Stratigraphy and fossil fauna of the marine Triassic System in Russian Far East

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The stratigraphy and paleontology of the Lower Triassic in South Primorye, Russia were thoroughly investigated in 2005, 2006 and 2007. The Lower Triassic is divided into the Lazurnaya and Zhitkov formations in ascending order. The Lazurnaya Formation unconformably covers the Permian rocks, and consists of basal conglomerate and shallow marine sandstones with hummocky cross stratification. The Zhitkov Formation is composed mainly of offshore black mudstone with slump deposit and turbiditic sandstone. Ammonites are abundant throughout the sequence, in which several ammonite zones were recognized successively from upper Lower Induan(Upper Griesbachian) to upper Lower Olenekian(Upper Smithian). The Induan ammonites include several species in common with South China and Canada, but the most Lower Olenekian ammonites are endemic species. This fact suggests that ammonites diversified regionally during the early Olenekian. Five species belonging to four genera of nautiloids were discovered from the Induan. They show a various shell form from involute, evolute and gyroconic coiling, and various siphuncle position from center to ventral. They belong to the same family, and it is very likely that they rapidly diversified from single ancestor survived at P/T extinction event during the early Triassic. The nautiloid diversity was the highest in the South Primorye. This fact suggests that the recovery of the nautiloid in South Primorye was much faster than the other area.