First discovery of nautilids from the Albian\u2013Cenomanian succession of the Koppeh Dagh Basin, NE Iran

Javad Sharifi,Amane Tajika,Alireza Mohammadabadi,Mohammad Hossein Tabari Abkuh

doi:10.1186/s13358-021-00230-2

Abstract

The Aitamir Formation, situated in the Koppeh Dagh Basin in the northeast of Iran, is known for its well-exposed Albian-to-Cenomanian succession. Although geologists previously documented a number of macro- and microfossils, no nautilids had been discovered until now to our knowledge. Here, we present lower Albian and middle Cenomanian nautilids from the Koppeh Dagh Basin for the first time. This discovery is also the first record of Cretaceous nautilids from Iran. We identified the specimens as Eutrephoceras clementianum (d’Orbigny 1840), E. sublaevigatum (d’Orbigny 1850), E. bouchardianum (d’Orbigny 1840) and Eutrephoceras sp. These specimens occur in horizons situated between several ammonite-bearing levels, which allowed us to more precisely constrain age estimates for the recovered nautilid specimens. E. clementianum could not be dated precisely but likely comes from between late Aptian ammonite index Hypacanthoplites uhligi and middle Albian Hoplites (Hoplites) baylei. E. sublaevigatum occurs just above the late Albian ammonites Mariella bergeri and Semenoviceras michalskii and below the Mantelliceras mantelli Zone. At the upper part of the section, E. bouchardianum and Eutrephoceras sp. were collected from lower Albian beds, which correspond to the Mantelliceras mantelli and Mantelliceras dixonii zones. These new findings contribute to our knowledge of the geographical distribution and stratigraphic range of Albian–Cenomanian nautilid species.

Highlights

The Nautiloidea is a group of ectocochleate cephalopods that has a long geological record, originating in the late Cambrian (Kröger et al, 2011)
The Koppeh Dagh Basin, in which the lower Albian to middle Cenomanian sedimentary rocks are exposed as Aitamir Formation, is known for its rich assemblage of diverse macro- and microfossils, but no nautilid fossils have been documented far
All studied specimens belong to the genus Eutrephoceras, which is the first record of this genus in the Koppeh Dagh Basin, as well as in Iran

Summary

Introduction

The Nautiloidea (de Blainville, 1825) is a group of ectocochleate cephalopods that has a long geological record, originating in the late Cambrian (Kröger et al, 2011). The genus Eutrephoceras (Hyatt, 1894) is one of the most common nautilid taxa in the Cretaceous and an interesting group to study because they survived the K/Pg mass extinction event (e.g., Landman et al, 2014). Most of the mid-Cretaceous occurrences of this taxon were recorded from European Albian and Cenomanian successions (AyoubHannaa et al, 2018; Jattiot et al, 2021; Kennedy et al, 2008; Machalski & Wilmsen, 2015; Tajika et al, 2017; Wilmsen, 2000, 2016). The Koppeh Dagh Basin (northeastern Iran), in which the lower Albian to middle Cenomanian sedimentary rocks are exposed as Aitamir Formation, is known for its rich assemblage of diverse macro- and microfossils, but no nautilid fossils have been documented far. Most of the important taxonomic and biostratigraphic studies of this formation have

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