HIGH RESOLUTION STRATIGRAPHY AND EXTINCTION PATTERNS OF THE AMMONOIDEA AT THE PERMIAN-TRIASSIC BOUNDARY OF NW IRAN

The classical Permian-Triassic (P-Tr) boundary sections in the vicinity of Julfa (NW Iran) have high potential for the study of the end-Paleozoic extinction event. The lithostratigraphic record of the P-Tr boundary beds in the Julfa area is rather uniform over a distance of 35 km. It comprises the Paratirolites Limestone (four meters of red nodular limestone), Boundary Clay (60-190 cm), and Elikah Formation (400 m gray platy limestones). Sections in the Aras Valley, at Kuh-e-Ali Bashi, and near Zal display very similar successions of latest Permian and earliest Triassic strata. Numerous index horizons, such as distinct color changes in the sediments, rhythmic changes in the clay content of the nodular limestones, characteristic fossil accumulations, occur. Excellent correlation of the sections can, thus, be achieved and enables the detailed documentation of the demise of the Paleozoic faunas. The Paratirolites Limestone, the youngest Permian carbonate formation that represents about the upper half (appr. 1.2 Ma) of the Changxingian stage, can be subdivided in terms of ammonoid biostratigraphy, complementing lithostratigraphic, carbonate microfacies, stable isotopes, and conodont data. The unit can be subdivided into at least four clearly separable ammonoid zones, allowing discrimination into ~300,000 year intervals. The study region can therefore serve as a standard for the Tethyan development of the P-Tr boundary. A stepwise temporal development of the Late Permian ammonoid faunas is observed. In contrast to earlier studies, the Paratirolites Limestone is not uniform in its ammonoid faunas but shows some distinct patterns: (1) the abundance of ammonoids decreases towards the top of the Paratirolites Limestone; (2) a morphological development from simple suture lines to more complex sutures with stronger frilled external lobes; (3) a simplification of the conch geometry (from trapezoidal to compressed whorl cross sections); (4) a smoothing of the conch ornament (loss of coarse sculpture); and (5) a size decrease of the ammonoid conchs (from 100 mm diameter to 30 mm). The top of the Paratirolites limestone shows the extinction horizon with numerous small ammonoids with simplified suture lines. These data indicate that the evolution of the ammonoids has severely been affected already within the Changxingian.