Fossil acanthodians had mineralized fin spines on several or all of their fins. Yet extant species of chondrichthyans only have spines on their dorsal fins, if at all. Fin spines serve multiple functions, sometimes stiffening a fin’s leading edge, but typically used as a defensive puncture tool. The diversity of fin spines among species begets the question whether certain morphotypes (recurved or straight) and macrostructures (serrations and keels) perform better for certain functions. To address the defensive providence of these spines, we compared puncture performance among three chondrichthyan species (ratfish Hydrolagus colliei, horn shark Heterodontus francisci, and dogfish Squalus suckleyi) using 3D-printed spine models mounted on a Material Testing System. We measured the force and work incurred during puncture and withdrawal of spines from gelatin mimicking animal flesh. Our results showed that thicker, blunter spines (horn shark) required the most force to puncture, therefore acting as the worst “stabbers” and best crack propagators. Recurved spines (dogfish, ratfish) and spines with serrations (ratfish) required the most force to withdraw, suggesting that these spines cause more damage upon removal or are not meant to be dislodged at all. Ratfish and posterior dogfish spines performed similarly, while the posterior and anterior spines of dogfish behaved differently, demonstrating functional convergence as puncture tools in the former and the alternative role of fin stiffeners in the latter. As many acanthodian spines were recurved and serrated, our findings in modern chondrichthyans suggest that these spines would have been difficult to dislodge and would have caused serious damage to extinct predators.