Here I assess the use of osteoderms in systematics with comparative material from fossil and extant tetrapod taxa. Putative differences among three groups (ankylosaurid, nodosaurid, and polacanthid) were evaluated. Archosaur osteoderms have cortices surrounding a cancellous core. Ankylosaurs are united by a superficial cortex distinguishable from the core, lack of Sharpey’s fibers, and mineralized structural fibers. Nododsaurids lack a deep cortex and have dense superficial cortical fibres. Ankylosaurid osteoderms are thinner than those of other ankylosaurs. Polacanthids (and some nodosaurids and ankylosaurids) have a cancellous core. Cortical thickness overlaps among groups, so a thick cortex is not diagnostic for polacanthids. Modified elements diverge histologically from the primitive condition for specific functions. Haversian bone in the core is not indicative of any group. Some shapes and superficial textures are diagnostic for specific taxa. Parsimony analyses show support for the Ankylosauridae and Nodosauridae, but not a monophyletic polacanthid clade.