Confuciusornithids, which only lived in the Early Cretaceous terrestrial Jehol Biota, are a very unusual group of early birds. In the nearly 30 years since the first specimen of Confuciusornis sanctus was reported in 1995, thousands of exceptionally well preserved Confuciusornis-like specimens have been unearthed from Early Cretaceous strata representing a considerable span of time (131 - 119 million years) and distributed across a 600 km x 300 km area centered on western Liaoning Province, northeastern China. As the earliest known birds possessing a toothless beak and a fused tail skeleton as seen in modern avians, confuciusornithids have attracted intense attention from the global scientific community ever since they were discovered. Numerous specimens have provided rich, multifaceted data that enhance our understanding of this group, and by extension of the early evolution of birds after Archaeopteryx. Over the past 30 years, in fact, confuciusornithids have been the most deeply and broadly studied avian group in the Jehol Biota. The appearance of a previously unknown confuciusornithid species could hardly have been expected, and my coauthors and I were surprised to encounter a fossil bird that clearly represented a new confuciusornithid, which we named Confuciusornis shifan.
The only known specimen of Confuciusornis shifan is from the youngest fossil-bearing strata of the Jehol Biota in western Liaoning. When I first saw it, I immediately felt that it was different from all confuciusornithid specimens that had previously been documented. Its skeleton appeared slender and lightly constructed, and displayed features which could otherwise only be seen in some members of Ornithothoraces, the group that includes modern birds and some of their closer Cretaceous relatives, For example, the deltopectoral crest on the upper end of the humerus was low rather than prominent in Confuciusornis shifan, and the pubic bone was strongly curved. Subsequently, more detailed observations and comparisons suggested that this bird not only belonged to a new species, but also had evolved a flight apparatus that was superior in many respects to those of other known confuciusornithids, and superior in some respects even to those of contemporaneous ornithothoracines. An analysis of estimated aerodynamic parameters within the framework of early avian phylogeny also suggested that confuciusornithids had undergone a change in their flight capabilities and strategy during their 12-million-year evolution, with Confuciusornis shifan representing a late confuciusornithid characterized by high aerial maneuverability and some level of specialization for slow, short flights.
This specimen also reveals an informative stage in the skeletal growth of confuciusornithids. As birds and many other vertebrates mature, some elements of the skeleton that were previously separated fuse together. In the Confuciusornis shifan specimen, nearly all of these compound bones are fully fused, but a partial exception involves the vertebrae at the end of the tail, which are only partially amalgamated into a tapering structure called the pygostyle. Openings, called intercentral foramina, are still present between the successive vertebrae, making it possible to infer with unprecedented confidence the number of vertebrae that contribute to the pygostyle in a confuciusornithid bird. Another skeletal element, however, kept us puzzled for a long time. As in other vertebrates, the hands of birds include metacarpals and phalanges, respectively equivalent to the bones that support the human palm and the bones of the fingers. In the Confuciusornis shifan specimen, the alular digit, corresponding to the thumb, contains a small cushion-shaped bone located between the metacarpal and the first phalanx. To our knowledge, such a bone has never been previously reported among birds or their dinosaur ancestors. In our judgment, the cushion-shaped bone most likely represents an epiphyseal secondary ossification, or in other words a small skeletal element that forms at the end of a long bone – in this case, the alular metacarpal – and would normally fuse with the shaft of that bone by the end of skeletal growth. In Confuciusornis shifan, such an ossification might have strengthened and stiffened the alular digit, perhaps compensating for slow growth of the metacarpal and/or strong locomotor stress associated with flight. We look forward to hearing how this interpretation is received by others.
The discovery of Confuciusornis shifan highlights the diversification of flight adaptation modes that took place within the early avian group Confuciusornithidae, parallelling a similar process in Ornithothoraces. Going forward, it will be important to comprehensively analyze flight-relevant parameters such as body mass in confuciusornithids and their relatives, within an appropriate temporal and phylogenetic framework. Analyses of this kind can be particularly fruitful for groups, such as the confuciusornithids, that are represented in the fossil record by numerous specimens and are characterized by high species diversity and a long evolutionary history.