First report of Middle Ordovician gastropod from Alborz Mountains, north of Iran

The euomphaloid gastropod Lesueurilla prima (Barrande in Perner) is reported for the first time from the Middle Ordovician (Darriwilian) Lashkarak Formation of eastern Alborz, northern Iran. This near planispiral hyperstrophic gastropod was recovered from the reddish-brown argillite and oolitic ironstone beds which belong to the Hajiabad Member in the Deh-Molla section, west of the Shahrud city. This is also the first record of the Lesueurilla prima co-occurrence with the Saucrorthis brachiopod fauna and associated trilobites which is indicative of the Darriwilian age. This species, which is considered as a component of the Mediterranean cold-water fauna, was previously reported from the Floian and Darriwilian of Central Iran, the Prague Basin in the Czech Republic, Montagne Noire in France, and Central Morocco. The presence of Lesueurilla prima in Alborz, extends the biogeographical distribution of this species along the northern peri-Gondwana margin. The observed pattern of the geographical distribution of this gastropod species was controlled by a South Subpolar Current running along the western coast of Gondwana and possibly, by a planktotrophic nature of the gastropod larva.