The origin and early evolution of birds: discoveries, disputes, and perspectives from fossil evidence

Abstract

The study of the origin and early evolution of birds has never produced as much excitement and public attention as in the past decade. Well preserved and abundant new fossils of birds and dinosaurs have provided unprecedented new evidence on the dinosaurian origin of birds, the arboreal origin of avian flight, and the origin of feathers prior to flapping flight. The Mesozoic avian assemblage mainly comprises two major lineages: the prevalent extinct group Enantiornithes, and the Ornithurae, which gave rise to all modern birds, as well as several more basal taxa. Cretaceous birds radiated into various paleoecological niches that included fish- and seed-eating. Significant size and morphological differences and variation in flight capabilities, ranging from gliding to powerful flight among early birds, highlight the diversification of birds in the Early Cretaceous. There is little evidence, however, to support a Mesozoic origin of modern avian groups. Controversy and debate, nevertheless, surround many of these findings, and more details are needed to give a better appreciation of the significance of these new discoveries.

Discovery of an ornithurine bird and its implication for Early Cretaceous avian radiation

“…Other Early Cretaceous ornithurine, such as Yixianornis and Gansus, were also wading birds living in and near water. Therefore, this discovery confirms that the aquatic environment had clearly played a key role in the origin and early radiation of ornithurines (3,9,17).…”

“…The bird featured unexpectedly preserved complete hyoid bones and a predentary bone in the lower jaws. Most of the known ornithurines from the Jehol Biota were from the Jiufotang Formation, the upper part of the Jehol Group, and only one genus, Liaoningornis, represented by an incomplete postcranial skeleton, has been discovered from the Yixian Formation (9). Therefore, the new bird represents only the second, but much more complete, ornithurine from the Yixian Formation, shedding light on our understanding of the origin and early radiation of ornithurines.…”

“…Although most of these birds are toothed forms like other Mesozoic birds, several taxa showed independent reduction or loss of teeth, such as Confuciusornis, which was reported as the earliest bird with a horny beak, and Jeholornis, which only retained three small teeth on the lower jaw as a specialization for seed-eating adaptation (9).…”

A new raptorial dinosaur with exceptionally long feathering provides insights into dromaeosaurid flight performance

“…HG B016 is (Table 1), which before discovery was the largest 'four-winged' dinosaur 3 . Small size has been previously noted as a potential key adaptation in microraptorines for aerial locomotion 3,18,23,24 . The morphology of Changyuraptor yangi suggests that aerial abilities (be it gliding, powered flight, incline running or other semi-aerial locomotion) were not necessarily limited to small-bodied animals but were also present among more sizable animals.…”

A new feather type in a nonavian theropod and the early evolution of feathers

“…Early Cretaceous ͉ filament ͉ display F eathers have been documented in most nonavian coelurosaurian theropod groups, based mainly on recent discoveries of exceptionally well-preserved specimens from the Early Cretaceous Jehol Group of western Liaoning, China (1)(2)(3)(4). The feathers present in these specimens can be categorized into several morphotypes, but all of them are composite structures formed by multiple slender filaments (5-7).…”