For early birds, evolution wasn't always about becoming better flyers. This conclusion is supported by the recent discovery of a remarkably well-preserved, 150-million-year-old bird fossil from Fujian Province, China, which is challenging long-held assumptions about early bird evolution.
A study published in Nature discusses the newly discovered species, Yuezhuornis yangyuanensis, which lived during the Late Jurassic period (150–148 million years ago). The research suggests that the evolution of the avian body plan was not a straightforward progression toward improved flight capabilities, but a complex process involving the exploration of different ecological niches, including some that did not require flight.
Discovered in the recently recognized Zhenghe Fauna of southeastern China by researchers from the Institute of Vertebrate Paleontology and Paleoanthropology (IVPP) of the Chinese Academy of Sciences (CAS) and the Fujian Institute of Geological Sciences, the crow-sized fossil represents the earliest known example of flight loss in the evolutionary history of birds.
Yuezhuornis challenges the traditional view that the avian body plan evolved almost exclusively under selective pressures associated with flight. Instead, it exhibits a striking mosaic of primitive and derived features, combining characteristic avian traits with features inherited from earlier dinosaur ancestors.
The fossil preserves closely packed, serrated teeth, which are typically found in deinonychosaurs but not birds. Most notably, Yuezhuornis possessed a hyperextensible second toe with a sickle-shaped claw, a characteristic feature of deinonychosaurs, including dromaeosaurids and troodontids.
"When we examined Yuezhuornis, we were struck by its extraordinary mix of features," said Dr. WANG Min, corresponding author of the study. "It possesses bird-like pelvic and hindlimb structures yet retains the specialized predatory foot and teeth of a small meat-eating dinosaur. It shows us that early bird evolution was far more complicated than we previously realized."
To determine where this enigmatic species fits on the evolutionary tree, the research team conducted comprehensive phylogenetic analyses using both maximum parsimony and Bayesian tip-dating methods.
"The results consistently placed Yuezhuornis firmly within Avialae (the group containing modern birds and their closest extinct relatives), just crownward of Archaeopteryx and as a sister taxon to Zhengheornis," said Dr. ZHOU Zhonghe, a co-author of the study.
Given its unique anatomy, the team investigated the ecological niche of Yuezhuornis using ecomorphometric and multivariate statistical analyses. The analyses revealed that the limb proportions of Yuezhuornis were similar to those of living terrestrial birds and some non-avialan paravians, suggesting a terrestrial, flightless lifestyle and shedding new light on the ecological diversification of early-diverging avialans.
The discovery of a flightless, ground-dwelling bird from the Late Jurassic challenges the notion that aerodynamic selection was the dominant force shaping early avialan evolution. Instead, it suggests that some early birds followed evolutionary paths that did not favor flight, highlighting the remarkable ecological and morphological diversity that emerged during the early evolution of birds.
The ongoing discovery of early avialans also highlights the potential of the Zhenghe Fauna to shed light on a poorly documented stage of Jurassic bird evolution.
