Filamentous impressions associated with locomotive theropod tracks in the Lower Jurassic Turners Falls Formation of western Massachusetts, U.S.A. represent the oldest evidence of feathered dinosaurs. Feather impressions are preserved with sitting traces which bear integumentary structures along the outlines of the pre-pubic and ischiadic impressions. Extant palaeognathous down feathers provide a valuable comparative model for these filamentous integumental structures and for similar structures described in Chinese theropods from younger deposits. The described morphologies are congruent with Stage II of Prum ('99) and support that plumulaceous morphologies evolved before the origin of the rhachis and the planar vane. Appearance of feathery appendages in theropods may be linked to evolution of higher metabolic rates, improved locomotory abilities, and/or distinct behavior(s) and visual communication. Development of feathery integument might have also played a crucial role in the competitiveness and successful radiation of maniraptoriform theropods and their actively flying descendants in the Jurassic. J. Exp. Zool. (Mol. Dev. Evol.) 302B:355–364, 2004. © 2004 Wiley-Liss, Inc.