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IntroductionDuring the Silurian and Devonian periods, parts of Iran (central Iran, Alborz, and Sanandaj-Sirjan) along with the Afghan and Turkish plates were attached to the Arabian and African plates and formed the northwestern margin of the Gondwana supercontinent and the southern margin of Paleo-Tethys (Berberian and King, 1981;Al-Husseini, 1991;Ruban et al., 2007;Al-Juboury and Al-Hadidy, 2009). The current structure and geological status of Iranian platuea has been shown in Figures 1a and 1b.The Devonian sedimentary horizons of Alborz-Azarbaijan zone show a variety of rock facies. These facies always display unique shapes, which reveal an independent basin with different sedimentary conditions. Also, the Devonian lithologic units are Juban Formation in Saudi Arabia, Kuwait (partly), and parts of Ora Formation in Iraq, Bahram Formation in central Iran, and parts of the Muli and Zakeen Formations in NW and

The Khoshyeilagh Formation located in the northeast of Iran yielded seven conodont species and sub-species and 19 macro- and microfossil taxa that allow recognition of two conodont biozones and one biozone based on calcareous microfossils. The latest Frasnian age (the Upper rhenana to linguiformis zones) is attributed to the topmost strata with Icriodus alternatus. Its replacement with I. cornutus indicates the Famennian (Lower triangularis to Lower crepida zones) corresponding to the Umbellina Zone. The fossil assemblages identified in the Khoshyeilagh Formation represent a shallow marine environment with a tropical climate at the time of deposition. The fossil species from the Khoshyeilagh Formation and the sedimentary basins in Algeria, Libya, Morocco, Poland, and Russia reveal a close connection between the Iranian plateau and the northern parts of Gondwana in the Late Devonian. The biofacies and lithofacies analyses show a sea-level decline at the end of Frasnian, followed by a considerable sea-level drop, as in other regions of the world. After a short time, in the Famennian, the deepening occurred in some parts of the area and the open marine facies (bioclast spicule wackestone-packstone) were deposited. This study is the first attempt to determine the Frasnian-Famennian boundary based on conodont assemblages and other fossil species such as umbellulids, tentaculites, and ostracods. The distribution of these species is interpreted in sedimentological, stratigraphic, sequence stratigraphy, and the global eustatic context. Keywords: biofacies, conodont, Frasnian-Famennian boundary, Late Devonian, NE Iran.