The First Record of the Goniatite Genus Paramexicoceras in the Changhsingian Deposits of the Upper Permian in Northeastern Asia

Ammonoids Paramexicoceras aldanense Popow were found at the base of the Nekuchan Formation (upper part of the Changhsingian) in the Dyby River basin, Verkhoyan region. The first discovery of goniatites of the Upper Permian in northeastern Asia made it possible to clarify the biostratigraphic subdivision of the Permian–Triassic boundary deposits. The Otoceras concavum zone was reduced in its lower part due to identification of the Paramexicoceras aldanense beds at the base of the Nekuchanian Regional Stage. The data obtained were indicative of the fact that the genus Paramexicoceras previously considered as long-lived belonged to the youngest and short-lived taxa of Cyclolobidae representing the terminal goniatite phase. This record made it possible to compare the Nekuchanian base in the Southern Verkhoyan region with the upper part of the Foldvik Creek Group in East Greenland.