Trilobites and the Lower-Middle Devonian boundary in southern West Siberia

Abstract On the basis of a study of Lower Devonian and Eifelian trilobites (Fam. Dechenellidae) of northeastern Salair and Mountain Altai, the author discusses the boundary between the Lower and Middle Devonian. As a result of the revision of Dechenellidae from the Gurievsk and Chumysh sections of Salair, in the north and central portions of the Mountain Altai, three new genera (Lacunoporaspis Yolkin, 1966; Khalfinella gen.n.; Ganinella gen.n.) and a number of new species are established. Representatives of the genera Khalfinella and Ganinella are widespread in Lower Devonian and Eifelian deposits of the regions under consideration. This has enabled the author to trace evolutionary trends in their development, and to outline the main stages of morphologic changes. Specific distinctions are based upon progressive changes in degree of separation of glabella and pygidium. Species of the genus Ganinella underwent gradual change throughout the period of time under consideration. The changes were expressed in the increase or decrease of glabella and pygidial separation. Species of the genus Khalfinella, in addition to a gradual change during the Lower Devonian Epoch and Eifelian Series, suffered more profound changes between Early and Middle Devonian time. Morphologic changes of Khalfinella at this time—stratigraphic level are connected with the appearance of distinct occipital lobes and the separation of pleural ribs on the pygidium. At the same time both Khalfinella and Ganinella preserve some ancient features at the beginning of the Early Devonian. The altered forms (Ganinella dombrowiensis dombrowiensis (Gurich) and Khalfinella elegantula gen. et sp.n.) are characteristic of the Salairkinsk beds of northeastern Salair and the Kireyevsk beds of Mountain Altai (Lower Eifelian). Stratigraphically higher they undergo further change. The interval of their vertical occurrence in the above section may be distinguished as the “Zone of Ganinella dombrowiensis dombrowiensis and Khalfinella elegantula”, which lies at the base of the Middle Devonian. The distribution of lithofacies and faunifacies is such that the author has not been able to distinguish this zone beyond the Salairkinsk and Kireyevsk beds, though on faunal grounds it possesses some of the features of “transition beds”. It might be more correctly termed “basal beds” of the Middle Devonian. Taking into consideration the entire fossil complex, the lower boundary of the Zone of Ganinella dombrowiensis dombrowiensis and Khalfinella elegantula (or the boundary between Lower and Middle Devonian in the southern part of west Siberia) may be correlated with the upper boundary of Upper Koneprus Limestone of Czechoslovakia and with the lower boundary of the zone of Paraspirifer cultrijugatus of Western Europe.