Origin and evolution of genus Youngquis tognathus Myshkina et Zhuravlev (conodonts, late Devonian)

The conodont biostratigraphy of the Late Devonian shallow-water deposits is insufficiently developed and, as a rule, is a regional in nature. The Late Devonian conodont genus Youngquistognathus is considered as promising for both the regional and global biostratigraphy. This genus, homeomorphic to Ctenopolygnathus, is characterized, and new species Y. praeangustidiscus is described. Phylogeny of the early representatives of the genus is reconstructed. The evolutionary sequence Y. praeangustidiscus - Y. angustidiscus - Y. posterus - Y. rossicus is proved with morphological data and presence of the transient forms. The first appearance (FAD) of Y.angustidiscus apparently marks a level close to the base of the Frasnian, FAD of Y. posterus corresponds to the base of the transitans Zone, and FAD of Y. rossicus corresponds to the middle part of the zone. Origin of the genus from Mehlina corresponds to the late Givetian.