Early Famennian bryozoan fauna from the Baqer-abad section, northeast Isfahan, central Iran

Palaeobiodiversity and Palaeoenvironments https://doi.org/10.1007/s12549-020-00417-4 ORIGINAL PAPER Early Famennian bryozoan fauna from the Baqer-abad section, northeast Isfahan, central Iran Andrej Ernst 1 2 & Ali Bahrami & Ayesheh Parast 2 Received: 15 April 2019 / Revised: 2 July 2019 / Accepted: 16 January 2020 # The Author(s) 2020 Abstract A bryozoan fauna from the Upper Devonian (lower Famennian) of the Bahram Formation of the Baqer-abad section in central Iran contains four species: three trepostomes and one rhabdomesine cryptostome. Two trepostome species and one genus are new: Anomalotoechus parvus sp. nov. and Zefrehopora asynithis gen. nov. et sp. nov. The trepostome Coeloclemis zefrehensis Ernst et al., 2017 and the rhabdomesine cryptostome Euthyrhombopora tenuis Ernst et al., 2017 were recorded previously from the Upper Devonian (Frasnian) of the Bahram Formation at the Zefreh section. The fauna is dominated by the erect ramose Euthyrhombopora tenuis, accompanied by relatively frequent Zefrehopora asynithis, which developed both the erect and encrusting colonies. The bryozoans indicate low to moderate water energy environment in a middle to outer ramp position. Low diversity and high abundance of one species indicate an environmental stress apparently caused by strong sediment deposition. No significant differences in the composition of the bryozoan assemblages of the Frasnian and lower Famennian of the Bahram Formation were observed mirroring global patterns. Keywords Bryozoa . Taxonomy . Early Famennian . Northeast Isfahan . Central Iran Introduction The late Middle and Late Devonian represents a relatively warm period with an acme in diversity, size and latitudinal distribution of reefs and associated shallow-water sediments in the Middle Devonian (Flügel and Kiessling 2002; Joachimski et al. 2009). On the other hand, the midPalaeozoic underwent dramatic change in Earth’s climate This article is registered in Zoobank under urn:lsid:zoobank.org:pub:DAD5D022-364E-4E12-938EFC2434767ABC * Ali Bahrami Andrej Ernst Ayesheh Parast 1 Institut für Geologie, Universität Hamburg, 20146 Hamburg, Germany 2 Department of Geology, Faculty of Sciences, University of Isfahan, Isfahan, Iran systems which resulted in changes in ocean chemistry and sea level. As a consequence of this, palaeoecosystems were impacted by several mass extinctions and ecological perturbations spanning millions of years (e.g. Talent et al. 1993). Fluctuations in physical palaeoenvironments and resultant mass extinctions were recorded in the sedimentological record by lithological changes and geochemical excursions which are also dependent on the depositional setting (Mottequin et al. 2017). Middle to Late Devonian strata are mainly composed of shallow-water facies and occur in isolated units in central Iran (e.g. Zahedi 1973; Soffel and Förster 1984; Wendt et al. 2005). The distribution of the upper Palaeozoic sediments around Isfahan (Fig. 1) is mostly limited to the northern Isfahan basin (Soh and Natanz regions Najhf, Negheleh, Varkamar, Northern Tar and Western Kesheh sections; (Zahedi 1973; Adhamian 2003; Ghobadipour et al. 2013; Bahrami et al. 2015), northeastern Isfahan basin (Zefreh, Chahriseh and Dizlu sections; Brice et al. 2006; Gholamalian 2003; Habibi et al. 2013; Königshof et al. 2017; Ernst et al. 2017; Bahrami et al. 2018) and southern Isfahan basin (Darchaleh and Ramsheh (in Shahreza region) sections; Boncheva et al. 2007; Leven and Gorgij 2008, 2011; Bahrami et al. 2014). The main objectives of this paper are to describe and interpret the bryozoan fauna from bryozoan- Palaeobio Palaeoenv Fig. 1 Structural map of the central Iran (the star marks the position of the investigated section; after Königshof et al. 2017) bearing horizons at the lower part of the Bahram Formation in the study area (Baqer-abad section, NE Isfahan). Geological setting and material The Baqer-abad section is located 46 km to the northeast of Isfahan (N 33° 2′ 38′′, E 51° 57′ 91′′ WGS coordinates; Fig. 2) in the area where most Devonian outcrops of Iran are exposed; the sequence includes some hiatuses due to erosion and/or tectonic activity. The entire section has a thickness of approximately 1000 m, ranging stratigraphically from the Devonian (Bahram Formation) to the Cretaceous (Aptian-Albian), whereas the studied section of the Bahram Formation has a thickness of 220 m representing mainly shallow-water palaeoenvironments. The section has been grouped in 13 units which include fossiliferous carbonate units with brachiopods, tentaculitids, corals, gastropods, crinoid remains and bryozoans, with a few shale (grey to black) and sandstone interbeddings. Bryozoans occur in distinct layers mainly in the lower part (triangularis to crepida conodont zones) of the section in unit 1 (samples P1–P6; Figs. 3, 4). Twentyseven rock samples were taken from this part of the section from which 54 thin sections of different size were made. Systematic palaeontology Bryozoans were studied in thin sections using a binocular microscope. Morphological character terminology is partly adopted from Anstey and Perry (1970) for trepostomes and from Hageman (1993) for cryptostomes. The spacing of structures is measured as the distance between their centres. Statistics are Palaeobio Palaeoenv Fig. 2 Geological map of the studied area with indication to accessible road to the studied section. (After Zahedi (1973), slightly modified) summarised using number of measurements (N), arithmetic mean (X), sample standard deviation (SD), coefficient of variation (CV) and minimum (MIN) and maximum (MAX) values. All the studied material is housed at Senckenberg Research Institute and Natural History Museum, Frankfurt am Main, Germany (prefix SMF). Phylum Bryozoa Ehrenberg, 1831 Class Stenolaemata Borg, 1926 Superorder Palaeostomata Ma, Buttler and Taylor, 2014 Order Trepostomata Ulrich, 1882 Suborder Amplexoporina Astrova, 1965 Family Stenoporidae Waagen and Wentzel, 1886 Genus Coeloclemis Girty, 1911 Type species: Coeloclemis tumida Girty, 1911. Fayetteville Shales (Upper Mississippian, Carboniferous); Westville, Oklahoma, USA. Palaeobio Palaeoenv Fig. 3 Lithological characteristics and stratigraphy of the Baqer-abad section based on conodont fauna Palaeobio Palaeoenv Fig. 4 a–b Field photographs of the Baqer-abad section. a Medium to thick-bedded sandy limestone just above the bryozoan bearing horizons (package 2). b Grey thick-bedded limestone with scattered bryozoan fauna (part of package 1). c–d Thin sections parallel (c) and across (d) bedding plane showing lithological characteristic of limestones at the Zefreh section. a SMF 23.899, b SMF 23.914. Scale bars 5 mm (c–d) Occurrence: Upper Devonian to Lower Carboniferous; Iran; USA. Diagnosis: Colonies encrusting, maculae unknown. Autozooecia usually cylindrical or prismatic, with short recumbent parts, growing from exterior basal colony walls. Autozooecial a (...truncated)