Boletín de la Sociedad Geológica Mexicana Volumen 70, núm. 3, 2018, p. 787 - 795 http://dx.doi.org/10.18268/BSGM2018v70n3a11 Short note

 First report of the rugose coral Hexagonaria davidsoni from the Khoshyeilagh Formation (Devonian), Alborz Mountains, Northeastern Iran

Mostafa Falahatgar¹, Andreas May², Mehdi Sarfi³  

¹Department of Geology, Payame Noor University, Shiraz, Iran, 71893-73111.
²Friedrich-List-Str. 66, D-59425 Unna, Germany.
³School of Earth Sciences, Damghan University, Damghan, Iran, 36716-41167.
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Abstract

The Middle to Late Devonian Khoshyeilagh Formation successions are here reported from the Khoshyeilagh neck, Alborz Mountains, Northeastern Iran. This formation is very fossiliferous. The rugose coral Hexagonaria davidsoni occurs within Frasnian strata and is the first report of this species from the Khoshyeilagh Formation.

Keywords: Devonian, Frasnian, rugose corals, Anthozoa, Alborz Mountains, Iran.

Resumen

En este trabajo se reportan las secuencias del Devónico Medio a Tardío de la Formación Khoshyeilagh en el cuello de Khoshyeilagh. Dicha formación es muy fosilífera en el área estudiada. La especie de coral rugoso Hexagonaria davidsoni de los estratos del Frasniense de esta formación es el primer reporte de esta especie en el Devónico de Irán y se describe aquí sistemáticamente.

Palabras clave: Devónico,Frasniense, corales rugosos,Anthozoa, Montañas Alborz,Irán.

1. Introduction

The Khoshyeilagh Formation is scattered in east­ern parts of the Alborz Mountains, northern Iran. Devonian corals in the Khoshyeilagh Formation have been studied by several paleontologists over the last several decades. Ghods (1982) studied four sections in the Alborz Range, northern Iran from the northwest to eastern parts of the Alborz. Wendt et al. (2005), reported rugose corals from the Upper Devonian Geirud Formation in the central Alborz. Ashouri et al. (2008), Abbasi et al. (2014a), and Abbasi et al. (2014b) described and studied some Givetian to Frasnian rugose corals from the Khoshyeilagh Formation in the eastern Alborz. Abbasi et al. (2014a) studied Givetian and lower Frasnian rugose corals at Till Abad village NE of Shahrood in NE Iran. Other fossil groups have been investigated in the Khoshyeilagh For­mation by several researchers, such as Bozorgnia (1973), Ahmadzadeh Heravi (1975), Jenny, (1977), Stampfli (1978), Ashouri (1994) and Ghavidel Syooki (1975, 2007). Up to now, the investigations on rugose corals from the Khoshyeilagh Forma­tion have focused more on solitary forms and descriptions of colonial corals are rare. The aim of this study is to systematically describe speci­mens of the colonial rugose coral Hexagonaria from the Khoshyeilagh Formation.

2. Geological setting

The studied section is situated in the eastern Alborz Mountains, northern Iran and is located in the Khoshyeilagh neck along the main road between Shahrood and Azad Shahr cities in NE Iran (Figure 1). This section is near the type section (Khoshyeilagh valley). The geographical coordinates of the studied section are 36° 50’ 44” N and 55° 20’ 05” E. At this section, the older rocks of the sequences consist of Silurian basalt of the Soltan Maidan Formation, the Early Devo­nian Padeha Formation, and younger overlying formations that rest on the Khoshyeilagh Forma­tionn respectively called the Mobarak, Dorud, and Ruteh formations (Figure 1).

Figure 1. (A, B) Geographical map of the studied area. (C) Geological map of the studied area. Abbreviations: Osh,s(Ordovician), Sv (Silurian Soltan Maidan Basalt), Dp (Padeha Fm.), Dkh (Khoshyeilagh Fm.), Cm (Mobarak Fm.), P (Permian), Js (Jurassic), Qt (Quaternary).

3. Materials and methods

The studied collection of corals contains 2 sam­ples of Hexagonaria. Five thin sections were pre­pared for this study which are housed at the Earth Science School of the Damghan University, Iran (acronym DKH1-5).

