Basal paravian functional anatomy illuminated by high-detail body outline

Abstract

Body shape is a fundamental expression of organismal biology, but its quantitative reconstruction in fossil vertebrates is rare. Due to the absence of fossilized soft tissue evidence, the functional consequences of basal paravian body shape and its implications for the origins of avians and flight are not yet fully understood. Here we reconstruct the quantitative body outline of a fossil paravian Anchiornis based on high-definition images of soft tissues revealed by laser-stimulated fluorescence. This body outline confirms patagia-bearing arms, drumstick-shaped legs and a slender tail, features that were probably widespread among paravians. Finely preserved details also reveal similarities in propatagial and footpad form between basal paravians and modern birds, extending their record to the Late Jurassic. The body outline and soft tissue details suggest significant functional decoupling between the legs and tail in at least some basal paravians. The number of seemingly modern propatagial traits hint that feathering was a significant factor in how basal paravians utilized arm, leg and tail function for aerodynamic benefit.

Additional information on the primitive contour and wing feathering of paravian dinosaurs

“…soft‐tissue‐joined fingers) is based on the reconstructed high‐detail LSF study of Wang et al . (). Present are the novel contour feathers described here, resulting in a ‘shaggy’ appearance to the plumage.…”

“…Wang et al . , ), and gliding rather than powered flight has been proposed for ‘four‐winged’ non‐avian theropods (e.g. Chatterjee & Templin ; Dyke et al .…”

“…Furthermore, the presence of a propatagium in Anchiornis , and other paravians like Serikornis , may have also helped to compensate for aerial locomotion‐related inefficiencies of the feathers and skeletomusculature (Wang et al . ; Lefèvre et al . ), and the presence of hindlimb wings and tail rectrices in addition to forelimb wings might have compensated by increasing surface area for lift or stability during gliding (Xu et al .…”

Powered flight potential approached by wide range of close avian relatives but achieved selectively

“…There is no evidence that the airfoil shapes of fossil taxa differed significantly from living taxa e.g. fossil taxa also possessed similar leading edge shapes with well-developed propatagia [56][57][58] . The long bone cross sections in the forelimbs of early birds and microraptorine dromaeosaurids have similar shapes and comparable bending strengths to those of living birds 59 .…”

“…138.2) with an apex located closer to its proximal end (char. 684.2) and the lack of a bony sternum (at least in Anchiornis 56,73 ). Paradoxically, Xiaotingia has a bowed rather than straight ulna, a feature linked with better takeoff potential in modern birds 74 .…”