The Confuciusornithids are a group of Birds known from the Early Cretaceous Jehol Biota. Unlike earlier Jurassic Birds, they have both a pygostyle (short tail made from fused bones) and a beak, key features we associate with modern Birds, although they are not closely related, being considered to be the sister group to the Ornithothoraces, the group which comprises both the Euornithes ('True Birds'; the group to which all modern Birds belong) and the Enatiornithes ('Opposite Birds'; the most commonly preserved group of fossil Birds from the Cretaceous, which were generally toothed rather than beaked). Although these Birds are only known from a single fossil lagerstatten, thousands of individual specimens with a temporal range of about 15 million years (the Jehol Biota comprises three succeeding formations, the Dabeigou, Yixian and Jiufotan), which have been assigned to five genera and eleven species (although some of these assignments are debatable), giving us a degree of understanding of the group.
In a paper published in the journal Communications Biology on 21 December 2022, Renfei Wang of the College of Earth Sciences at Jilin University, and the Key Laboratory for Evolution of Past Life in Northeast Asia at the Paleontological Museum of Liaoning, Dongyu Hu, also of the Key Laboratory for Evolution of Past Life in Northeast Asia at the Paleontological Museum of Liaoning, Meisheng Zhang, also of the College of Earth Sciences at Jilin University, Shiying Wang, again of the Key Laboratory for Evolution of Past Life in Northeast Asia at the Paleontological Museum of Liaoning, Qi Zhao of the Key Laboratory of Vertebrate Evolution and Human Origins at the Institute of Vertebrate Paleontology and Paleoanthropology of the Chinese Academy of Sciences, Corwin Sullivan of the Department of Biological Sciences at the University of Alberta, and the Philip J. Currie Dinosaur Museum, and Xing Xu, once again of the Key Laboratory for Evolution of Past Life in Northeast Asia at the Paleontological Museum of Liaoning, and the Key Laboratory of Vertebrate Evolution and Human Origins at the Institute of Vertebrate Paleontology and Paleoanthropology, and of the Center for Vertebrate Evolutionary Biology at Yunnan University, describe a new species of Confuciusornithid Bird from the Xiaotaizi Village exposure of the Jiufotang Formation in Jianchang County, Liaoning Province.
The new species is placed in the genus Confuciusornis, and given the specific name shifan, which means 'paragon of all teachers', and is an honorific applied to Confucius, and also refers to the 70th anniversary of the opening of Shenyang Normal University (瀋陽師範大學 or Shenyang Shifan Daxue). The species is described from a single specimen, PMoL-AB00178, which is a nearly complete and mostly articulated skeleton, preserved on a single slab.
Examination of the skeleton of Confuciusornis shifan indicates that it is an adult specimen, with all neurocentral sutures are closed without any trace, the sacral vertebrae are completely fused together to form a synsacrum, and nearly all of the compound bones (e.g., the metacarpus, tibiotarsus, and metatarsus) are fully formed. Examination of a cross section of the right femur revealed four lines of arrested growth (indicative of seasonal pauses in growth, similar to tree rings), which is considered indicative of having reached the final adult size in a Confuciusornithid.
Despite this, Confuciusornis shifan is small for a Confuciusornithid Bird, with an estimated body mass of 174 g (around the size of a large Thrush or small Dove), with only one species, Changchengornis hengdaoziensis, having a smaller adult specimen (with an estimated body mass of 138 g).
The pygostyle is one of the defining features of modern Birds, but was absent in the long-tailed Birds of the Jurassic, with the Cretaceous-Recent Birds which share this characteristic being considered to comprise a single clade, the Pygostylians. In more advanced members of the group, such as the Sapeornithids, Pengornithid Enantiornithines, and Ornithuromorphs, this is a short, plough-shaped structure, but in earlier members of the group, including the Confuciusornithids, Jinguofortisids, and the majority of Enantiornithines, the pygostyle was longer and rod-shaped. It has generally been assumed that this longer pygostyle comprises a larger number of bones than the pygostyle of modern Birds, although the structure is seldom preserved well enough to be able to count the co-ossified bones which make it up. However, the pygostyle of Confuciusornis shifan is perforated (thought to be the result of incomplete fusion between adjacent neural arches), which leaves the intervertebral foramina (gaps between vertebrae) visible. The specimen has ten intervertebral foramina, implying a minimum of eleven vertebrae are fused, more than has previously been recorded in any Confuciusornithid Bird (the previous record-holder was a juvenile specimen of Confuciusornis sanctus with eight), and significantly more than is known from any non-Confuciusornithid Pygostylian Bird, with 5-6 being typical in even early members of the group.
This has interesting implications for the evolution of the Avian tail. Both Confuciusornis shifan and Confuciusornis sanctus have seven free caudal (tail) vertebrae in addition to the fused vertebrae of the pygostyle, giving Confuciusornis shifan a total caudal vertebrae count of eighteen. Five to eight free caudal vertebrae seems to be typical of early Pygostylian Birds, with 5–6 in Jinguofortisids, 6–8 in Enantiornithines, and 5–7 in early Ornithuromorphs (modern Birds have 5-6). The exception appears to be the Sapeornithids, which are thought to have about 12. Since the pygostyle typically comprises another 5-6 vertebrae, this means that most Pygostylian Birds have a total of no more than 14 caudal vertebrae, with the exception of the Confuciusornithids and Sapeornithids, which have about 18. This is still less than in the non-Pygostylian Birds, with Archaeopteryx having 21-23 free caudal vertebrae, and Jeholornis having 27. This implies a general tendency towards a reduction of the number of caudal vertebrae in Pygostylian Birds, with exceptions in groups such as the Confuciusornithids and Sapeornithids, and possibly the Enantiornithines (which had a long pygostyle, presumably made up of a higher number of fused vertebrae, although no actual data is available). It is known that at least some of the long-pygostyled Birds used this structure to support large, ornamental tail feathers, whereas the shorter plough shaped pygostyles of more advanced groups are used to support retractable tail fans, which greatly increase aerodynamic lift and maneuverability.
Typical Confuciusornithids have long wings with strongly asymmetrical flight feathers, strut-like coracoids, a keeled sternum, and enlarged major manual digits, all features which have been used to argue that they were strong flyers, although some functional analyses have suggested that their capabilities would have been more limited, and it is generally accepted that their flight capacity would have been weaker than modern Birds, or even Early Cretaceous Ornithothoracines.
Although clearly closely related to Confuciusornis sanctus, Confuciusornis shifan shows some flight adaptations dissimilar to those seen in other Confuciusornithids. Typical Confuciusornithids have an elongated forearm, giving an increased wing-area, a useful adaptation to long-distance flight. Confuciusornis shifan is significantly smaller than most Confuciusornithids, and shows enlargement of many of the skeletal processes to which muscles would have attached. This would have given greater control over the tips of the wings during flight, increasing maneuverability during flight.
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