A new Middle Ordovician arthropod fauna (Trilobita, Ostracoda, Bradoriida) from the Lashkarak Formation, Eastern Alborz Mountains, northern Iran

The first Middle Ordovician trilobites, ostracods and a bradoriid from northern Iran are documented from the Lashkarak Formation. The discovery provides new data bearing on the biogeographical distribution of arthropods and the palaeogeographical setting of northern Iran during the Ordovician. The ostracods include the palaeocopes Cerninella aryana and Ordovizona amyitisae that provide a biostratigraphical tie with horizons of Middle Ordovician (Darriwilian) age in the upper Shirgesht Formation of east central Iran, whilst the trilobites comprise Neseuretus aff. tristani that resembles material from the Tabuck Formation of Saudi Arabia, and Ningkianites sp., a genus characteristic of south Chinese Ordovician faunas. The appearance of Neseuretus in the Lashkarak Formation coincides with the termination of carbonate accumulating facies interpreted as a fall in sea level. The geologically late occurrence of a bradoriid may represent the vestige of a once globally distributed Cambrian arthropod group.