fossil genera, species are difficult to define, as differences between species can often not be readily distinguished from variation that occurs within a species. In the case of Confuciusornis, only C. sanctus is universally accepted. Confuciusornis chuonzhous was named by Hou in 1997 based on specimen IVPP V10919, originally a paratype of C. sanctus. The specific name refers to Chuanzhou, an ancient name for Beipiao. C. chuonzhous is now generally considered synonymous with C. sanctus. Confuciusornis suniae, named by Hou in the same 1997 publication, was based on specimen IVPP V11308. The specific name honours madam Sun, the wife of Shikuan Liang who donated the fossil to the IVPP. C. suniae is now usually considered synonymous with C. sanctus. Confuciusornis dui was named by Hou and colleagues in 1999. The specific name again honours the donating collector, Du Wengya. The holotype specimen (IVPP V11553) is a nearly complete skeleton of an adult that includes a pair of long tail feathers and the impression of the horny beak. A second specimen, the paratype IVPP 11521, is fragmentary and includes some vertebrae and ribs, tail, sternum and pelvis, and femora. According to Hou and colleagues, C. dui was smaller and more gracile than most other Confuciusornis specimens, with the holotype being ca. 15% smaller than the holotype of C. sanctus and ca. 30% smaller than larger individuals of that species. The jaw tips were more pointed than in C. sanctus, and the mandible lacked the underside keel that is distinct in the latter species. Further differences to the type species can be found in the postcranium: The claw on the first digit was not enlarged as in C. sanctus. The sternum was more elongate and differed in anatomical details, and the lower segment of the hind limb (the tarsometatarsus) was shorter than the pygostyle of the tail. A statistical analysis by Marguán-Lobon and colleagues in 2011 revealed no significant differences to specimens referred to the smallest size class of C. sanctus, suggesting that the supposed differences are individual variations of a single species. However, these authors could not re-locate the C. dui holotype, which is possibly lost, and therefore had to rely on a cast of that specimen for their measurements. A re-study of the C. dui specimens would be required in order to evaluate the validity of the species. Confuciusornis feducciai was named in 2009 by Zhang Fucheng and colleagues, the