Abstract(#br)Micropaleontological studies of calcareous dinoflagellates, calpionellids and planktonic foraminifera have provided new knowledge on the stratigraphic age and detailed characteristics of the Lar Formation and the K u b rock unit in the Marghsoo section in the central Alborz Zone of northern Iran, within the Central Neo-Tethys region. The Colomisphaera fibrata , Cadosina parvula , Stomiosphaera moluccana , Cadosina semiradiata semiradiata , Colomisphaera tenuis, and Colomisphaera fortis calcareous dinoflagellate zones, ranging upper Oxfordian to upper Tithonian, have been identified in the Lar Formation, and calpionellid assemblages in the uppermost Lar Formation can be identified to the Tintinnopsella remanei Subzone of late Tithonian age. Poorly preserved planktonic... foraminifera occur at a level assigned to upper Kimmeridgian. These observations document the opening of a major seaway connecting Iran to the Central Atlantic during late Kimmeridgian seafloor widening, to provide the pathway for planktonic foraminifera to migrate from the Central Atlantic to the Central Neo-Tethys of Iran in the late Kimmeridgian.(#br)Twelve species of Late Cretaceous planktonic foraminifera in the K u b rock unit indicates Dicarinella concavata Zone (uppermost Turonian to upper Coniacian), with close similarity to material described from the Central Atlantic, southern Europe and the Middle East.