title: Homology and Potential Cellular and Molecular Mechanisms for the Development of Unique Feather Morphologies in Early Birds.
abstract: At least two lineages of Mesozoic birds are known to have possessed a distinct feather morphotype for which there is no neornithine (modern) equivalent. The early stepwise evolution of apparently modern feathers occurred within Maniraptora, basal to the avian transition, with asymmetrical pennaceous feathers suited for flight present in the most basal recognized avian,Archaeopteryx lithographica. The number of extinct primitive feather morphotypes recognized among non-avian dinosaurs continues to increase with new discoveries; some of these resemble feathers present in basal birds. As a result, feathers between phylogenetically widely separated taxa have been described as homologous. Here we examine the extinct feather morphotypes recognized within Aves and compare these structures with those found in non-avian dinosaurs. We conclude that the "rachis dominated" tail feathers ofConfuciusornis sanctusand some enantiornithines are not equivalent to the "proximally ribbon-like" pennaceous feathers of the juvenile oviraptorosaurSimilicaudipteryx yixianensis. Close morphological analysis of these unusual rectrices in basal birds supports the interpretation that they are modified pennaceous feathers. Because this feather morphotype is not seen in living birds, we build on current understanding of modern feather molecular morphogenesis to suggest a hypothetical molecular developmental model for the formation of the rachis dominated feathers of extinct basal birds.
authors.affiliation: Key Laboratory of Evolutionary Systematics of Vertebrates, Institute of Vertebrate Paleontology and Paleoanthropology, 142 Xizhimenwai Dajie, Beijing 100044, China ; Dinosaur Institute, Natural History Museum of Los Angeles County, 900 Exposition Boulevard, Los Angeles, CA 90007, USA.
authors.forename: Jingmai K
authors.initials: JK
authors.lastname: O'Connor
authors.forename: Luis M
authors.initials: LM
authors.lastname: Chiappe
authors.forename: Cheng-Ming
authors.initials: CM
authors.lastname: Chuong
authors.forename: David J
authors.initials: DJ
authors.lastname: Bottjer
authors.forename: Hailu
authors.initials: H
authors.lastname: You
keywords: Confuciusornis; Mesozoic birds; Similicaudipteryx; dinosaur integument; feathers; molecular development; rectrix