A bizarre “spine-brush complex” is found only in extinct symmoriiform chondrichthyans (Order Symmoriiformes). Its structural complexity makes the spine-brush complex a rich source of morphological characters for phylogenetic analysis, and it has been used as a basis for their family-level subdivision. Development of the complex is sex-linked in one of the three symmoriiform families customarily recognized (Falcatidae), but its distribution within members of the other two families (Symmoriidae, Stethacanthidae) is poorly documented. Consequently, earlier family-level diagnoses (which were largely based on the presence or absence of the spine-brush complex) are probably misleading. Some features of the spine-brush complex can be homologized with structures found in “normal” shark dorsal fins, suggesting that the spine-brush complex is a highly specialized first dorsal fin. The spine-brush complex may have arisen by co-option of the first dorsal fin module, possibly involving its suppression in females and postdisplacement (delayed development) in males, plus acquisition of several apomorphic traits, unlike in chimaeroids and actinistians where co-option of the first dorsal fin module is not sex-linked or developmentally postdisplaced. The evolutionary history of dorsal fin modularity is examined. In gnathostomes, co-option is typically confined to the first module, whereas the second is more highly conserved, retaining its functional role in aquatic locomotion.