Replacement names for two species of Orthacanthus Agassiz, 1843 (Chondrichthyes, Xenacanthiformes), and discussion of Giebelodus Whitley, 1940, replacement name for Chilodus Giebel, 1848 (Chondrichthyes, Xenacanthiformes), preoccupied by Chilodus Müller & Troschel, 1844 (Actinopterygii, Characiformes)

Three species assigned to the same nominal genus of Paleozoic xenacanthiform shark have been combined with the name Orthacanthusgracilis (Chondrichthyes, Xenacanthiformes, Orthacanthidae). Orthacanthusgracilis (Giebel, 1848), which was originally combined as Chilodusgracilis Giebel, 1848, is the senior synonym; it has priority over both Orthacanthusgracilis (Newberry, 1857), which was originally combined as Diplodusgracilis Newberry, 1857, and Orthacanthusgracilis Newberry, 1875a. Proposed species-group replacement names are Orthacanthuslintonensisnom. nov. for O.gracilis (Newberry, 1857) and Orthacanthusadamasnom. nov. for O.gracilis Newberry, 1875a. Chilodusgracilis Giebel, 1848 is designated as the type species of Chilodus Giebel, 1848; this species becomes the type species for Giebelodus Whitley, 1940, which is a replacement name for Chilodus Giebel, 1848 (preoccupied by Chilodus Müller & Troschel, 1844, Actinopterygii). Giebelodus Whitley, 1940 is a junior subjective synonym of Orthacanthus Agassiz, 1843.