1. The Institute of Vertebrate Paleontology and Paleoanthropology, Academia Sinica, P.O. Box 643, Beijing 100044, People's Republic of China
2. Natural History Museum, Changchun University of Science and Technology, 6 Ximingzhu Street, Changchun 130026, People's Republic of China
3. Vertebrate Morphology Research Group, Department of Biological Sciences, The University of Calgary, 2500 University Drive NW, Calgary, AB T2N 1N4, Canada

Correspondence to: Xiao-Chun Wu^1,3 Correspondence and requests for materials should be addressed to X.-C.W. (Email: e-mail: xiaow@tyrrellmuseum.com or xcwu@ucalgary.ca).

Abstract

Dromaeosaurids, despite their notoriety, are poorly characterized meat-eating dinosaurs, and were previously known only from disarticulated or fragmentary specimens¹. Many studies have denied their close relationship to birds^{2, 3}. Here we report the best represented and probably the earliest dromaeosaurid yet discovered, Sinornithosaurus millenii gen. et sp. nov., from Sihetun, the famous Mesozoic fish–dinosaur–bird locality in China^{4, 5}. Sinornithosaurus not only greatly increases our knowledge of Dromaeosauridae but also provides evidence for a filamentous integument in this group. It is remarkably similar to early birds postcranially. The shoulder girdle shows that terrestrial dromaeosaurids had attained the prerequisites for powered, flapping flight⁶, supporting the idea that bird flight originated from the ground up^{7, 8}. The discovery of Sinornithosaurus widens the distribution of integumentary filaments among non-avian theropods^{5, 9, 10}. Phylogenetic analysis indicates that, among known theropods with integumentary filaments or feathers^{2, 5}, Dromaeosauridae is the most bird-like, and is more closely related to birds than is Troodontidae.