Calcareous foraminifers from the Bashkirian stratotype (Middle Carboniferous, south Urals) and their significance for intercontinental correlations and the evolution of the Fusulinidae

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 foraminiferGlobivalvulina bulloides(Brady) (=G. moderataReitlinger) and the conodontIdiognathodus 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 fusulinidPseudostaffella(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, becausePs.(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 theEostaffella postmosquensisplexus in the late Serpukhovian.Plectostaffella jakhensis, in turn, gave rise toPs.(Semistaffella)variabilisin the early Bashkirian (late Siuransky), from which evolvedPs.(Ps.)antiquashortly thereafter (earliest Akavassky). An as yet unidentified but advanced species ofPs.(Pseudostaffella) is the most likely ancestor to late BashkirianNeostaffella ivanovi. The evolutionary series leading from theE. postmosquensisplexus to primitiveNeostaffellaapparently developed exclusively in the Eurasian–Arctic faunal realm, asPl. jakhensis, Ps.(Semistaffella)variabilis, andPs.(Ps.)antiquaare unknown in the Midcontinent–Andean region. DiversePs.(Pseudostaffella) spp. appeared in the latter area pursuant to an adaptive radiation aided by periodic interchange between faunal realms.