| Gansus | |
|---|---|
 | |
| Fossil specimen,Beijing Museum of Natural History. | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Clade: | Dinosauria |
| Clade: | Saurischia |
| Clade: | Theropoda |
| Clade: | Avialae |
| Family: | †Gansuidae |
| Genus: | †Gansus Hou & Liu, 1984 |
| Type species | |
| †Gansus yumenensis Hou & Liu, 1984 | |
| Other species | |
Gansus is agenus ofaquatic birds that lived during theAptian age of the EarlyCretaceous (Aptian-Albian) period in what are nowGansu andLiaoning provinces, westernChina. The rock layers from which their fossils have been recovered are dated to 120 million years ago.[2] It was first described in 1984 on the basis of an isolated left leg.[3] It is the oldest-known member of theOrnithurae, the group which includes modern birds (Neornithes) and extinct related groups, such asIchthyornis andHesperornithes.[3][4]
The genusGansus contains a single species,G. yumenensis,[3] which was about the size of apigeon and similar in appearance toloons anddiving ducks.[6] It had many features common among modern birds, and also retained some basal traits such as its clawed wings.[6]
Gansus was discovered in the form of a single fossil foot in 1981. Five more well-preserved fossils were found in 2003–2004 inmudstone at the site of an ancient lake at Changma, Gansu; the geological stratum in which the fossils were found is theXiagou Formation. Their bodies had settled in anoxic mud and were soon covered with further extremely fine silty sediments. Without oxygen, their remains resisted decay: these specimens preserved remains offlight feathers and traces of the webbing between their toes.[6] In 2011, there were described nine additional specimens, that supported on the basis of a statistic analysis of the sternum and elements of the legs, the hypothesis thatGansus was a volant bird.[7]
Youet al. (2006) concluded that the anatomical characteristics ofGansus were similar to foot-propelled diving birds, such asHesperornis (from theCretaceous) and the loons (Gaviidae) and grebes (Podicipedidae).[6] On the other hand, Liet al. (2011) concluded thatGansus showed a more similar morphology to ducks.[7] Two years later, Nuddset al. (2013) showed that the pectoral limb length proportions ofGansus were most similar to swifts and hummingbirds (Apodiformes), while the pelvic limb length proportions fell within the modern birds (Neornithes), showing similarities with grebes (Podicipedidae), albatross (Diomedeidae) and cormorants (Phalacrocoracidae), suggesting thatGansus was both volant and capable of diving to some degree using either foot-propelled or, perhaps, both its wings and its feet for underwater locomotion.[8]
Gansus was described as the oldest knownornithuran. TheOrnithurae, however, has been given several very different definitions. In the definition used by You and colleagues (that is, theclade containing all living birds plusHesperornis andIchthyornis),Gansus is indeed the oldest known member. However, several birds from the olderYixian Formation and contemporaryJiufotang Formation are considered ornithurans under other definitions. Under any definition, all living birds, includingtaxa as diverse asostriches,hummingbirds andeagles, are descended from basal ornithurans, many of which were semi-aquatic. It is now thought possible that all modern birds descended specifically from a semi-aquatic bird similar toGansus. Thus, whileGansus is not necessarily a direct ancestor of today's birds, it is closely related to such an ancestral species.[6] This hypothesis was corroborated by later phylogenetic studies that included this taxon.[4][9]
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