﻿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)

﻿Abstract 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.


Introduction
Three species of xenacanthiform sharks described from Carboniferous strata have been assigned to the same nominal genus and combined with the name Orthacanthus gracilis (Chondrichthyes, Xenacanthiformes, Orthacanthidae), either originally or subsequently.The basionym of the senior synonym, in its original combination, Chilodus gracilis Giebel, 1848, is homonymous with the name of an extant species of characiform fish, Chilodus gracilis Isbrücker & Nijssen, 1988 (Actinopterygii, Characiformes, Chilodontidae).Chilodus is a genus-group name that was proposed for two different nominal genera.One is a genus of characiform fish (Müller and Troschel 1844: 85-86) and the other is a genus of extinct xenacanthiform shark (Giebel 1848: 352).
The purpose of this paper is to clarify, detangle, and stabilize the nomenclature of these genus-group and species-group names.
Etymology.The species refers to Linton, Ohio, the type locality.
Detailed study of xenacanthiform materials from the Linton Lagerstätte is needed, and the type specimens need to be re-examined.Much of the systematic work on fish taxa described from Linton after 1900 has involved non-type specimens.Indeed, most published illustrations of Linton fish types are line-art drawings (e.g.Newberry 1873Newberry , 1874Newberry , 1875aNewberry , 1875b; herein, Fig. 1B, C), often with generous "restoration;" few of the types, even the ones whose repositories are known, have been photographically illustrated.Pending restudy of the type specimens of xenacanthiform sharks from the Linton Lagerstätte, O. lintonensis nom.nov. is proposed here as an available name that can compete in priority with other names, not as a junior synonym of any other species (compare Hotton 1952;Hook and Baird 1986;Johnson 1999).Etymology.Adamas (Latin, diamond), in allusion to the Diamond Coal Mine, where the species was first collected.

Orthacanthus adamas
Remarks.The new species-group name Orthacanthus adamas nom.nov.replaces Orthacanthus gracilis Newberry, 1875a, which is a junior homonym of Orthacanthus gracilis (Giebel, 1848).Newberry (1875a: pl. LIX, fig. 7;1875b: pl. LIX, fig. 7) illustrated this species with a composite figure based on syntypic dorsal spines.This species should not be confused with the other xenacanthiform species from Linton bearing the species epithet gracilis, based on teeth, and also referred to Orthacanthus, as discussed above.Replacement names for both taxa will reduce potential confusion.Cope (1881) and Case (1900) extended the stratigraphic range of this species into the Permian.
According to Articles 23.3.5, 52, 57, and 60.3 of the International Code of Zoological Nomenclature (International Commission on Zoological Nomenclature 2000), this species has priority over two species named by Newberry (1857, 1875a) (see below) that have the name Orthacanthus gracilis originally or after recombination.2. Diplodus gracilis Newberry, 1857 (Fig.