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

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.


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 mid-Palaeozoic underwent dramatic change in Earth's climate 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; Gorgij 2008, 2011;Bahrami et al. 2014). The main objectives of this paper are to describe and interpret the bryozoan fauna from bryozoan-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 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  Königshof et al. 2017) 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).
Family Atactotoechidae Duncan, 1939Genus Anomalotoechus Duncan, 1939(= Stereotoechus Duncan, 1939see Boardman 1960;Astrova 1978) Type species: Anomalotoechus typicus Duncan, 1939. Traverse Group (Middle Devonian); Michigan, USA. Diagnosis: Encrusting, massive, less commonly branched colonies. Autozooecia with polygonal to rounded-polygonal apertures. Diaphragms abundant in exozones, straight or inclined. Exilazooecia rare, short. Acanthostyles abundant. Autozooecial walls thin in the endozone; merged, without visible zooecial boundaries, strongly and irregularly thickened in the exozone, often with monilae-shaped thickenings.  Diagnosis: Colonies branched. Autozooecia radially diverging in a broad endozone and forming more or less distinct bundle. Autozooecial apertures oval or circular, regularly spaced. Acanthostyles of two sizes: macroacanthostyles in junctions of autozooecial walls and paurostyles surrounding autozooecial apertures. Diaphragms and hemisepta few or absent in most species (modified after Yang, Hu and Xia. 1988). Remark: Euthyrhombopora differs from Rhombopora Meek, 1872 in the radial budding pattern of autozooecia in endozone as against the radial or spiral pattern in Rhombopora, as well as in the presence of hemisepta. Euthyrhombopora differs from Nicklesopora in the arrangement of autozooecia in form of the  Euthyrhombopora tenuis Ernst et al., 2017 (Figs. 4a-b, 7a-h; (Ernst et al. 2017). The bryozoan assemblage is represented by encrusting and erect branched growth forms with the latter dominating the fauna numerically (66.7%). Encrusting colonies occur in form of hollow erect tubes, which are apparently cavariiform, i.e. encrusting ephemeral cylindrical objects (Coeloclemis zefrehensis) and as secondary overgrowths of erect branched species (Zefrehopora asynithis, Euthyrhombopora tenuis). Anomalotoechus parvus developed exclusively erect branched colonies. Euthyrhombopora tenuis is clearly the dominant species represented by numerous fragments (Fig. 4c-d). Zefrehopora asynithis and Coeloclemis zefrehensis are less abundant, whereas Anomalotoechus parvus occurs in a few thin sections. Fenestrate and cystoporate bryozoans are completely absent in the Bahram Formation.
The studied bryozoan fauna shows low diversity and high abundance of one species (Euthyrhombopora tenuis). The same pattern has been observed in the Frasnian of the Bahram Formation (Ernst et al. 2017). Low diversity and high abundance one or few species are usually signs of an environmental stress (e.g. Bone and Wass 1990;Bone 1991;Butler and Cuffey 1996). The position of the studied fauna within the middle to outer ramp setting Ernst et al. 2017) implies a soft and unstable substrate with relatively high rates of fine sedimentation. Erect colonies of the dominant species Euthyrhombopora tenuis are relatively immune to such sediment influx, and can tolerate wide range of water energy (e.g. Nelson et al. 1988;Amini et al. 2004). In contrast, encrusting species are strongly affected by strong sediment precipitation.
Bryozoans do not show significant extinctions during the bioevents at the Frasnian-Famennian transition (Bigey 1988;Morozova et al. 2002). During these bioevents, bryozoan faunas experience rather taxonomic shifts rather than reduction in diversity and abundance (Ernst 2013). Bryozoans from the Bahram Formation display a similar pattern. Both the Frasnian (Zefreh section) and lower Famennian (Baqer-abad section) assemblages are represented by four species, but two trepostome species (Cyphotrypa definita Morozova, 1960 and Anomalotoechus ramosus Morozova, 1960) of the Frasnian were replaced by the trepostomes Anomalotoechus parvus sp. nov. and Zefrehopora asynithis gen. nov. et sp. nov. in the Famennian assemblage. Coeloclemis zefrehensis and Euthyrhombopora tenuis occur in both assemblages.

Conclusions
The bryozoan fauna from the lower Famennian of the Bahram Formation at the Baqer-abad section, central Iran, contains four species: three trepostomes Coeloclemis zefrehensis Ernst et al., 2017, Anomalotoechus parvus sp. nov. and Zefrehopora asynithis gen. nov. et sp. nov., and one rhabdomesine cryptostome Euthyrhombopora tenuis Ernst et al., 2017. The latter species is numerically dominant. Two species are previously known from the Frasnian of the Bahram Formation at the Zefreh section. The studied assemblage shows lower diversity and high abundance of one species (Euthyrhombopora tenuis) existing apparently in stressful environment. Environmental stress was apparently caused by soft substrate and high sediment precipitation in the middle to outer ramp setting. Dominance of erect branched colonies against encrusting ones is explained as adaptation for an environment with high sediment influx. No difference in the diversity and abundance of bryozoans in the Frasnian and lower Famennian assemblages of the Bahram Formation was observed. The transition between Frasnian and Famennian is marked by a replacement of two trepostome taxa.