Upper Permian Brachiopods from the Nesen Formation, North Iran

The two members of the Nesen Formation in North Iran are very rich in brachiopods, which, collected bed-by-bed along six sections and in two localities, comprise 48 species of 31 genera of the orders Productida, Orthotet- ida, Orthida, Rhynchonellida, Athyridida, Spiriferida, and Terebratulida. One subfamily (Transcaucasathyriinae) and three species are new. Brachiopod settling preferences show a significant change through the formation, with semi-infau- nal productids dominating in the lower member and more diversified pedicle attached and cemented taxa increasing in the upper member. This suggests a shallowing upward trend from settings below storm-wave base in the lower member to shallow water, higher nutrient substrates around fair- weather wave base in the upper member, in agreement with lithological and sedimentological data. The two biozones of the lower member, the Tyloplecta persica and Araxilevis inter- medius Biozones, are Wuchiapingian in age and can be cor- related with the Araxilevis Biozone of Transcaucasia. More difficult is the correlation of the biozones of the upper member, the Permophricodothyris ovata and Enteletes latero- plicatus Biozones, which are Changhsingian in age and more diversified than the Changhsingian ‘Comelicania’(= Gruntal- lina) beds of Transcaucasia. This may be related to a palaeo- ecological control, as the North Iran sediments were deposited at shallow water depths, whereas the Transcauca- sian succession records a shift to deeper water settings. How- ever, the two biozones of the upper member show a significant generic similarity with the Changhsingian shal- low-water brachiopod faunas of South China. The Nesen brachiopod fauna disappears about twenty metres below the Permo ⁄ Triassic boundary, with a single occurrence of a spe- cies of Ombonia in this almost barren interval. This pattern has been recorded in many other locations in the western Tethys and it contrasts with the brachiopod distribution pat- tern of South China, which has more diverse and abundant faunas up to the end-Permian extinction crisis and even in the survival interval.