The paper examines four areas of marked inflectional morphology (affinity between the past and the progressive, aspiration inversion, tonal inversion, ablaut) in the formation of verb stems in the Puxi dialect of Shangzhai, an underexplored Tibeto-Burman language of northwestern Sichuan. Partial parallels in other languages of the rGyalrong Tibetans suggest retention of relic alternation patterns confined to these particular Tibeto-Burman languages. The morphological archaisms uncovered in this study not only vindicate the intimate ties between Horpa and Shangzhai noted earlier by this author (J. T.-S. Sun 2000), but constitute crucial individual-identifying evidence pointing to a unique low-level parent language Proto-rGyalrongic shared by rGyalrong proper, Lavrung, and Horpa-Shangzhai, paving the way for rigorous comparative rGyalrongic linguistics.