Geologicheskii Vestnik

This paper briefly reviews the history of the study of the Sterlitamak shikhans and the Early Permian ammonoids discovered there in the time span from the expedition of Murchison, Verneuil and Keyserling in 1841–1842 to the present day. The first description of a small collection of Asselian-Sakmarian ammonoids from the Shikhans was published by Gerasimov in 1937, and these materials were then re-examined by Ruzhencev in 1938 and 1951. Late Artinskian ammonoids from the shikhans were mentioned among others in Ruzhencev’s 1956 monograph. During the 2015–2022 fieldwork of the laboratory of mollusks of the Borissiak Paleontological Institute of the Russian Academy of Sciences led by A.V. Mazaev, representative ammonoid material comprising more than 400 specimens was collected from both stratigraphic levels (Asselian-Sakmarian and late Artinskian). This material was examined and described by colleagues from the laboratory of mollusks, showing the taxonomic, morphological, paleoecological and biogeographic features of each of these two ammonoid assemblages. The older reef assemblage is by far the richest taxonomically among all coeval assemblages in the world. It is very diverse morphologically, containing various morphotypes: involute, semi-involute and evolute, from platiconic and pachyconic to cadiconic and ophioconic; sutures vary from simple “goniatitic” to the most complex “ammonitic” (Properrinites sp.). Shell sizes also vary widely. The ecological structure of the reef assemblage differs from the open sea communities in a higher proportion of nektobenthic forms. The assemblage contains the ultra-endemic species Shikhanites singularis and Protopanoceras sublahuseni and several endemic species suggesting its isolation from the adjacent basins. The presence of the Tethyan genus Properrinites suggests connections with the Tethyan basins. The Late Artinskian Shikhan ammonoid assemblage is similar in taxonomic, morphological, and functional characteristics to other South Ural assemblages. The complete absence of the previously widespread genus Agathiceras combined with the dominance of the family Paragastrioceratidae, which became most abundant in the Ural Early Permian basins, indicates the closure of seaways with the Paleotethyan Ocean.

Bashkortostan, Sterlitamakian Shikhans, Ammonoidea, taxonomic composition, morphology, life forms, biogeography, Lower Permian, Asselian, Sakmarian, Artinskian, Upper Artinskian

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