Four-winged Norellraptor gained its flight anatomy in a different order from birds

Andrea Cau of the OPHIS Paleontological Museum and Herpetological Center in Italy and his colleagues took an old question to a new animal ^[3]. The question is whether flight began in an ancestor shared by microraptors and birds, or whether feathers, and perhaps flight, evolved more than once ^[6]. Microraptors are close relatives of birds but not their ancestors; birds belong to a neighbouring clade, Avialae ^[7]. Norellraptor barsboldi, a small predator with four wings, is a microraptor ^[14].

Counting shared traits cannot settle that alone, because inheritance and convergence both leave two groups looking alike. The order in which traits appear can help. When the team compared Norellraptor with other fossils, the features the two groups share had appeared in a different sequence in each ^[1]. New Scientist reports the researchers also argue the two groups took to the air for different reasons ^[2]. Rui Pei, a palaeontologist at the Chinese Academy of Sciences' Institute of Vertebrate Paleontology and Paleoanthropology who was not involved, said: "This demonstrates that microraptors acquired traits convergent with the early evolutionary stages of true birds, albeit through a distinct evolutionary pathway, supporting the hypothesis that flapping flight evolved independently in non-avialan dinosaurs and birds." ^[8][9]

The overlap is around 30% of the 194 anatomical changes the team identified across microraptor evolution, or about 58 changes that also arose in the bird lineage ^[4][22]. They include bony projections on the ribs fused into a rigid torso, forelimbs sturdy relative to the hind limbs, and an ulna, the main weight-bearing bone of a wing, longer than the humerus ^[5]. The coverage does not report how much overlap would be expected between two groups this closely related. On its own, the percentage fits either history; the ordering is what points to separate origins ^[1].

A second kind of evidence comes from inside the bone. Jacqueline Nguyen of the Australian Museum in Sydney said analyses of the specimen's bone microstructure, growth patterns and evolutionary relationships show that the sequences leading to flight in living birds and in bird-like dinosaurs were quite different ^[10][11]. "Its forelimb growth pattern is quite different from that of living birds," she said ^[12].

The thing this doesn't tell you is whether Norellraptor itself flew, or how well. The researchers think its feathers may have been associated with flight, New Scientist reports ^[21]. Pei told Live Science by email that "it is reasonable to expect that these non-avialan theropods were capable of behaviors comparable to those of modern avian species" ^[23]. Scott Hartman, a vertebrate palaeontologist at the University of Wisconsin-Madison who was not involved, suggested a mix of gliding and flapping ^[13][24]. He pointed to a large breastbone that could allow for larger pectoral muscles. "They probably have a pretty solid downstroke, which is what generates the thrust to keep it going forward," he said ^[25].

Getting airborne is harder to read from bones. "I doubt they could climb trees like a squirrel to gain height because their forelimbs don't have that range of motion," Hartman said ^[26]. He suggested they might have taken off after running down a slope, or leapt from hills or cliffs ^[27]. In his reading the hind wings sat directly under the body, for stabilising flight, braking and control in tight turns ^[28].

The paper compares two lineages, microraptors and birds ^[1]. Hartman's endorsement is broader. "I'm very happy that they've come to the conclusion that there were probably independent origins of flight amongst various groups," he said. "I strongly agree with them." ^[15] Nguyen was also persuaded. "This is interesting, because it shows that the evolutionary path to flight was not just a linear one, but that these evolutionary innovations appeared independently," she said ^[16].