| Aurornis | |
|---|---|
 | |
| Life restoration | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Clade: | Dinosauria |
| Clade: | Saurischia |
| Clade: | Theropoda |
| Family: | †Anchiornithidae |
| Genus: | †Aurornis Godefroitet al.,2013 |
| Type species | |
| †Aurornis xui Godefroitet al., 2013 | |
Aurornis is anextinctgenus ofanchiornithidtheropoddinosaurs from theJurassic period ofChina. The genusAurornis contains a single knownspecies,Aurornis xui (/aʊˈrɔːrnɪsˈʃuːi/).[1]Aurornis xui may be the mostbasal ("primitive")avialan dinosaur known to date, and it is one of the earliest avialans found to date. The fossil evidence for the animal pre-dates that ofArchaeopteryx lithographica, often considered the earliest bird species, by about 10 million years.[1][2]
Aurornis xui was first described and named by Pascal Godefroit, Andrea Cau, Hu Dong-Yu, François Escuillié, Wu Wenhao and Gareth Dyke in2013. Thegeneric name is derived from theLatin wordaurora, meaning "daybreak" or "dawn", and theAncient Greek ὄρνις (órnis) meaning "bird". Thespecific name,A. xui, honorsXu Xing.[1] A recent study of specimens of the avialanAnchiornis has found that the traits exhibited byAurornis fall within the range of variation inAnchiornis, warranting their synonymization.[3]
Aurornis was roughly the size of a modernpheasant, with a length of 50 cm (20 in). It had clawed wings and a long bony tail. Its leg bones were similar to those ofArchaeopteryx, but overall its anatomy was more primitive.[2]Aurornis lived roughly 160 million years ago, roughly 10 million years beforeArchaeopteryx, which often has been described as the first bird.[2]
Aurornis was described from asedimentary rock fossil in 2013. The fossil was purchased from a local dealer who said it had been unearthed in Yaoluguo in westernLiaoning, China. Subsequent analysis confirmed it came from theTiaojishan Formation, which has been dated to the lateJurassic period (Oxfordian stage), approximately 160 million years ago.[1][4][5] The fossil features traces of downy feathers along the animal's tail, chest, and neck. It was only partially prepared at the time of purchase with the feathers not showing, and bore no signs of forgery.[2]
On 7 June 2013, Science Magazine published an article that noted thatPascal Godefroit, the paleontologist who led the team that describedAurornis, reported that he is uncertain if the fossil material came from Liaoning province's 160-million-year-old Tiaojishan Formation, as the information provided by the fossil dealer indicated, or from the province's 125-million-year-old Yixian Formation, which is known to have produced several ancient bird fossils. The failure to secure rigorous provenance information casts doubt on the claim thatAurornis is 160 million years old and predatesArchaeopteryx. Godefroit's team will attempt to confirm the specimen's provenance, and its age, by conducting mineralogical and botanical analysis on the shale slab and then publishing their findings.[6]
A 2017 study suggested thatAurornis may be ajunior synonym ofAnchiornis,[3] a conclusion followed by a different set of researchers two years later.[7]
Aphylogenetic analysis ofAurornis published in 2013 found that it belongs in the bird lineage, in a morebasal position thanArchaeopteryx.[2] The analysis was based on "almost 1,500 [anatomical] characteristics."[8] On the other hand, a phylogenetic analysis conducted by Brusatteet al. (2014) recoveredAurornis outside Avialae; it was recovered as atroodontid closely related toAnchiornis,Xiaotingia andEosinopteryx.[9] In 2017 re-evaluation of the HarlemArchaeopteryx specimen,Aurornis is found to be an anchiornithid.[10]
The classification ofA. xui as a bird is somewhat contentious, however, due to the various differing definitions of the word "bird". Recent discoveries "[emphasize] how grey the dividing line is between birds and [non-avian] dinosaurs", says Paul Barrett of theNatural History Museum inLondon. "There's such a gradation in features between them that it's very difficult to tell them apart ... [Aurornis xui] is certainly an older member of the bird lineage thanArchaeopteryx, and it's fair to call it a very primitive bird. But what you call a bird comes down to what you call a bird, and a lot of definitions depend onArchaeopteryx."[2] Bird evolution specialistLawrence Witmer called the new analysis compelling, but said it remains difficult to distinguish birds from birdlike dinosaurs: "All of these little feathered species running and flapping around ... were all very similar."[11]
American paleontologist Luis Chiappe said thatA. xui's forelimb is too short for this species to be a true bird. It "is very birdlike, but it is not yet a bird," he concluded.[12]
In his 2002 bookDinosaurs of the Air,Gregory S. Paul tried to conceptually model a "pro-avian". In his view, the direct ancestors of birds cannot have been completely arboreal, because in that case they would probably have used membranes to fly. He thought they must have represented an intermediate ecological stage, in which the hindlimbs still had largely cursorial adaptations whereas the arms had been elongated in order to climb. Feathers, originally serving the insulation of an already warm-blooded animal, would by elongation have turned the arms into wings in order to fly. More generally, the proavians would, in view of their basal theropod forebears and bird descendants, have been typified by long necks, a short trunk, long fingers with opposable digits, a decoupling of thelocomotor functions of the forelimbs and hindlimbs, a lack of apropatagium, a shallow tail, and a weight of about one kilogramme. Paul illustrated his analysis with a skeletal diagram, accompanied by a life illustration of a "proavis".[13] WhenAurornis was described in 2013, it was at the time the most basal known member of theAvialae, the group consisting of birds and their closest relatives. The Italian paleontologistAndrea Cau remarked it bore an uncanny resemblance to Paul's "proavis".[14][15]
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