Origin of the propatagium in birds

Uno and Hirasawa 2023 reported,
“Avian wings as organs for aerial locomotion are furnished with a highly specialized musculoskeletal system compared with the forelimbs of other tetrapod vertebrates. Among the specializations, the propatagium, which accompanies a skeletal muscle spanning between the shoulder and wrist on the leading edge of the wing, represents an evolutionary novelty established at a certain point in the lineage toward crown birds. However, because of the rarity of soft tissue preservation in the fossil record, the evolutionary origin of the avian propatagium has remained elusive.”

Unfortunately the authors never state what the propatagium is used for. Nor do they mention bats or pterosaurs, which both developed a propatagium by convergence.

Peters 2002 reported,
“Prolonged tree-clinging may have contributed to the development of the propatagium and pteroid bone as passive aids to prevent overextension of the elbow—but only if the pterosaur held its torso away from the tree trunk, like a sloth, a flying lemur or a lumberjack, rather than embracing it closely. Brown et al. (1995) showed that a tendon in the leading edge of the propatagium in birds prevents elbow extension due to aerodynamic drag in flight. The origin of this tendon and its associated propatagium may be traced to early Cretaceous birds, such as Confusciusornis (Clarke et al., 2001), which could grapple tree trunks with their wing claws, like a pterosaur.”

The authors do not cite Brown et al 1995.
Nor do they cite Clarke et al 2001.
Nor do they mention ‘over-extension’ or ‘cling’.
They do mention ‘grasping items such as prey’, but a propatagium is not needed for that.

Uno and Hirasawa 2023 continue,
“Here we focus on articulated skeletons in the fossil record to show that angles of elbow joints in fossils are indicators of the propatagium in extant lineages of diapsids (crown birds and non-dinosaurian diapsids), and then use this relationship to narrow down the phylogenetic position acquiring the propatagium to the common ancestor of maniraptorans.”

That’s one way to do it: measuring elbow angles.

“Our analyses support the hypothesis that the preserved propatagium-like soft tissues in non-avian theropod dinosaurs (oviraptorosaurian Caudipteryx and dromaeosaurian Microraptor) are homologous with the avian propatagium, and indicate that all maniraptoran dinosaurs likely possessed the propatagium even before the origin of flight.”

No doubt. Rapid flapping forces make elbows extend. That needs to be limited by a propatagium, both in flight and in non-volant displays.

BTW… The propatagium (with feathers in birds) also adds coverage (shade, weather protection) for eggs in nests, but that aftereffect is also overlooked by the authors.

References
Brown RE, Baumel JJ and Klemm RD 1995. Mechanics of the avian propatagium: flexion-extension mechanism of the avian wing. Journal of Morphology 225, 91–105.
Clarke JA, Gauthier JA, Norell MA and Ji Q 2001. The origin and significance of a propatagium in flying dinosaurs. Journal of Vertebrate Paleontology 21 (Supplement to No. 3), 41A.
Peters D 2001. A New Model for the Evolution of the Pterosaur Wing—with a twist. Historical Biology 15:277–301.
Uno Y and Hirasawa T 2023. Origin of the propatagium in non-avian dinosaurs. Zoological Letters 9(4): https://doi.org/10.1186/s40851-023-00204-x