How Can We Infer That the Long-Feathered Anchiornis huxleyi Was Flightless?

A dinosaur with long feathers and wing-like forelimbs was not necessarily capable of flight. Anchiornis huxleyi, which lived approximately 160 million years ago during the Late Jurassic and was recovered from the Tiaojishan Formation of northeastern China, offers a particularly complex example. Its body was covered in feathers, and both the forelimbs and hindlimbs bore long feathers, giving it an appearance strikingly reminiscent of early birds. For many years, however, researchers have disagreed over whether this animal was actually capable of flight.

Reconstruction model of Anchiornis huxleyi (National Museum of Natural History, Luxembourg)（Image source：TB0815， CC BY 4.0 ）

The feathered surface of a wing is the part of a flying animal that interacts directly with the air. Whether that surface can generate lift efficiently depends on more than the skeleton and muscles: it is also shaped by the number, length, arrangement, overlap, and mechanical strength of the feathers themselves. Feathers are subject to an additional physiological limitation. Once a feather is fully grown, its blood supply and living cells disappear; from that point onward, it can neither repair itself nor continue growing. When it becomes damaged through abrasion, sunlight, or impact, it must be replaced through molt. Molting strategy is therefore closely linked to flight ability. An animal that depends heavily on flight must avoid any molt pattern that would suddenly compromise the functional surface of its wings.

Wing-feather replacement in modern birds can broadly be divided into three strategies. Most flying birds undergo a gradual, sequential molt, replacing only a small number of primary feathers at a time, usually in a broadly symmetrical pattern between the two wings. This allows the wings to remain functional for flight while the feathers are being renewed. Some waterbirds, or birds living in habitats that offer greater protection, replace all of their flight feathers simultaneously. They temporarily lose the ability to fly, but complete feather renewal within a shorter period. The third strategy is irregular molt, in which feathers are replaced without a fixed sequence and the two wings may not molt synchronously. This pattern is essentially restricted to flightless birds such as ostriches, the Flightless Cormorant (Nannopterum harrisi), and the Kākāpō (Strigops habroptilus). Detecting irregular molt in a fossil animal therefore provides strong evidence that it may have been flightless.

The research team examined 226 fossils of Anchiornis huxleyi housed in the Shandong Tianyu Museum of Nature. Among them, nine specimens preserved sufficiently clear feather outlines and traces of plumage coloration to allow the wing structure to be analyzed. The wing feathers of this dinosaur had a distinctive color pattern: they were generally light in color, with dark patches at their tips. When the feathers were fully grown, these dark tips aligned into neat transverse bars across the wing. The wings of Anchiornis carried four such dark bars. Three were formed by the dark tips of three series of primary coverts, while the fourth was formed by the tips of the primary feathers themselves. This regular pattern provides a natural indicator of feather growth. When a feather was still growing, its dark tip remained closer to the postpatagium, causing a gap or displacement in what would otherwise have been a continuous wing bar. Even though the fossils did not preserve every feather contour perfectly, the positions of these dark bands allowed the researchers to identify feathers that had not yet reached full length.

Feathers of Anchiornis huxleyi: 1st primary covert series (1st PC series), 2nd primary covert series (2nd PC series), 3rd primary covert series (3rd PC series), and primary feathers（Image source：Kiat Y et al. (2025)， CC BY-NC-ND 4.0 ）

All nine exceptionally preserved specimens retained evidence of short, growing feathers. In most individuals, as many as five primary feathers were still incompletely grown at the same time. These short feathers occurred in different positions along the wing, and the two wings were often asymmetrical. One specimen, STM0-143, showed especially uneven feather lengths, indicating that it was undergoing a more intensive stage of molt when it died. The absence of a fixed replacement sequence, the coexistence of feathers at different stages of growth, and the asymmetry between the two wings all correspond to the irregular molt pattern observed in modern flightless birds. The researchers also examined photographic data from the Flightless Cormorant. Among 143 randomly sampled individuals, 113 were actively undergoing irregular molt, and asynchronous molt between the two wings was common. This living example provides a useful comparison for interpreting the wing-feather condition preserved in Anchiornis.

