Stratigraphic position of the ammonoid Paramexicoceras Popow in the Permian-Triassic deposits of the Southern Verkhoyanie, Northeast Russia

Widespread Upper Permian pre-Otoceras deposits of northeastern Russia contain rich assemblages of marine invertebrates (brachiopods, bivalves, and foraminifera), but are not characterized by ammonoids. Only single finding of the Late Permian ammonoids is known in the Southern Verkhoyanie. In the fifties of the last century, a cyclolobid shell was found here in alluvial or deluvial deposits, later described as the holotype of the type species Paramexicoceras aldanense Popow, 1970. There was no stratigraphic position for this goniatite, and for more than half a century of searching, the researchers failed to find it again. A variety of assumptions have been made about the stratigraphic position of South Verkhoyanian Paramexicoceras, and their synchronism or asynchrony with the Paramexicoceras found in the Changhsingian deposits of the East Greenland has also been discussed. Most researchers agreed that South Verkhoyanian Paramexicoceras comes from the Imtachan Formation, but the new data discussed in this article refute these assumptions. Numerous shells of Paramexicoceras aldanense were found in the lower part of the Nekuchan Formation in the Dyby River basin. This finding belongs to the Changhsingian Stage, which approximately corresponds in geological age to the distribution level of the Greenlandic Paramexicoceras. All Otoceras in the Allakh-Yun structuralfacies zone are found above the Paramexicoceras level, therefore, the deposits with Changhsingian cyclolobids are separated into Paramexicoceras aldanense Beds, which underlie the Otoceras concavum Zone. Despite the Changhsingian age, the invasion of Paramexicoceras is historically associated with the initial phase of the Early Triassic sedimentation cycle in the Verkhoyanian basin. Paramexicoceras (goniatite) and early Otoceras (ceratite) probably belonged to a single warm water ammonoid community that came to South Verkhoyanian basin at the beginning of the Nekuchanian time.

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