Abstract

We re-examine the partial skeleton of the enigmatic small feathered theropod Yixianosaurus longimanus from the Lower Yixian Formation, China. A phylogenetic analysis recovers Yixianosaurus as a basal maniraptoran, in a polytomy with Alvarezsaurus, Therizinosauria, Alvarezsauroidea excluding Alvarezsaurus, and a well-resolved clade of derived maniraptorans including Oviraptorsauria and Paraves. The blend in Yixianosaurus of primitive traits, such as a low brachial index and robust digit III, and derived traits, such as a subrectangular coracoid, suggests a more complicated mosaic of forelimb evolution within Maniraptora than previously proposed. The robust forelimb bones and massive, recurved, and pointed manual unguals suggest Yixianosaurus was predatory, although this assessment remains tentative until more complete fossils of this taxon are found. The basal maniraptoran phylogenetic position implies a larger range of forelimb morphologies at this point in coelurosaurian evolution. In many ways, the robust forelimb and hypertrophied recurved unguals are similar to those of Tanycolagreus and Coelurus, and may offer transitional morphologies between these taxa and basal therizinosaurs and oviraptorosaurs. The presence of large, veined, penneacous, contour feathers on Yixianosaurus indicates the origin of these integumentary structures may be deeper in coelurosaur phylogeny than previously reported. Finally, the robust, elongate forelimbs suggest a different ecological role than inferred for other sympatric small theropods, and lend support for niche partitioning within the small non-avian theropods of the Jehol fauna.