Description and speciesEdit
Zhou and Hou (2001) distinguished Cathayornis from the similar Sinornis by the former's larger size, a shorter, straighter, finger number I, with a slightly longer claw (ungual), the absence of an antitrochanter, and other features. The type and possibly only species is Cathayornis yandica.
Other species – C. aberransis, C. chabuensis and C. caudatus – have been described. However, their validity and/or assignment to the genus Cathayornis has been questioned in subsequent evaluations. Jingmai O'Connor and Gareth Dyke (2010) found that many of the supposedly distinct features of C. aberransis (such as the base of a crest on the skull) had been inaccurately described, casting doubt on the few remaining features separating it from C. yandica, and suggested that further study was needed to determine its validity. Similarly, C. caudatus was so named for its supposedly bony tail lacking a pygostyle, and was further differentiated by its small size. O'Connor and Dyke re-examined the specimen and showed that the specimen is in fact only slightly smaller than the type specimen of C. yandica, and that a normal enantiornithine tail with a pygostyle is clearly visible in one of the fossil slabs, parts of the hip bones having been mistaken for unfused tail vertebrae. O'Connor and Dyke therefore considered C. caudatus a nomen dubium. On the other hand, C. chabuenis is clearly distinct from C. yandica and cannot be assigned to the cathayornithiformes, so it most likely represents a new genus.
Paul Sereno et al., in 2002, considered Cathayornis a junior synonym of Sinornis. They interpreted the anatomies of the two as very similar and sharing key autapomorphies of the pygostyle.
The first thorough review of Sinornis and Cathayornis was published by Jingmai O'Connor and Gareth Dyke in 2010. O'Connor and Dyke concluded that despite the earlier opinion of Sereno and colleagues, the two birds were not synonyms and in fact differ in several clear ways, including different proportions in the wing claws and digits, differences in the pelvis, and size of the pygostyle.
Other species of similar birds from the same location have been regarded as synonymous with C. yandica by some researchers, including Largirostrornis sexdentoris and Cuspirostrisornis houi, though this has yet to be supported by rigorous study. O'Connor and colleagues noted that Longchengornis sanyanensis, also synonymized with C. yandica by some authors, seems to show distinct anatomy not shared with at least that species of Cathayornis.