Parallelisms in the morphology and evolutionary transformations of two bones of the visceral skeleton in fossil and recent freshwater sculpins of the genus Triglopsis (Pisces: Cottidae)

The article considers the morphogenesis of two visceral skeleton bones, which are most important for identification, in five freshwater forms of the genus Triglopsis Girard, 1851. These forms evolved as a result of colonization of fresh waters by marine fishes in different geological periods and continents. On American continent, two fossil species, Triglopsis idahoensis (Smith, 1975) and T. antiquus (Smith, 1975), from the Pliocene Lake Idaho and one recent species T. thompsonii Girard, 1851 from Lake Michigan were studied. On European continent, two freshwater populations of one species T. quadricornis (Linnaeus, 1758) from Lakes Ladoga and Onega were investigated. Comparative analysis of the external features of the preopercular and dentary bones showed that all five studied forms have common characters inherent in the genus Triglopsis. The differences are in the size, shape and orientation of the spines; and hollows of the preopercular-mandibular sensory canal. The fossil species T. idahoensis and T. antiquus have wide, slightly curved preopercle with spines of similar length. Significant differences are found in the morphology of the sensory canal. The most primitive state of this canal is observed in T. antiquus. It is characterized by small size of hollows and wide bone bridges between them. Evolutionary transformations in morphology of the canal in fossil and recent species go in the direction of increasing passage of the canal in the bones. In recent forms, passage of the sensory canal occupies almost the entire width of the bone, which is significantly greater than in fossil species. Geographically remote and isolated fossil and recent forms of the genus Triglopsis, each with its own evolutionary fate, have a single trend of evolutionary transformations of the studied bones. Despite the fact that the lake forms were at different stages of differentiation, evolutionary transformations of the morphology of the studied bones proceed in parallel and in the same direction.