More than 250 million years ago, weigeltisaurids became the first vertebrates to glide through the air, with a flight system unlike anything alive today.
Long before birds, bats or even dinosaurs, a small reptile lineage became the first known vertebrates to glide through the air. Weigeltisaurids achieved flight using an unusual wing system built from entirely new bones anchored to the side of their abdomen, and new research shows these early fliers were capable, well-engineered gliders.
The first known gliding reptile, a member of the family Weigeltisauridae, lived during the Late Permian, about 260 million years ago. Its capacity to glide depended on a structure called the patagium, a membranous structure stretching between the limbs and body, affording aerodynamic lift and, thus, a finite form of flight. Flying Draco lizards are the best living analogs for the extinct Weigeltisauridae, whose wing anatomy and gliding mechanism has been well studied. These frilled creatures glide through Southeast Asian tropical canopies with the help of patagia supported by elongated ribs. But the structure and mechanics of gliding in the extinct Weigeltisauridae has been less clear, until now.
With the help of advanced imaging, researchers were able to help settle the debate on the true nature of this ancient gliding reptile’s patagia. Valentin Buffa, a researcher at the University of the Witwatersrand in South Africa, and colleagues examined the structure of the patagial skeletons of two specimens found in Germany, along with comparisons to well-preserved specimens from Germany and Madagascar.
Each wing was supported by 25 to 30 slender bony rods on either side of the body, making up the patagial skeleton. When the researchers looked closely, they noticed that every one of these wing bones was connected to a single bone in the gastralia, often considered “belly ribs”. Gastralia form a basket-like structure that supports the abdomen and helps with breathing but, unlike true ribs, they don’t attach to the backbone and instead connect to each other and surrounding muscles.
Together with the distinct joint linking each pair of bones from the patagium and gastralia, the bones are also different at a more fundamental level, through the lens of a microscope. This challenges the view that patagial bones are an offshoot of belly ribs. So, the patagial bones in Weigeltisauridae, while possessing the same function as those found in Draco lizards, have a completely different origin story.
Given the length of the patagials, the researchers think it unlikely that they were embedded in the abdominal muscles. It is much more likely that they were anchored in the external oblique muscles. That positioning placed the wings lower on the body than in Draco lizards, whose rib-supported patagia extend from the back, and it means weigeltisaurids had to deploy their wings in a different way.
“By comparison to modern aircraft, this configuration would offer more maneuverability but less stability than a high-wing configuration such as that known for Draco,” write the researchers. The oblique muscles ran partially along the patagials, likely helping unfold the wing, while the flying reptile used its clawed forelimbs to keep the wing prised open. “However, the likely involvement of the forelimb in keeping the patagium unfolded during flight might have also helped in keeping the wing in a dihedral position, which would have stabilized flight and thus at least partially counterbalanced this trade-off.”
The researchers also revised long-standing assumptions about the size of the weigeltisaurids. Instead of weighing around 270 grams, one species, Coelurosauravus elivensis, appears to have weighed just 135 grams, making it far lighter and better suited for gliding than previously thought. With its svelter form, the reptile was a skilled glider, much like the extant northern flying squirrel, and faster even than Draco lizards.
Taken together, these findings portray weigeltisaurids as aerial animals with a wing design found nowhere else among vertebrates. “This work represents a critical step toward understanding the wing structure and gliding mechanism in weigeltisaurids,” add the researchers, “paving the way for future morphofunctional or biomechanical studies on the locomotion of the world’s first flying vertebrates.”
Reference: V. Buffa et al., Morphology and osteo-histology of the weigeltisaurid wing: Implications for aerial locomotion in the world’s first gliding reptiles, Journal of Anatomy (2025). DOI: doi.org/10.1111/joa.70058
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