Stratigraphy and taphonomy of tubular macrofossils in Terreneuvian rocks of the Soltanieh Formation, Soltanieh Mountains, Northern Iran

Unmineralized tubular remains with variable morphology and preservability are common in Ediacaran and Cambrian rocks, providing important records of early animal evolution. Here we report a new occurrence of such fossils in the Soltanieh Mountains of northwestern Iran. A large, densely packed tube population is present in the lowermost part of low-energy, outer shelf mudrocks of the Fortunian Upper Shale Member (USM) of the Soltanieh Formation in the Jashn Sara section. Morphologically, these tubes are unbranched and straight to sinuously curved, with uniform diameters of 0.8–1.1 mm and lengths up to 34 mm. The fossils are preserved as orangish to cream-colored structures with low positive relief. X-ray diffraction and SEM-EDS reveal that Soltanieh tubes were largely infilled by variable mixes of pyrite framboids (and their oxidation products), silica, and clay minerals. In contrast, the sediment matrix is dominated by K-Al-silicate with lesser amounts of an Al-oxide/(oxy)hydroxide phase. The abundance of pyrite in the tubes indicates a prominent role for sulfate-reducing bacteria in the post-mortem decay of organic materials within the tubes, and abundant chert along the tube's inner margins further indicates the propensity of silica to precipitate on relatively recalcitrant organic structures. While the newly discovered population shares features in common with Sabellidites cambriensis, known from Terreneuvian strata in northern Europe and Avalonia, the Iranian fossils do not preserve the transverse annulations characteristic of Sabellidites walls. With this in mind, we leave our new population in open nomenclature. Nonetheless, the occurrence of abundant organic-walled tubes in the USM of the Soltanieh Formation adds to both the diversity of animal fossils found regionally in Terreneuvian rocks and the broader paleogeographic distribution of tubular metazoans in early Cambrian oceans. •Newly discovered tubular fossils from Fortunian mudrocks in Iran.•Preservation by early diagenetic pyrite and silica emplacement with later oxidation.•Key beds for stratigraphic integration of shelly, unmineralized, and trace fossils.•Faunal similarities among Gondwana, Baltica, and Avalonia in the earliest Cambrian.