Morphological,Ontogenetic And Aerodynamic Analyses Of The Early Cretaceous Confuciusornithids From Western Liaoning

The earliest-known beaked confuciusornithids are the only species-rich and morphologically diverse clade of Early Cretaceous birds,appearing the longest time span from the Dabeigou Formation(about 131 Ma),the Yixian Formation to the Jiufotang Formation(about 120 Ma).Thousands of exceptionally preserved specimens that collectively provide rich information on confuciusornithid morphology,taxonomy,flight ability,growth,diet,and ecology,and significantly advances our knowledge of early avian evolution.However,there are still some problems in the taxonomy,osteology,functional morphology,phylogeny and ontogeny of these Early Cretaceous birds.Here,we report a new confuciusornithid species,Confuciusornis shifan sp.nov.,from the Jiufotang Formation.Confuciusornis shifan sp.nov.,differs from other confuciusornithids in a number of morphological and developmental features.Fortunately,as the holotype of Confuciusornis shifan sp.nov was preserved at a very rare developmental stage,the pygostyle of the earliest known pygostylians is confirmed for the first time to being from a fusion of 11 caudal vertebrae.The available data suggest a gradual reduction in caudal vertebral number in early bird evolution and a proportionally much shorter tail in pygostylians than in other early birds.Confuciusornis shifan sp.nov.has the secondary ossification center in manual bones.Such feature never occurred in non-avian archosauriforms and non-ornithuromorph birds,and even in the forelimbs of crown-group birds.The unusual occurrence of the secondary ossification center in the wing bones is possibly a combinative result of the flight-performing demand and a comparative primitive growth mode.The new find thus provide a striking example showing the morphological,developmental and functional diversity in the earliest-known beaked birds.We restudied the holotype of Confuciusornis jianchangensis(PMo L-AB00114).PMo L-AB00114 is a big size individual,from the Jiufotang Formation.It was in early growth stage at the time of death.Some morphological differences between this specimen and all known Confuciusornithid,but this specimen does not show obvious autopomorphies,and the preservation is not complete.Therefore,we believe that there is no reliable basis to judge this specimen as an effective species.In order to further assess the flight capabilities and strategies of confuciusornithid birds,we estimated BMs and flight parameters for 11 confuciusornithid specimens.Our analyses under phylogeny indicate that varying modes of flight adaptation emerged across the diversity of confuciusornithids,and to a lesser degree over the course of their ontogeny,and specifically suggest that both a trend towards improved flight capability and a change in flight strategy occurred in confuciusornithid evolution.