Introduction
Hello again r/DebateEvolution. A few weeks ago was revealed a new avialan dinosaur originating from the Nanyuan formation in the fujian province in the southeastern China.
The paper is titled "Jurassic avialan reveals stepwise evolution of bony tail in birds"
The new genus and species was named Zhengheornis buyu referring Zhenghe County, where the holotype specimen was, the suffix "ornis" means "bird". The specific name "buyu" is mandarin for "unexpected" referring to its unusual combination of characters.
Zhenghenornis was found in sediments form the titonian in the upper jurassic between 150 and 148 million years ago. the Nanyuan formation has recently yielded some important discoveries of avialan dinosaurs from this period.
Why is fossil this important?
Diagnostic traist (autopomorphies) of this taxon include: middle and posterior caudal vertebrae less elongate; final two caudal vertebrae box-like; manual phalanx III-1 shorter than III-2, manual phalanx III-3 50% longer than III-2. But above all a short tail comprising 15 vertebrae that is shorter than the hindlimb.
And is this character than become a new transitional form. Basal theropod dinosaurs had long tails with 40 and 50 caudal vertebrae, while the modern birds have a short tail with a pygostile, a structure where the last caudals are fused.
If the evolution is true and the birds evolve from theropod dinosaurs we predict a gradual reduction from long tails to short tails, and then the origin of the pygostile. That is exactly that we see.
Stepwise tail reduction in the bird lineage:
- Theropods (~230 million years ago): almost 50 caudals, no pygostyle
- Coelurosaurs (~190-180 million years ago): approximately 40 caudals, no pygostyle
- Maniraptorans (>170 million years ago): between 30 and 26 caudals, more slender tail, no pygostyle
- Avialae (>160 million years ago): 26-20 caudals, not pygostyle
- Zhengheornis buyu (~150-148 million years ago): 15 vertebrae, not pygostyle
- Avebrevicauda and pygostylia(~140-130 million years ago): 8-6 free caudals and 10-5 in the pygostyle (at the origin long and with neural spines).
- Neornithes (modern birds) (~100-90 million years ago, according to the molecular clock): 9-5 free caudals and 7-3 in the pygostyle.
How we can see Zhengheornis is transitional between ancient birds like archaeopteryx and more recent birds like confuciusornis in the number of free caudal vertebrea, also when can see that the tail's legth is intermidiate.
Phylogenetic position
An important detail that we cannot overlook is that the paper situe the new taxon in a more derived position that archaeoptery and anchiornitids, but in a less derived position than jeholornis, pygostylia, and Baminornis zhenghensis, another short-tailed bird of the same formation and age, however the baminornis' pygostyle have been reinterpreted like a synsacrum, but this does not seem to have affected his phylogenetic position.
If I offer my opinion both taxa (mainly Baminornis) can be related with a japanese bird from the kitadani formation from the aptian in the early cretaceous Fukuiteryx prima that is in a similar position (between archaeopteryx and Jeholornis + pygostylia) and prensent a long and primitive pygostyle that preserves the neural spines.
This raises an interesting question: why do Jeholornithiformes retain long tails with many vertebrae if more basal birds like zhengheornis had already reduced them?
Conclusion
Zhengheornis buyu is a new important find that helps us understand how the birds lost the long theropod like tail and evolve the short tail and the pygostyle. Its discovery fills a gap in the evolutionary history of the avian lineage.
References:
Wang, Min; Tang, Jianrong; Deng, Ke; Dong, Liping; Xu, Liming; Xu, Xing; Lin, Min; Du, Honggang; Lin, Ganmin; Chen, Runsheng; Zhang, Chi; Zhou, Zhonghe (2026). "Jurassic avialan reveals stepwise evolution of bony tail in birds" https://www.science.org/doi/10.1126/sciadv.aeb5202
Senter P, Kirkland JI, DeBlieux DD, Madsen S, Toth N (2012) "New Dromaeosaurids (Dinosauria: Theropoda) from the Lower Cretaceous of Utah, and the Evolution of the Dromaeosaurid Tail". https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0036790#amendment-0
Felice, Ryan N; O'connor, Patrick M (2014) "Ecology and Caudal Skeletal Morphology in Birds: The Convergent Evolution of Pygostyle Shape in Underwater Foraging Taxa" https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0089737#abstract0
Chen, Runsheng; Wang, Min; Dong, Liping; Zhou, Guowu; Xu, Xing; Deng, Ke; Xu, Liming; Zhang, Chi; Wang, Linchang; Du, Honggang; Lin, Ganmin; Lin, Min; Zhou, Zhonghe (2025). "Earliest short-tailed bird from the Late Jurassic of China" https://www.nature.com/articles/s41586-024-08410-z
Huang, Jiandong; Wang, Xuri; Cau, Andrea; Mao, Lei; Liu, Yichuan; Wang, Yang (2026). "A new euornithine from the Lower Cretaceous (Aptian) of China reveals the first radiation of fish-eating birds" https://www.sciencedirect.com/science/article/pii/S0195667125001673?via%3Dihub
Imai, T., Azuma, Y., Kawabe, S., Shibata, M., Miyata, K., Wang, M., & Zhou, Z. (2019). "An unusual bird (Theropoda, Avialae) from the Early Cretaceous of Japan suggests complex evolutionary history of basal birds" https://www.nature.com/articles/s42003-019-0639-4