Gansus
Gansus is a genus of aquatic birds that lived during the Aptian age of the Early Cretaceous (Aptian-Albian) period in what are now Gansu and Liaoning provinces, western China. The rock layers from which their fossils have been recovered are dated to 120 million years ago. It was first described in 1984 on the basis of an isolated left leg. It is the oldest-known member of the Ornithurae, the group which includes modern birds (Neornithes) and extinct related groups, such as "Ichthyornis" and Hesperornithes. Description. The genus "Gansus" contains a single species, "G. yumenensis", which was about  the size of a pigeon and similar in appearance to loons and diving ducks. It had many features common among modern birds, and also retained some basal traits such as its clawed wings. "Gansus" was discovered in the form of a single fossil foot in 1981. Five more well-preserved fossils were found in 2003–2004 in mudstone at the site of an ancient lake at Changma, Gansu; the geological stratum in which the fossils were found is the Xiagou 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 of flight feathers and traces of the webbing between their toes. 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 that "Gansus" was a volant bird. You "et al." (2006) concluded that the anatomical characteristics of "Gansus" were similar to foot-propelled diving birds, such as "Hesperornis" (from the Cretaceous) and the loons (Gaviidae) and grebes (Podicipedidae). On the other hand, Li "et al." (2011) concluded that "Gansus" showed a more similar morphology to ducks . Two years later, Nudds "et al." (2013) showed that the pectoral limb length proportions of "Gansus" 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 that "Gansus" 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. Classification. "Gansus" was described as the oldest known ornithuran. The Ornithurae, however, has been given several very different definitions. In the definition used by You and  colleagues (that is, the clade containing all living birds plus "Hesperornis" and "Ichthyornis"), "Gansus" is indeed the oldest known member. However, several birds from the older Yixian Formation and contemporary Jiufotang Formation are considered ornithurans under other definitions. Under any definition, all living birds, including taxa as diverse as ostriches, hummingbirds and eagles, 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 to "Gansus". Thus, while "Gansus" is not necessarily a direct ancestor of today's birds, it is closely related  to such an ancestral species. This hypothesis was corroborated by later phylogenetic studies that included this taxon.