Berriasian ammonites of supposed Tethyan origin from the type 'Ryazanian', Russia: a systematic re-interpretation.
Résumé
For more than a century, a number of ammonite taxa of supposed Tethyan origin or affinity have been reported from the Berriasian condensed deposits of Russia (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. To accommodate the systematic issues, we introduce a new ammonite genus: Mittaites n. gen. (type species: Mazenoticeras ceccai). We also provide re-assessment for the genera Tauricoceras (= Subriasanites), Riasanella, Riasanites, Prorjasanites, and Karasyazites. Considering the strong affinities between these genera (except for Karasyazites), resctricted palaeobiogeographic distribution and a problematic phyletic origin, we erect a new family Riasanitidae n. fam. Our re-examination suggests that the occurrence of western Tethyan migrants in the type ‘Ryazanian’ should be ruled out. Pending new investigation, correlation of the ammonites of the type ‘Ryazanian’ beds with the Berriasian part of the Standard Mediterranean Ammonite Scale (SMAS) should be treated with caution.
Domaines
Paléontologie
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