Yuanjiawaornis is an extinct genus of large enantiornithean bird known from the early Cretaceous of present-day China. It is monotypic, with only type species Y. virisosus known.[1]
In 2005, near the village of Yuanjiawa, near the town of Dapingfang in Chaoyang in western Liaoning province, the fossil of a bird was unearthed. Exceptionally, this was done by a paleontologist and not by the illegal fossil trade, which poaches by far the majority of fossils in the area.[1]
In 2015, the type species Yuanjiawaornis viriosus was named and described by Hu Dongyu, Liu Ying, Li Jinhua, Xu Xing and Hou Lianhai. The genus name connects the name of the finding site with the Ancient Greek ornis, "bird". The species name means "powerful" in Latin and refers to its relatively large body size.[1]
The holotype, 'PMOL AB00032, has been found in the Yuanjiawa strata of the Jiufotang Formation dating to the Aptian, about 120 million years old. It consists of a skeleton without a skull, flattened on a plate. Apart from the skull, the cervical vertebrae, the frontal vertebrae, and most of the phalanges of the fingers and toes are missing. The fossil is largely in bandages, although the pelvis has shifted. Nothing of the plumage has been preserved, which is exceptional for bird fossils from Liaoning. However, the pelvis is partly covered by what appears to be a layer of preserved soft tissue. It is probably a young adult specimen. It is part of the collection of the Paleontological Museum of Liaoning, which also collected and prepared it.[1]
Yuanjiawaornis is a remarkably large species. Most enantiornithines are sparrow- or starling-sized birds, but Yuanjiawaornis must have had a wingspan of nearly half a metre. The femur has a length of forty-three millimetres. That makes the species about the same size as Zhouornis and only slightly smaller than Pengornis, considered to be a giant among the Chinese enantiornithines from the Lower Cretaceous.[1]
No autapomorphies, unique derived features of Yuanjiawaornis, could be determined. However, there was an unusual combination of non-unique characteristics. The body size is large. The wings and hindlimbs are roughly the same length. The synsacrum shows a longitudinal groove on the underside instead of ridges. The last vertebra of the sacrum has robust and long lateral protrusions, rather than the penultimate vertebra. The scapula has a processus acromialis that tapers obliquely to the front and above. The lateral edge of the coracoid is straight instead of convex. The branches of the wishbone turn towards each other, tapering instead of running parallel with a widening. The sternum has an oval profile, while the outer posterior protrusions are slightly widened at their tip instead of having an angular profile or strongly widened protrusions. The head of the humerus is flat rather than convex or concave. The deltopectoral crest of the humerus passes downward gradually into the shaft rather than abruptly.[1]