The structure of the wing itself provides another important line of evidence. Flying modern birds generally possess only two series of primary coverts. In the study, measurements taken from 31 volant species of modern neornithines showed that the longer primary covert series covered an average of 47.9% of wing length, with values ranging from 38% to 62%. Anchiornis, in contrast, possessed three series of primary coverts, and the longest series extended across more than 80% of the wing. This means that much of its primary feather length was concealed beneath unusually long coverts. Such extensive coverage may have made the feathered forelimb thicker and produced a wing profile unlike the aerodynamic surface suited for generating lift. Among modern birds, the clearest examples of multiple primary covert series covering most of the forelimb are penguins. Their wings have evolved into flippers used for underwater propulsion, with five to six series of primary coverts covering more than 95% of wing length. Yet Anchiornis did not possess flipper-like forelimbs adapted for swimming. Its wing morphology has no direct counterpart among living animals, suggesting that its forelimb feathers may have served a function that remains unknown.

Percentage of wing length covered by the primary coverts: volant Neornithes (blue), Anchiornis huxleyi (yellow), and Aptenodytes penguins (green)（Image source：Kiat Y et al. (2025)， CC BY-NC-ND 4.0 ）

The number and form of the primary feathers also support the interpretation that Anchiornis was flightless. In modern flying birds and many other non-avian pennaraptoran dinosaurs examined in previous work, the wing typically carries about nine to eleven primary feathers. Most Anchiornis specimens, however, possessed between 20 and 25 primaries, while specimen STM0-65 preserved as many as 28. Its primary feathers were also relatively narrow and nearly symmetrical, unlike the asymmetrical remiges characteristic of most birds capable of effective flight. The study identified several additional traits inconsistent with volant locomotion, including an unusually high number of secondary feathers relative to ulna length, distinctive primary covert proportions, remiges that were at least partly open-vaned, and the absence of a postpatagium capable of supporting the wing surface. The number of primary feathers also varied among individuals. In modern flying birds, the number of wing feathers within a species is generally under strong functional constraint, and substantial variation is uncommon. Once the wing surface no longer performs a precise aerodynamic role, the constraints on feather number and arrangement may be reduced.

The study also attempted to reconstruct the evolutionary history of molt strategies among paravian dinosaurs. The analysis included Anchiornis, Microraptor, Confuciusornis, and modern birds, while accounting for two alternative placements of Anchiornis on the phylogenetic tree because its evolutionary position remains debated. The results indicated that the common ancestor of paravians most likely underwent a gradual, sequential molt, with an estimated probability of 89.2%. Irregular molt appears to have evolved independently in Anchiornis and in several modern bird lineages that lost the ability to fly. Both Microraptor and Confuciusornis have been identified as possessing sequential molt, a condition consistent with interpretations that they were capable of flight.

Evolutionary tree of molt strategies among paravian dinosaurs: sequential molt (blue), irregular molt (yellow), and flightlessness in Neornithes (green)（Image source：Kiat Y et al. (2025)， CC BY-NC-ND 4.0 ）

Many questions remain unanswered, yet Anchiornis huxleyi offers an important reminder that the evolutionary transition from dinosaurs to birds was far more complicated than a simple progression in which feathers gradually formed wings and ultimately enabled flight. The available evidence strongly suggests that Anchiornis had already lost the ability to fly. Through subtle clues preserved in its wing-feather arrangement and molt pattern, researchers are now able to move closer to reconstructing the actual life of this remarkable Jurassic dinosaur.

Author: Shui-Ye You

Reference:

Kiat Y et al. (2025). Wing morphology of Anchiornis huxleyi and the evolution of molt strategies in paravian dinosaurs. Communications Biology.