The classical Permian-Triassic (P-Tr) boundary sections in the vicinity of Julfa (NW Iran) and in the Baghuk Mountain area (Central Iran) have high potential for the study of the end-Palaeozoic 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 and at Kuh-e-Ali Bashi display very similar successions of latest Permian ammonoids. Sections in the vicinity of Abadeh in Central Iran show a similar succession and faunal composition. Excellent correlation of the sections can 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 Changhsingian 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 at the top of the Paratirolites Limestone; only the top part of this rock unit shows sometimes a mass occurrence of small ammonoids. (2) A morphological development from simpler suture lines (with unserrated prongs of the external lobe) at the base of the Paratirolites Limestone towards more complex sutures with stronger frilled external lobes in the middle portion of the rock unit, and finally a rebound towards simpler suture lines (succession of the genera Dzhulfites, Paratirolites, and Abichites). (3) A general simplification of the conch geometry from trapezoidal to compressed whorl cross sections. (4) A smoothing of the shell ornament (loss of coarse sculpture such as ventrolateral nodes with a development to faint ribs). (5) A conspicuous size decrease of the ammonoid conchs (from up to 200 mm diameter to 30 mm). This size reduction can be seen in the paratirolitid lineage (genera Paratirolites and Abichites) but also in the immigration of small-sized ammonoids such as Neoaganides. 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 Changhsingian. The data demonstrate the complex morphological evolution of the latest Permian ammonoids prior to the mass extinction event.

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