Semionotiformes is an order of ray-finned fish known from the Middle Triassic (Anisian) to the Late Cretaceous (Maastrichtian). Their closest living relatives are gars (Lepisosteidae), with both groups belonging to the clade Ginglymodi within the Holostei. The group includes both freshwater (Semionotidae) and marine (Callipurbeckiidae, Macrosemiidae) adapted forms. Many members of the family Macrosemiidae (which are usually included in Semionotiformes but sometimes placed in their order), had elongated dorsal fins, often associated with an adjacent area of skin which was free of scales. These fins were likely undulated for use in precision swimming. The body morphology of macrosemiids suggests that they were slow swimmers who were capable of maneuvering around complex topography, such as reef environments.