Troodontid dinosaurs form one of the most avian-like dinosaur groups. Their phylogenetic position is hotly debated, and they have been allied with almost all principal coelurosaurian lineages. Here we report a basal troodontid dinosaur, Sinovenator changii gen. et sp. nov., from the lower Yixian Formation of China. This taxon has several features that are not found in more derived troodontids, but that occur in dromaeosaurids and avialans. The discovery of Sinovenator and the examination of character distributions along the maniraptoran lineage indicate that principal structural modifications toward avians were acquired in the early stages of maniraptoran evolution.
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We thank Z.-H. Zhou for help; H.-J. Wang and B. Long for specimen preparation; R.-S. Li for drawings; M. Ellison for photographs; and members of the Liaoxi expedition team of the IVPP. This work was supported by grants from the Special Funds for Major State Basic Research Projects of China, the National Geographic Society, the Chinese Natural Science Foundation and the Chinese Academy of Sciences. M.A.N. and P.J.M. are supported by the American Museum of Natural History and the Field Museum, respectively.