In Western Kopeh-Dagh, the lower Cretaceous carbonate series contains distinctive assemblages of calcareous algae, chiefly Dasycladales which are primarily studied in this work. Based on the significant presence of certain species, the Zard Formation, assigned to the Valanginian and, in its upper part, to the Hauterivian or base of the Barremian, is subdivided into two intervals, respectively A and B. Higher up, the Urgonian-type Tirgan Formation, assigned to the Barremian and Lower Aptian, is subdivided into intervals C and D. On this occasion, the taxonomy and systematics of 17 species of Dasycladales and one incertae sedis found in the series are discussed, in relation to some recent standpoints. Out of this inventory, the biogeographic distribution of 11 species is particularly interesting. During the Lower Cretaceous, the Kopeh-Dagh region was quite isolated, northeast of the Izanca, a branch of the Tethys. Assemblages encountered in this region are compared with other areas, encompassing Iberia to the northern margin of West Gondwana. For two species, under any reserve, distribution seems to be related to the genus, therefore with the systematics. On the other hand, the algal habitat, which depending of the species, may be open marine or rather sheltered, appears to be only partially related to biogeography. Finally, the stratigraphic and/or biogeographic distribution of two species appears to be partially related, in one case to the Valanginian OAE1 event, in another case to one or more obstacles in relation to the paleotectonic context.

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