UPPER ORDOVICIAN TRILOBITES FROM NORTHERN IRAN AND THEIR BIOGEOGRAPHIC AFFINITIES

An extremely diverse undescribed trilobite fauna from northern Iran sheds new light on the Early Paleozoic geography of the Alborz terrane. The structural history of the Alborz region is well known, though faunal data for the region are sparse, especially for the Ordovician. Trilobites, brachiopods, conodonts, bryozoans and echinoderms occur in a red grainstone/packstone near the village of Tatavrud, 35 km southwest of Bandar-e-Anzali. The fossils point to a Late Ordovician age, although lithologically similar outcrops in northern Iran are Silurian. Trilobite genera occurring include Trinodus, Geragnostus, Illaenus, Panderia, Phorocephala, Ovalocephalus, Dicranopeltis, Symphysops, Cyclopyge, and Sphaerexochus. Similar assemblages have been reported from the Late Ordovician of Ireland, Spain, Poland, Norway, NW China, Kazakhstan and Turkistan. Reconstructions of Late Ordovician geography place the Alborz terrane near the eastern margin of Gondwana, but there is doubt as to its precise location. Some workers have considered it part of the margin while others have considered it a separate terrane. The presence of the genera listed above confirms a peri-Gondwanan position for the Alborz terrane. However, several species present, including Ascetopeltis? incredibilis (Petrunina, 1975) and Phorocephala ulugtana (Petrunina, 1975) occur elsewhere only in Turkistan. In addition, the isocolid genus Paratiresias has only been reported from Iran, Turkistan and NW China. The presence of these species/genera indicates that the Alborz terrane was in close proximity to these regions during the Late Ordovician and casts doubt on its attachment to eastern Gondwana at this time.