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 looked like 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 as against 71 mm, which might indicate that it stood on its hind limbs and could use its forelimbs for flapping. 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 basalavians.
Classification
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 altherefore 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 Avescurrently 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. In 2017 re-evaluation of the Harlem Archaeopteryx specimen, Xiaotingia is found to be an anchiornithid.
