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The oldest feathers known to date have been found in archosaurs capable of flight. However, some of them (scansoriopterygids) flew by the use of a membrane rather than feathers. We therefore propose a new mechanism for the origin of avian flight by the use of membranous wings on both the forelimbs and the hindlimbs. It complements Beebe’s (1915) prediction of the tetrapteryx stage. Paleontological and embryological evidence suggest that feathers are a modification of reptilian scales. Scansoriopterygids were covered mostly by down-like feathers which seemingly acted as thermal isolation rather than being adapted for flight. Certain early birds, including scansoriopterygids, possessed elongate shafted tail feathers, which were probably used principally for display and resembled elongate scales. We suppose that display is the primary function of early feathers, which were preadapted for thermal isolation and also flight. The body of theropods was covered mostly by typical reptilian scales, yet some ornithischian dinosaurs possessed filamentous integumentary structures which might have had a comparable display function. However, it is doubtful that these structures in dinosaurs were homologous with avian feathers. Early birds probably shared with theropod dinosaurs an incipient endothermy, which was inherited from their common ancestor in the Triassic Period.
This is a new preprint version with minor corrections: (a) Praeornis and Zhongornis are discussed in more detail and new references are added, (b) the Fig. 1 is slightly modified, and (c) the English Language is improved.