Comparative Osteology of Representative Salmonid Fishes, with Particular Reference to the Grayling (Thymallus arcticus) and its Phylogeny

The purpose of this study is to describe the morphology, particularly the osteology, of the grayling (Thymallus arcticus), in relation to that of the other salmonid fishes, in order to assess the phylogenetic position of the grayling and to synthesize the natural classification of the Salmonidae.Fresh and preserved material of 33 species in 9 genera of Salmonidae were studied. The study materials included dried whole skeletons from fresh fishes, stained bones of preserved specimens, and cross sections of larval stages. Effort was made to eliminate bias due to ontogenetic and sexual differences.The Salmonidae are soft-rayed teleost fishes belonging to the suborder Salmonoidei of the generalized order Clupeiformes (Isospondyli). Prior classifications of the salmonids have arranged them into one, two, or three families. As herein visualized, the family Salmonidae contains those salmonoid fishes that have three upturned caudal vertebrae. They are divided into three subfamilies: Salmoninae, the trouts and salmons, with an orbitosphenoid bone and a suprapreopercular bone, a basibranchial plate, teeth on the maxilla, no dermosphenotic bone, and parietals separate at the midline; Thymallinae, the graylings, with no orbitosphenoid, suprapreopercle or basibranchial plate, a dermosphenotic bone, teeth on the maxilla, and parietals meeting at the midline; and Coregoninae, with orbitosphenoid and dermosephnotic bones, no suprapreopercle, no teeth on the maxilla, and parietals meeting at the midline.The grayling possesses only two invariable morphological differences from other salmonids. These are the absence of an orbitosphenoid bone and the presence of seventeen or more dorsal fin rays. In other characters, there is overlap with one or the other subfamilies.Three osteological characteristics are thought to be more fundamental than the others: the toothless maxilla in the Coregoninae, the lack of an orbitosphenoid in the Thymallinae, and the separation of the parietals by the supraoccipital in the Salmoninae. Each character is distinctive for only one subfamily, it being common to the other two. The complete bony and cartilaginous skeletal system of the grayling is illustrated, as are many osteological features of the trouts and whitefishes.Within the subfamily Salmoninae, 5 genera (Brachymystax, Hucho, Salvelinus, Salmo, and Oncorhynchus) are recognized and defined. The Thymallinae consists of a single genus Thymallus with 4 species. Three genera of Coregoninae are recognized: Prosopium, Coregonus, and Stenodus. The discovery that Prosopium has a basibranchial plate, taken together with previously known characters, justifies recognition of generic rank for this group. The group commonly ranked as a genus Leucichthys does not display enough difference to warrant such segregation and is synonymized with the genus Coregonus. Stenodus is found to have distinctive characters not heretofore noted and may rank as a genus separable from but related to Coregonus.