Five Spathian (late Olenekian) ammonoids are newly reported from the Zhitkov Formation in the Artyom section, South Primorye, Russian Far East. They include the early Spathian taxa Bittnerites pacificus and Tirolites longilobatus from the middle part of the formation, and Ussuriphyllites amurensis, Leiophyllites sp. and Keyserlingites sp. from the upper part. The Ussuriphyllites amurensis Zone was previously correlated with the lower Anisian, but we herein correlate it with the upper Spathian. As a consequence of this age assignment, the weakly bioturbated offshore mudstone in the middle and upper parts of the formation is correlated with the Spathian. In contrast, the Smithian (early Olenekian) offshore laminated mudstone in the lower part of the formation lacks any sign of bioturbation. These mudstones suggest that the oxygen-deficient sea floor was transformed into an aerobic environment inhabitable to benthic organisms at the Smithian-Spathian boundary.
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Vol. 20 • No. 1