Parahoplitid ammonites and narrowing down the Aptian/Albian boundary interval in northern Iran

Abstract

Aptian and Albian parahoplitid ammonites from the Sanganeh- and Aitamir formations of the Koppeh Dagh Basin in northern Iran are described and illustrated, some taxa are recorded from Iran for the first time. The earlier established view of a close palaeobiogeographic affinities of the Koppeh Dagh Basin with the European faunal realm during the late Aptian–early Albian is consolidated by our data. The idea of an assumed faunal exchange particularly with Europe across the Russian Platform and Transcaspia is boosted by the new records of Parahoplites irregularis, Colombiceras (Colombiceras) tobleri, Protacanthoplites abichi and Diadochoceras sp. from Iran. The stratigraphic succession of the ammonite faunas is used for a biostratigraphic appraisal of the Sanganeh- and Aitamir formations and conform earlier ideas that both formations are diachronous. The Tutli section is particularly relevant in this respect since a high level in the Aitamir Formation is dated here as middle upper Aptian whereas the other sections investigated indicate a lower stratigraphic level, but are attributed to the underlying Sanganeh Formation. Most ammonites described indicate fairly long ranges in the upper Aptian according to the literature, except for those of the P. melchioris group characterising the middle part of the upper Aptian. We record the index of the eponymous P. melchioris Zone as well as the closely allied Parahoplites cf. transitans and Parahoplites cf. multicostatus. Only one species recorded, Hypacanthoplites elegans, probably indicates a level above the Aptian/Albian boundary.