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Berriasian ammonites from the type "Ryazanian" (Central Russia) of supposed Tethysian origin: A systematic re-interpretation
Camille Frau, William A.P. Wimbledon, Christina Ifrim, Luc G. Bulot, Alexandre Pohl

Building: Muséum d'Histoire Naturelle de Genève
Room: Lobby /Hall/
Date: 2018-12-05 11:05 AM – 11:08 AM
Last modified: 2018-12-02

Abstract


In the past fifteen years, a number of ammonite taxa of supposed Tethysian origin or affinity were reported from the Berriasian condensed deposits of Central Russia (regionally referred to as "Ryazanian''). These occurrences have been used to constrain long-distance correlation and palaeobiogeographic interpretation of the Russian Platform during the earliest Cretaceous. We revise these taxa herein. We provide re-assessment for the genera Riasanella, Riasanites (= Subriasanites), Prorjasanites and Karasyazites. To accommodate the systematic issues, we introduce a new ammonite genus: Mittaites gen. nov. (type species Mazenoticeras ceccai). Considering the strong affinities between these genera (except with Karasyazites), their restricted palaeobiogeographic distribution and a potential phyletic origin from the GechicerasTauricoceras lineage, we propose the introduction of the new family Riasanitidae fam. nov. Our re-examination suggests that the occurrence of Tethysian migrants in Central Russia should be ruled out. Pending new investigation, correlation of the Ryazanian-type beds with the Standard Mediterranean Ammonite Scale (SMAS) for the Berriasian Stage should be regarded with caution.


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