Orbitolinid biostratigraphy of the top Taft Formation (Lower Cretaceous of the Yazd Block, Central Iran)

The top parts of the Taft Formation of Central Iran in its type area southwest of Yazd (Central Iran) contain an orbitolinid assemblage consisting of Dictyoconus? pachymarginalis Schroeder, 1965, Mesorbitolina birmanica (Sahni, 1937), and Mesorbitolina ex gr. texana (Roemer, 1849), assigned to the late Gargasian (mid-late Aptian). M. birmanica belongs to a group of mesorbitolinids (e.g., M. pervia, M. ovalis) exhibiting a rather large and complex embryo with a plano-convex protoconch. It has been largely overlooked in the literature and its integration into the early Aptian (Bedoulian) to early Cenomanian Mesorbitolina lineage (M. lotzei–M. aperta) is discussed. The occurrence of mesorbitolinids with a comparatively large and complex embryonic apparatus exhibiting a plano-convex protoconch already in the Gargasian is in conflict to the current phylogenetic concept of the genus Mesorbitolina. Further studies and emendation of this concept are needed. The new biostratigraphic data from the top of the Taft Formation indicate a stratigraphic hiatus of ca. 5 myr at the contact to the overlying, ammonite-dated Darreh-Zanjir Formation, comprising the latest Aptian to earliest Albian.