its phylogenetic relationships". Atti della Societa Italiana di Scienze Naturali e del Museo Civico di Storia Naturale in Milano. 149: 293–324.
12. Zhou, Z.; Clarke, J.; Zhang, F. (2008). "Insight into diversity, body size and morphological evolution from the largest Early Cretaceous enantiornithine bird". Journal of Anatomy. 212: 565–577. doi:10.1111/j.1469-7580.2008.00880.x. PMC 2409080. PMID 18397240.
13. O'Connor, J.; Dyke, G. (2010). "A reassessment of Sinornis santensis and Cathayornis yandica (Aves: Enantiornithes)". Records of the Australian Museum. 62: 7–20. doi:10.3853/J.0067-1975.62.2010.1540.