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Branched integumental structures in Sinornithosaurus and the origin of feathers

Abstract

The evolutionary origin of feathers has long been obscured because no morphological antecedents were known to the earliest, structurally modern feathers of Archaeopteryx1. It has been proposed that the filamentous integumental appendages on several theropod dinosaurs are primitive feathers2,3,4,5,6; but the homology between these filamentous structures and feathers has been disputed7,8,9, and two taxa with true feathers (Caudipteryx and Protarchaeopteryx) have been proposed to be flightless birds8,10. Confirmation of the theropod origin of feathers requires documentation of unambiguously feather-like structures in a clearly non-avian theropod. Here we describe our observations of the filamentous integumental appendages of the basal dromaeosaurid dinosaur Sinornithosaurus millenii, which indicate that they are compound structures composed of multiple filaments. Furthermore, these appendages exhibit two types of branching structure that are unique to avian feathers: filaments joined in a basal tuft, and filaments joined at their bases in series along a central filament. Combined with the independent phylogenetic evidence supporting the theropod ancestry of birds11,12,13, these observations strongly corroborate the hypothesis that the integumental appendages of Sinornithosaurus are homologous with avian feathers. The plesiomorphic feathers of Sinornithosaurus also conform to the predictions of an independent, developmental model of the evolutionary origin of feathers14.

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Figure 1: Sinornithosaurus millenii, holotype (IVPP 12811).
Figure 2: Filamentous integumental appendages of S. millenii dissociated from the integument and preserved overlying each other in the matrix near the skull.
Figure 3: Filamentous integumental appendage of S. millenii from the dorsal surface of the snout.
Figure 4: Dissociated filamentous integumental appendage of S. millenii from near the skull. a, Arrows indicate the distal tips of some component filaments.
Figure 5: Filamentous integumental appendages of S. millenii indicating the presence of larger central filaments, or shafts, and secondary branched filaments.
Figure 6: The predicted transition series of feather morphologies from an independent, developmental model of the evolutionary origin of feathers14.

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Acknowledgements

We thank M. Chang and Y. Wang for assistance during the work; S.-h. Xie for preparation of the holotype specimen of Sinornithosaurus; and members of the Liaoxi expedition team of the IVPP, who participated in the excavation of the specimen. The paper benefited from discussions with C.-M. Chuong, P. Currie, A. Deane, L. Martin, B. Timm, G. Wagner, L. Witmer and F. Zhang; and was improved by comments from W. Boles, P. Stettenheim and L. Witmer. We acknowledge financial support from the Chinese Academy of Sciences, Chinese Natural Science Foundation, Chinese Special Funds for Major State Basic Research Projects (X.X., Z.Z.), and the United States National Science Foundation (R.O.P.). Order of authorship was assigned randomly.

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Correspondence to Richard O. Prum.

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Xu, X., Zhou, Zh. & Prum, R. Branched integumental structures in Sinornithosaurus and the origin of feathers. Nature 410, 200–204 (2001). https://doi.org/10.1038/35065589

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