Incisor enamel microstructures of three euharamyidans, Shenshou lui, Xianshou linglong, and X. songae, from the early Late Jurassic Yanliao Biota, Liaoning Province, China, are reported. The enamel of the three species consists of columnar divergence units that are delimited by planes of crystallite convergence and have irregular shapes and sizes, but there is no distinct line or plane along the divergent axis of crystallites in the unit. Of the three species, the enamel of S. lui is most primitive in having simpler enamel units that are roughly perpendicular to the enamel dentine junction. In Xianshou, the enamel units are oblique apically, and crystallites in the inner zone of enamel show greater differentiation to form erratically spaced clusters that resemble incipient prism-like; seam-like and sheath-like structures are also present. This enamel type may represent a transitional stage between prismless and prismatic enamel. Mapping enamel types from selected taxa of basal mammaliaforms on a simplified phylogeny, the columnar enamel in Thomasia, Shenshou, and some ‘plagiaulacoid’ multituberculates is interpreted as the plesiomorphic condition for allotherians, from which evolved the transitional enamel, as represented by Xianshou and the ‘plagiaulacoid’ Paulchoffatia, and the plesiomorphic prismatic enamel as in some post-‘plagiaulacoid’ multituberculates and gondwanatherians. The prismatic enamel in advanced multituberculates and other gondwanatherians may have evolved independently. Despite the new enamel morphologies revealed in euharamiyidans, the amelogenesis mechanism of how prisms, seams, and sheaths evolved within mammals remains unclear.