Preserved soft anatomy confirms shoulder-powered upstroke of early theropod flyers, reveals enhanced early pygostylian upstroke, and explains early sternum loss

Abstract

Anatomy of the first flying feathered dinosaurs, modern birds and crocodylians, proposes an ancestral flight system divided between shoulder and chest muscles, before the upstroke muscles migrated beneath the body. This ancestral flight system featured the dorsally positioned deltoids and supracoracoideus controlling the upstroke and the chest-bound pectoralis controlling the downstroke. Preserved soft anatomy is needed to contextualize the origin of the modern flight system, but this has remained elusive. Here we reveal the soft anatomy of the earliest theropod flyers preserved as residual skin chemistry covering the body and delimiting its margins. These data provide preserved soft anatomy that independently validate the ancestral theropod flight system. The heavily constructed shoulder and more weakly constructed chest in the early pygostylian Confuciusornis indicated by a preserved body profile, proposes the first upstroke-enhanced flight stroke. Slender ventral body profiles in the early-diverging birds Archaeopteryx and Anchiornis suggest habitual use of the pectoralis could not maintain the sternum through bone functional adaptations. Increased wing-assisted terrestrial locomotion potentially accelerated sternum loss through higher breathing requirements. Lower expected downstroke requirements in the early thermal soarer Sapeornis could have driven sternum loss through bone functional adaption, possibly encouraged by the higher breathing demands of a Confuciusornis -like upstroke. Both factors are supported by a slender ventral body profile. These data validate the ancestral shoulder/chest flight system and provide insights into novel upstroke-enhanced flight strokes and early sternum loss, filling important gaps in our understanding of the appearance of modern flight.

Decoupling body shape and mass distribution in birds and their dinosaurian ancestors

“…Rahonavis, Scansoriopteryx) are yet, to our knowledge, to be analysed by volumetric modelling approaches, particularly where specimens are unrepresented by near-complete three-dimensionally preserved specimens. Unusual morphologies and the limitations of fossil preservation, and particularly the challenges of reconstructing biomechanical performance from fossilised hard tissue alone 12,13,16,19,[36][37][38][39][40][41] , mean that the locomotor capabilities of these taxa remain somewhat controversial 42,43 , although recent description and analysis of paravians with preserved muscle and body segment outlines have provided key insights into early flight evolution 44 . Given their skeletal proportions and likely phylogenetic positions, analyses of mass distribution in these groups potentially could refine or add a higher degree of complexity to the trajectory of CoM evolution recovered here between Maniraptoriformes and Avialae (Fig.…”

Reconstruction of the Hindlimb Locomotion of Confuciusornis (Aves) and Its Implication for the Origin of Avian Flight

“…It is known for its richly preserved specimens, extremely long period of existence, and being the first to evolve the beak in birds [1,11,17]. Locomotion-related structures of Confuciusornis, such as the sternum, forelimb, hindlimb, feathers, soft tissues et al, have been studied in detail [13,[20][21][22][23][24][25][26][27][28][29][30][31][32], which provides valuable information to our understanding of the habit of early birds. Although Confuciusornis are primitive for modern birds, they already possessed many relatively advanced avian anatomical characteristics.…”

“…Although Confuciusornis are primitive for modern birds, they already possessed many relatively advanced avian anatomical characteristics. They have imbricate arranged wing feathers [24], long and asymmetrical primary feathers with robust rachises [24], the proand postpatagium [25], the propatagium ligament [32], the larger ossified sternum, flexible hands [23]. These factors suggest that Confuciusornis was capable of powered flight and was probably a better flyer than Archaeopteryx.…”

Paleoecology of extinct species

“…A prime example is using Laser-Stimulated Fluorescence on fossils inspired by confocal microscopy. This has revealed otherwise hidden soft tissues, providing key new insights beyond existing evidence [ 1 ], especially in early flight studies [ 2 ]. Another example is dental microwear analysis to study extinct species, which has improved with methodological developments and data from modern animals.…”

“…hidden soft tissues, providing key new insights beyond existing evidence [1], especially in early flight studies [2]. Another example is dental microwear analysis to study extinct species, which has improved with methodological developments and data from modern animals.…”