Bipedalism as a preadaptive stage for bird’s flight is considered. We attribute the formation of full bipedalism in bird ancestors with pelvic limbs transition from segmental to parasagittal position. This transition was fast enough. We can assume that the pectoral limbs freed from the support remained while laterally spaced and gave set of transformations with different degrees of reduction. Thus morphologically “winglike” version of the thoracic limbs could appear. Parasagittal pelvic limbs allowed birds ancestors fast and maneuverable running, while the movements of free and highly movable thoracic limbs (feathered unrelated to flight) provided dynamic stability of the animal. In addition, their fluttering movements facilitate hopping from one branch to another and the descent from the trees. On the bottom branches protobirds could jump with perching just by the pelvic anisodactyl limbs, not by thoracic as had supposed earlier. Active interaction of the primary simple feathers with air as well as its protective function could become an impetus for their transformation into differentiated structures. Unlike gliding (as preadaptive stage for active flight) bipedalism with free feathered forelimbs provides per se parallel development of two autonomous enough locomotor systems of birds (flight and terrestrial locomotion) and extensive adaptive radiation of representatives of the class.
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