Revision of the Zonal Brachiopod Species Jakutoproductus protoverkhoyanicus Kashirtsev from the Carboniferous–Permian Boundary Deposits of Verkhoyanie, North-Eastern Russia

The brachiopod genus Jakutoproductus of the Upper Kygyltasian Regional Substage (Gzhelian) of Verkhoyanie is revised. Previously, these specimens of Jakutoproductus were identified as Jakutoproductus protoverkhoyanicus Kashirtsev, 1959, whose presence distinguished the same-named biostratigraphic zone. Based on geological reports, the holotype of Jakutoproductus protoverkhoyanicus Kashirtsev, 1959 comes from the Lower Kygyltas Subformation, rather than from the Solonchan Formation, although the latter was indicated as the type locality by the author of the species, and not from the Upper Kygyltas Subformation, as was assumed by subsequent workers. A systematic study of collections of Jakutoproductus from the Upper Kygyltasian Regional Stage showed that all Late Kygyltasian representatives of the genus Jakutoproductus differ morphologically from the holotype of J. protoverkhoyanicus and do not belong to this species. Due to the absence of reliable finds of J. protoverkhoyanicus in the eponymous zone, the use of this biostratigraphic unit is impractical. Since the holotype of J. protoverkhoyanicus was lost, new brachiopod material from the type locality of the species in the upper reaches of the Arkachan River in Western Verkhoyanie and subsequent selection of a neotype are necessary. Two brachiopod assemblages have been established in the uppermost Kygyltasian Regional Substage, separated by powerful flora-bearing sandstones of the Upper Kygyltas Formation. The first (Upper Kygyltasian) assemblage is dominated by small representatives of Jakutoproductus with a strongly elongated shell and a well-developed deep sinus of the ventral valve. These specimens are assigned to a new species J. rimmae sp. nov. In the second (Kygyltasian–Khorokytian) assemblage, brachiopods of the genus Jakutoproductus are distinguished by a significantly larger shell and a less developed sinus of the ventral valve. Their important common feature is the presence in the anterior part of the ventral valve of several rows of straight spines with ring-shaped ridges at the base. Two new species are distinguished among the Kygyltasian–Khorokytian specimens of Jakutoproductus: J. budnikovi sp. nov. and J. allaraensis sp. nov. Instead of the Jakutoproductus protoverkhoyanicus Zone, the Jakutoproductus rimmae Zone with a stratotype on the Setlandya River of the Upper Reaches of the Barajy River of the Western Verkhoyanie is proposed. In the upper part of the Jakutoproductus rimmae Zone, Jakutoproductus budnikovi Beds are recognized.