New Cretaceous Fossil Reveals the Evolution of the Avian Respiratory System: Uncinate Processes Are More Complex Than Previously Thought
Birds possess one of the most specialized respiratory systems among vertebrates. In addition to a pair of lungs, they have multiple air sacs distributed throughout the body. Working together with the ribs, sternum, and associated muscles, these structures enable air to flow unidirectionally through the lungs, greatly increasing the efficiency of gas exchange.
An often-overlooked but crucial component of this respiratory system is the uncinate process—a bony projection extending posterodorsally from the ribs. Present in most living birds, the uncinate process serves as an attachment site for respiratory muscles, helping move the ribs and sternum and thereby facilitating the expansion and contraction of the air sacs during breathing.
Previous studies have shown that the evolutionary origin of the uncinate process can be traced back to early archosaurs (Archosauria). However, except for more derived maniraptoran theropod dinosaurs, uncinate processes in most archosaurs remained cartilaginous, making them less likely to be preserved in the fossil record.
Most birds possess five or six pairs of uncinate processes. They are generally long and rod-like, with the first one or two pairs typically being noticeably shorter. Nevertheless, their morphology varies considerably among species. For example, the Alagoas curassow (Mitu mitu) has triangular uncinate processes, the hoatzin (Opisthocomus hoazin) has relatively reduced ones, and the blue-and-yellow macaw (Ara ararauna) possesses uncinate processes with broad rectangular bases. Even within a single individual, uncinate processes at different rib positions may differ in shape.
The uncinate process increases the mechanical advantage of the muscles attached to it, allowing muscles to generate greater force with less effort. For example, the appendicocostalis muscles exhibit approximately two- to fourfold greater mechanical advantage when an uncinate process is present, suggesting that these structures improve the efficiency of thoracic and air sac movements.
The uncinate process may also be associated with different avian lifestyles. Because the external intercostal muscles are mainly active during walking, whereas the appendicocostalis muscles are more active during resting ventilation, researchers have suggested that differences in uncinate process morphology may reflect adaptations to flight, terrestrial locomotion, or resting respiration. Longer uncinate processes may also increase the range of motion between the ribs and sternum, thereby enhancing respiratory efficiency.
Despite their important roles in respiration and locomotion, the reasons why birds evolved such diverse uncinate process shapes remain poorly understood. Ecological factors such as habitat, diet, lifestyle, and body size have all been proposed as possible influences, but no large-scale comparative analysis had previously addressed this question.
To investigate this issue, a recent study focused on a new Early Cretaceous enantiornithine (Enantiornithes) specimen, STM11-128, from the Jehol Biota of China, which preserves an unusual uncinate process morphology. Combining data from both extant birds and Mesozoic birds, the researchers applied geometric morphometrics and phylogenetic comparative analyses to examine the relationships between uncinate process morphology and ecological variables, while also reconstructing the evolutionary history of this structure from the Mesozoic to modern birds.
The researchers first analyzed uncinate process length in living birds. Length ranged from approximately 1.9 mm in the Puerto Rican tody (Todus mexicanus) to 48.7 mm in the emperor penguin (Aptenodytes forsteri), representing remarkable variation. Body mass proved to be the strongest predictor of uncinate process length, showing a strong positive correlation and explaining nearly 80% of the observed variation. Although habitat, diet, lifestyle, and flight ability also had detectable effects, their influence was much weaker than that of body size.
When Mesozoic birds were incorporated into the analysis, the same relationship between body mass and uncinate process length remained evident. The uncinate process of STM11-128, measuring approximately 4.75 mm, closely matches those of several living bird species with comparable body sizes.
The researchers then examined uncinate process shape, identifying major sources of variation, including basal width, overall robustness, ventral expansion of the base, and the orientation of the distal tip.
Including Cretaceous birds in the comparison revealed that Mesozoic birds occupied a considerably smaller region of morphospace than living birds, indicating more limited morphological diversity. The uncinate processes of STM11-128 most closely resemble those of the torrent duck (Merganetta armata), cuckoos (Coccyzus), and cinereous tinamou (Crypturellus cinereus), all of which possess relatively square, laterally expanded bases.
Unlike length, uncinate process shape showed little association with ecological variables. Diet, flight capability, body size, and phylogenetic relationships each explained only a small proportion of the observed variation. This suggests that no single ecological factor can adequately account for the evolution of uncinate process shape. Instead, previously underexplored factors such as respiratory efficiency or metabolic rate may have played a more important role.
In contrast, uncinate process length appears to be more closely linked to function. The researchers propose that longer uncinate processes provide larger muscle attachment areas or longer lever arms, thereby improving the mechanical efficiency of muscles involved in respiration and locomotion. Consequently, length may be more important than shape in determining respiratory and locomotor performance.
After incorporating Mesozoic birds into the analysis, the researchers found that the relationship between uncinate process length and body mass had already been established by the Early Cretaceous, indicating that this scaling relationship has persisted for more than 120 million years. In the future, uncinate process length may even help estimate the metabolic capacity of extinct birds, although additional research is needed to validate this possibility.
The researchers also reconstructed the evolutionary history of the uncinate process. Most Mesozoic birds retained simple, slender uncinate processes, whereas modern birds evolved much greater morphological diversity, including triangular, rectangular, basally expanded, and even accessory-projection morphologies.
The uncinate processes of STM11-128 are particularly unusual. Some are slender, whereas others possess distinctly expanded rectangular bases. Apart from another early ornithuromorph, Chaoyangia beishanensis, this morphology has not yet been reported in other Mesozoic birds, although it closely resembles that of several living species.
The rectangular-based uncinate process appears to have evolved independently at least three times, indicating repeated convergent evolution in different avian lineages. However, the ecological or functional pressures responsible for driving this morphology remain uncertain.
Overall, the study suggests that uncinate process shape is less closely related to flight ability and other ecological traits than previously expected, whereas uncinate process length is more strongly associated with respiratory and locomotor function.
The researchers further propose that uncinate process shape may be influenced primarily by physiological factors such as respiratory frequency and metabolic rate rather than by ecological variables such as habitat or diet. They also suggest that understanding its evolution and function will require integrated analyses of additional skeletal structures, including the sternum and pectoral girdle.
Finally, the study found that while most Mesozoic birds possessed relatively simple and slender uncinate processes, living birds evolved a much broader range of morphologies. However, because fossilized uncinate processes are extremely rare in Mesozoic birds, some of the observed differences may also reflect biases in fossil preservation.
（Author: Bai Leng）
Reference：
Kuo, P.-C., Wang, Y., Clark, A. D., Sullivan, C., Wang, X., Zheng, X., O'Connor, J. K. (2026). Evolutionary and ecological correlates of avian uncinate process morphology: insight from a bizarre shape found in Early Cretaceous Enantiornithes. All Earth.
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