Xu et al., 2011
Xu et al., 2011
Xiaotingia is known from the holotype STM 27-2, an articulated and almost complete skeleton including the skull. It was probably collected in the Linglongta area, Jianchang, from the Tiaojishan Formation. 
Xiaotingia was first named by Xu Xing, You Hailu, Du Kai and Han Fenglu in 2011 and the type species is Xiaotingia zhengi. The generic name and specific name together honour paleontologist Zheng Xiaoting. 
Xiaotingia lived around 155 million years ago which preceded Archaeopteryx that lived around 144 million years ago.
Xiaotingia was morphologically similar to Archaeopteryx and was the size of a hen. It was about 60 cm long and weighed an estimated 0.82 kg. It was a small feathered dinosaur that lived in an arboreal environment. Like Archaeopteryx it had long forelimbs. Its femur was longer than its humerus, 84 mm compared to 71 mm, which might indicate that it stood on its hind limbs and could flap its forelimbs to achieve flight. 
Xiaotingia had feathers on its head, body, forelimbs and hind limbs. The feathers on the femur were quite long, measuring 55 mm. It also had long pennaceous feathers on its tibia and metatarsus. If Xiaotingia could fly short distances it might also have used its hind limbs as wings. 
Xiaotingia had a dentary tooth count probably less than 10 and teeth similar in morphology to those of basal avians. 
The initial analysis by Xu et al. showed that Xiaotingia formed a clade with Archaeopteryx, Dromaeosauridae and Troodontidae to the exclusion of other groups traditionally seen as birds. Xu et al. therefore (re)defined the concepts of Deinonychosauria and Avialae to the extent that Archaeopteryx and Xiaotingia belonged to the Deinonychosauria in the clade Archaeopterygidae.  This led to popular reports that "Archaeopteryx is no longer a bird",  although Xu et al. noted that there are several competing definitions of the clade Aves currently in use, pointing out that their definitions are compatible with a traditional Aves with Archaeopteryx as a specifier.  This was challenged by an analysis using different methods published several months later however, in which Archaeopteryx was again recovered as an avialan, while Xiaotingia remained closely allied to Anchiornis within the Troodontidae.  In 2012, an expanded and revised version of the initial analysis also found Archaeopteryx to be avialan and Anchiornis to be troodontid, but recovered Xiaotingia as the most primitive member of the clade Dromaeosauridae rather than a close relative of Anchiornis within Troodontidae. 
Cladogram following the results of a phylogenetic study by Lefèvre et al., 2017. 
- Zhang, H.; Wang, M.; Liu, X. (2008). "Constraints on the upper boundary age of the Tiaojishan Formation volcanic rocks in West Liaoning-North Hebei by LA-ICP-MS dating". Chinese Science Bulletin. 53 (22): 3574–3584. Bibcode: 2008SciBu..53.3574Z. doi: 10.1007/s11434-008-0287-4.
- Xing Xu; Hailu You; Kai Du & Fenglu Han (28 July 2011). "An Archaeopteryx-like theropod from China and the origin of Avialae" (PDF). Nature. 475 (7357): 465–470. doi: 10.1038/nature10288. PMID 21796204. S2CID 205225790. Archived from the original (PDF) on 16 January 2016.
- Foth, C.; Rauhut, O.W.M. (2017). "Re-evaluation of the Haarlem Archaeopteryx and the radiation of maniraptoran theropod dinosaurs". BMC Evolutionary Biology. 17 (1): 236. doi: 10.1186/s12862-017-1076-y. PMC 5712154. PMID 29197327.
- Ulysse Lefèvre, Andrea Cau, Aude Cincotta, Dongyu Hu, Anusuya Chinsamy, François Escuillié & Pascal Godefroit (2017). A new Jurassic theropod from China documents a transitional step in the macrostructure of feathers. The Science of Nature, 104: 74 (advance online publication). doi: 10.1007/s00114-017-1496-y
- "Feathers fly in first bird debate". BBC News. 27 July 2011.
- Lee, M.S.Y. & Worthy, T.H. (2011). "Likelihood reinstates Archaeopteryx as a primitive bird". Biology Letters. 8 (2): 299–303. doi: 10.1098/rsbl.2011.0884. PMC 3297401. PMID 22031726.
- Senter, P.; Kirkland, J.I.; DeBlieux, D.D.; Madsen, S. & Toth, N. (2012). "New Dromaeosaurids (Dinosauria: Theropoda) from the Lower Cretaceous of Utah, and the Evolution of the Dromaeosaurid Tail". PLOS ONE. 7 (5): e36790. Bibcode: 2012PLoSO...736790S. doi: 10.1371/journal.pone.0036790. PMC 3352940. PMID 22615813.