Previous descriptions of the postcranial skeleton of the primitive stegosaur Huayangosaurus taibaii (Middle Jurassic: People's Republic of China) are insufficient for character determinations in cladistic analysis. Reexamination of the postcranium has revealed several important characters, most of which are retained plesiomorphies that have not been identified previously. For example, Huayangosaurus retains ossified tendons, the loss of which has been used as a synapomorphy of Stegosauria. These purported stegosaurian synapomorphies now serve to unite the monophyletic clade Stegosauridae, defined as all stegosaurs more closely related to Stegosaurus than to Huayangosaurus. In addition, some anatomical characters previously used as ‘ankylosauromorph’ synapomorphies have now been identified in Huayangosaurus: this suggests that these characters occur more basally within Thyreophora. The plesiomorphic status of Huayangosaurus suggests that substantial convergence between ankylosaurs and stegosaurs occurred during their respective evolutionary histories, reflecting coincident functional morphological changes associated with the adoption of obligate quadrupedality.