Published online by Cambridge University Press: 02 September 2016
The stratotype for the Bashkirian Stage of the Soviet Middle Carboniferous is located on the Askyn River in Gornaya Bashkiria (western slope of south Urals). Twenty-four rock samples, mostly from the lower part of the section, yielded abundant and diverse assemblages of calcareous foraminifers which are systematically described and illustrated here for the first time.
The foraminiferal assemblage from the Siuransky Horizon at the base of the Bashkirian is essentially identical to that from the top of the underlying Lower Carboniferous Serpukhovian Stage. Thus, foraminifers do not provide a basis for identifying the Soviet Lower–Middle Carboniferous boundary. This notwithstanding, the presence of the foraminifer Globivalvulina bulloides (Brady) (=G. moderata Reitlinger) and the conodont Idiognathodus parvus (Dunn) in both the upper Serpukhovian and Bashkirian indicates that the base of the Bashkirian can be no older than medial to late Morrowan of the North American succession. The primitive fusulinid Pseudostaffella (Pseudostaffella) appears at the bases of the lower Bashkirian Akavassky Horizon and the North American Atokan Series. The base of the Akavassky is interpreted to be somewhat older than early Atokan, however, because Ps. (Pseudostaffella) appeared in the Urals in phylogenetic continuity with its immediate ancestor, whereas in most of North America it was an immigrant.
The type Bashkirian succession contains a seemingly complete phylogeny from advanced eostaffellids to primitive fusulinids. Plectostaffella jakhensis, immediate ancestor to the fusulinids, arose from a member of the Eostaffella postmosquensis plexus in the late Serpukhovian. Plectostaffella jakhensis, in turn, gave rise to Ps. (Semistaffella) variabilis in the early Bashkirian (late Siuransky), from which evolved Ps. (Ps.) antiqua shortly thereafter (earliest Akavassky). An as yet unidentified but advanced species of Ps. (Pseudostaffella) is the most likely ancestor to late Bashkirian Neostaffella ivanovi. The evolutionary series leading from the E. postmosquensis plexus to primitive Neostaffella apparently developed exclusively in the Eurasian–Arctic faunal realm, as Pl. jakhensis, Ps. (Semistaffella) variabilis, and Ps. (Ps.) antiqua are unknown in the Midcontinent–Andean region. Diverse Ps. (Pseudostaffella) spp. appeared in the latter area pursuant to an adaptive radiation aided by periodic interchange between faunal realms.
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