Anatomy Of Pectoral Girdle And Forelimb Of Anchiornis Huxleyi

Since1990s, a large number of feathered dinosaur and primitive bird fossils have beendiscovered from the Early Cretaceous “Jehol Biota” in western Liaoning and its neighboringregions, northeastern China. The finds provide powerful evidence for the origin of birds fromtheropod dinosaurs, and also important information on the origin of feather and flight in birds.However, the feathered dinosaurs are younger than the earliest known bird---Archaeopteryxfrom Germany, and near-avian theropod fossils were scarcely found from the older strata thanArchaeopteryx’s era in the world. The facts have been ever used as last evidence by somescholars to argue against the theropod hypothesis of bird origin. As a feathered non-aviantheropod older than Archaeopteryx, the discovery of Anchiornis huxleyi in2009solved the lastevidence of the objectors, so that the theory that birds evolved from dinosaurs was finallyreccepted.Anchiornis huxleyi not only bridges a critical gap from dinosaurs to birds, but also furthersupports the arboreal hypothesis of flight origin and the hypothesis that early evolution of flighthad ever experienced a "four-winged" stage. As an important taxon for the research on the originof birds and the origin of bird flight, a detailed description and comparison in morphology andfunction, especially in comparison with Archaeopteryx, are very significant and necessary. In theprevious study, however, the skeletons of Anchiornis huxleyi were preliminarily described andthe discussion on the morphological function of the skeletons was not enough.In this study, the pectoral girdle and forelimb are described in detail and compared withArchaeopteryx, non-avian theropods and other primitive birds; a functional analysis on themorphology of the skeletons is also conducted. In conclusion, Anchiornis huxleyi is more similarwith Archaeoptery in morphology and the proportions between the elements of limbs. However,some morphological differences are also distinct between both; Anchiornis huxleyi has shortercoracoid, not deeply arched furcula, more ventrally oriented glenoid of scapular, smallerdeltopectoral crest, straight ulna, ulnare significantly larger than the radiale, long manual,indicting that Anchiornis huxleyi is more primitive than Archaeoptery. The functional analysis ofthe pectoral girdle and forelimb suggests that Anchiornis huxleyi was likely arboreal, and had not flight ability or had very weak flight ability. The weak flight feathers also support the result ofthis analysis.