A collection of marine gastropods from the Late Triassic (Norian) Nayband Formation of Iran near Isfahan consists of 207 specimens representing 9 species (3 species are new). Rarefaction analyses and diversity indices indicate that the diversity of the studied fauna is much lower than that from the vicinity of Ali Abad (Tabas region) and somewhat less diverse than that of Dizlu (Isfahan region). All known gastropod faunas from the Nayband Formation are dominated by more or less high-spired caenogastropods. However, species composition and rank abundance of the most common taxa differ strongly in all three gastropod collections. The new species are Acilia? basistriata, Teutonica? natanzensis and Protorcula iranica. The present gastropod fauna is dominated by the caenogastropods Coelostylina conica, Protorcula iranica and Teutonica? natanzensis.•
Following detailed stratigraphic work on the Mississippian marlstone and bioclastic limestone of the Mobarak Formation of the Alborz Mountains in North Iran, forty-eight of the most important brachiopod taxa are here systematically described and illustrated. The ranges of the taxa are given along the Abrendan and Simeh Kuh stratigraphic sections, located north of Damgham. The examined brachiopod species date the base of the Mobarak Formation to the Tournaisian, in absence of age-diagnostic foraminifers. Change in brachiopod settling preferences indicates a shift from high energy, shallow-water settings with high nutrient supply in the lower part of the formation to quieter, soft, but not soppy substrates, with lower nutrient supply in the middle part of the Mobarak Formation. Brachiopod occurrence is instead scanty at its top. The palaeobiogeographic affinity of the Tournaisian brachiopods from North Iran indicates a closer relationship to North America, Western Europe and the Russian Platform than to cold-water Australian faunas, confirming the affinity of the other biota of the Alborz Mountains. This can be explained by the occurrence of warm surface-current gyres widely distributing brachiopod larvae across the Palaeotethys Ocean, where North Iran as other peri-Gondwanan blocks acted as staging-posts.
The new nuculid bivalveTrigonucula(Gonionucula)aciloidesnew subgenus and species from the upper Triassic Nayband Formation of Central Iran is described.Gonionuculadiffers fromTrigonuculas.s. by the presence of oblique ribs, which either deviate from a concentric pattern at the anterior shell part, or, as in the newly described type species, consist of a complex pattern of chevron-like ribs on the central part of the flank, plus an additional set of opisthocline to more or less acline ribs on posterior part of the flank. The oblique ribs ofGonionuculaprobably aided energy-efficient burrowing, which was advantageous in competing for food resources with less efficiently burrowing detritus feeders. The number of burrowing bivalve species with oblique ribs generally rose towards the end of the Triassic, which probably reflects increasing competition at a time when most niches were re-occupied after the end-Permian mass extinction event. The fact that oblique ribs were rare in Paleozoic bivalves suggests that the level of competition was higher in post-Paleozoic biota, which is in accordance with a general increase in metabolic rates, predation pressure, and ecospace occupation in the course of the Mesozoic.
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