Confuciusornithidae

Confuciusornithidae is an extinct family of pygostylian avialans known from the Early Cretaceous, found in northern China. They are commonly placed as a sister group to Ornithothoraces, a group that contains all extant birds along with their closest extinct relatives. Confuciusornithidae contains four genera, possessing both shafted and non-shafted (downy) feathers. Some specimens probably referable to this clade represents one of the earliest known fossil evidence of primary feather moulting. They are also noted for their distinctive pair of ribbon-like tail feathers of disputed function.

The wing anatomy of confuciusornithids suggests an unusual flight behavior, due to anatomy that implies conflicting abilities. They possessed feathers similar to those of fast-flapping birds, which rely on quick flapping of their wings to stay aloft. At the same time, their wing anatomy also suggests a lack of flapping ability. Confuciusornithids are also noted for their beak and lack of teeth, similar to modern birds. Both predators and prey, confuciusornithid fossils have been observed with fish remains in their digestive systems and have themselves been found in the abdominal cavities of Sinocalliopteryx, a compsognathid predator.

Confuciusornithidae was first named by Hou et al. in 1995 to contain the type genus, Confuciusornis, and assigned to the monotypic clade Confuciusornithiformes within the class Aves. The group was given a phylogenetic definition by Chiappe, in 1999, who defined a node-based clade Confuciusornithidae to include only Changchengornis and Confuciusornis.

There are a number of features that define the clade. The most significant is the presence of a toothless jaw, which shows a more birdlike adaptation compared to Archaeopteryx. The other defining features are as follows, according to Chiappe et al. (1999):

Confuciusornithidae is the most basal group of the clade Pygostylia, whose members possess a pygostyle, a fused set of caudal vertebrae at the end of the tail. The pygostyle replaced the longer, unfused tail found in more primitive avialans such as Archaeopteryx, and may have served to improve flight. Pygostylia includes all modern birds, the only living members of the clade.

Additional members have been added to Confuciusornitidae since 1999. Jinzhouornis was added by Hou, Zhou, and Zhang in 2002, and in 2008, Zhang, Zhou and Benton assigned the newly described genus Eoconfuciusornis to the family.

Most confuciusornithids are known from the upper Jehol group, the Yixian Formation and Jiufotang Formation, dating from 125 to 120 million years ago. Eoconfuciusornis, however, predated the other confuciusornithids by 6 million years, dating to 131 Ma ago.

The entire body of confuciusornithids was covered in contour feathers, except for the foot, base of beak, and the tarsometatarsus, the bone directly attached to the foot. It appears that they may also have had down feathers.