We studied the classical Permian–Triassic (P–Tr) boundary sections in the vicinity of Julfa (NW Iran) and in the Baghuk Mountain area (Central Iran). The youngest Permian ammonoid-bearing carbonate formation is the Paratirolites Limestone (four metres of red nodular limestone), which represents about the upper half (ca. 1.2 Ma) of the Changhsingian stage. A 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) A morphological development from complex sutures with strongly frilled external lobes towards simpler suture lines. (2) A general simplification of the conch geometry from trapezoidal to compressed whorl cross-sections. (3) A smoothing of the shell ornament (loss of coarse sculpture such as ventrolateral nodes with a development to faint ribs). (4) A conspicuous size decrease of the ammonoid conchs (from up to 250 mm diameter to 25 mm), meaning a body mass reduction of three orders of magnitude. These data indicate that the evolution of the ammonoids has severely been affected already within the Changhsingian stage. These data demonstrate the complex morphological evolution of the latest Permian ammonoids prior to the mass extinction event.

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