Sawfish and sawsharks are aptly named for their long, serrated snouts. These jags may look like teeth, but they’re actually modified versions of denticles—the hardened scales that cover the skin of sharks and rays. Recently, scientists have wondered whether the structures could provide clues to the origin of true teeth, which also may have evolved from skin denticles. In a new study, researchers used x-ray computed tomography (CT) imaging to produce 3D depictions of real and saw teeth belonging to embryos and adults of prehistoric and present-day saw-bearing fishes (such as the one pictured). They found clear differences in how saw teeth and oral teeth grow. Similar to the skin denticles from which they are derived, saw teeth develop by filling in open spaces created as the snout grows. Alternatively, oral teeth develop in a highly ordered sequence mediated by genes. Replacement teeth also form differently; growth of new saw teeth begins only after one is lost, whereas rows of replacement oral teeth are preformed in the jaw before tooth loss. The differences add to the body of evidence suggesting dermal denticles are unrelated to present-day molars, the team reports online today in Royal Society Open Science.