CLARKE, J. A.; GAUTHIER, J. A.; NORELL, M. A.; QIANG, Ji: Justified inference and the origin of an avian propatagium

Most versions of the arboreal theory for the origin of avian flight assume that a propatagium – a membrane spanning the gap between shoulder and wrist – was associated a gliding phase prior to active flight feather-powered flight. While a propatagium has long been central to such theories, surprisingly, no attempts have been made to investigate its distribution outside of the avian crown clade. New material offers the possibility to test this prediction. A propatagium does not appear present in a new non-avialan dinosaur from China with preserved integumentary structures. In fact, the first avialan with evidence of propatagium is Confuciusornis. Feathers appear to have preceded both the origin of flight and the development of a propatagium. There is currently no evidence that a propatagium preceded the origin of flight. Thus, a propatagium may have more to do with perfecting the flight apparatus than with the development of an airfoil to support gliding. Finally, all quantitative estimates of Archaeopteryx‘s aerodynamic abilities have relied on surface-area estimates that presume the presence of a propatagium. The observed distribution a propatagium among extant birds amply justifies the inference that this structure was present in the last common ancestor of the crown clade (Aves). In the absence of evidence to the contrary, however, a propatagium cannot justifiably be inferred to have been present in any more inclusive clade, for example at the origin of Archaeopteryx+Aves. If that structure was absent, total wing area must be reduced by around 4%; aerodynamic parameters should be recalculated. Widely used taxon names such as Aves should be restricted to crown clades to dissuade this and other unjustified inferences.