Latest Ordovician–early Silurian chitinozoans from the eastern Alborz Mountain Range, Kopet–Dagh region, northeastern Iran: biostratigraphy and palaeobiogeography

Abstract

Chitinozoans were extracted from latest Ordovician to early Silurian strata in the Alborz Mountain Range, northeastern Iran; 24 species were identified and six local biozones established. Five new chitinozoan species are described: Ancyrochitina bojnourdensis, Ancyrochitina fatemae, Ancyrochitina longifilosa, Angochitina iranica, and Angochitina multipodspina. Correlation with existing formal biozonations suggests that the uppermost part of the Ghelli Formation is of mid-Ashgill age and that Member I of the Niur Formation ranges from early Rhuddanian to late Telychian. The recovered chitinozoan fauna is dominated by species which are endemic to the study area. A significant proportion of taxa is shared with typical Baltican assemblages, suggesting palaeobiogeographical affinities between Iran and the Baltica palaeocontinent. Taxa shared with typical North Gondwanan assemblages are only a minor component of the Iranian chitinozoan fauna. The present results confirm previous observations that current appreciation of chitinozoan provincialism may be affected by sample-and (palaeo)geographic coverage biases, particularly for the Silurian. Similarly, the application of existing biostratigraphic schemes for the dating of the Iranian Silurian sediments proved problematical, confirming the need of incorporating more data from little known areas (such as Iran) in global databases.