4. Stratigraphy

The Khoshyeilagh Formation is scattered in the eastern parts of the Alborz Mountains, northern Iran, and was first described by Bozorgnia (1973). This formation is the thickest and most fossiliferous formation of the Devonian sequences of Alborz Mountains (Aghanabati, 2008). The Khoshyeilagh Formation is subdivided into four parts including the lower terrigenous member, the lower carbon­ates member, the upper terrigenous member and the upper carbonates member (Aghanabati, 2008), however locally these respective members are changed. In this study, the Middle to Upper Devonian succesions of the Khoshyeilgah Formation were measured and described in the Khoshyeilagh neck near the type section (Khoshyeilagh village). This formation is about 1235 m thick divided into 11 rock units from below to top including: Unit 1-47 m, which consists of conglomerate grading upwards into sandstone overlies the Padeha Forma­tion. Unit 2-76 m, consists of alternating thin- to medium bedded limestones and shales; Unit 3- 82 m, consist of medium to thick-bedded limestones with brachiopods, solitary corals and bryozoans; Unit 4-147 m, consists of thin-to medium-bedded dolomitic limestones with minor debris of brachio­pods and crinoids; Unit 5- 58 m, consists of alter­nating limestone and shale containing bryozoans, brachiopods, tentaculites and solitary corals with small fragments of bryozoans; Unit 6-152 m, con­sists of bedded limestones with a variety of fauna such as solitary corals, tentaculites, crinoid debris and bryozoans with a few molds of trilobites; Unit 7- 158 m, alternating medium bedded limestones with interbedded shales with brachiopods solitary corals, crinoids and the rugose coral Hexagonaria collected from two beds of this unit. This unit corresponds to a Frasnian age because these two levels can be compared with those sequences that Ashouri (2006) assigned a Frasnian age based on a conodont falsiovalis Zone; Unit 8-147 m, consists of sandstones; Unit 9-117 m, consists of limestones interbedded with shales containing brachiopods and crinoids; Unit 10-99m, consists of thin-bedded sandy limestones with bryozoans and crinoids; Unit 11-152 m, consists of alter­nating inter bedded limestones and shales with brachiopods, bryozoans and solitary corals. The Khoshyeilagh Formation is overlain conformably by lower Tournaisian limestones of the Mobark Formation. (Figure 2).

Figure 2. Stratigraphic column of the Khoshyeilagh Formation in the Khoshyeilagh neck, eastern Alborz, Northern Iran.

5. Systematic paleontology

For terminology and systematic classification see Hill (1981), Birenheide (1978) and May (1993). 
Phylum CNIDARIA Hatschek, 1888 
Class ANTHOZOA Ehrenberg, 1834 
Subclass Zoantharia Blainville, 1830 
Superorder Rugosa Milne-Edwards and Haime, 1850 
Order Stauriida Verrill, 1865 
Suborder Columnariina Soshkina, 1941 
Family Disphyllidae Hill, 1939 
Subfamily Hexagonariinae Bulvanker, 1958
Genus Hexagonaria Gürich, 1896

Type species: Cyathophyllum hexagonum Goldfuss, 1826 Hexagonaria davidsoni (Milne-Edwards and Haime, 1851) Figure 3
1851 Acervularia davidsoni Milne-Edwards and Haime, p.418, pl. 9 figs. 4, 4a, 4b. 
1924 Acervularia bassleri Webster and Fenton, in Fenton and Fenton, p. 58-59, pl. 13 fig. 2, pl. 14 fig. 4-5. 
1981 Hexagonaria davidsoni (Milne-Edwards and Haime) Rohart and Semenoff-Tian-Chansky, p. 4-12, pl. 1-3. 
1993 Hexagonaria davidsoni (Milne-Edwards and Haime) Wrzolek, p. 240-243, fig. 16-17. 
1994 Hexagonaria davidsoni (Milne-Edwards and Haime) Coen-Aubert, p. 33-34, pl. 2 fig. 1. 
1998 Hexagonaria bassleri bassleri (Webster and Fen­ton, in Fenton and Fenton) Sorauf, p. 64-67, pl. 3 fig. 7, pl. 30 fig. 1-6, pl. 31 fig 1-6; pl. 32 fig. 1. 
2010 Hexagonaria davidsoni (Edwards and Haime) McLean, p. 63-66, pl. 27 fig.3-4, pl.28-30; cum syn. 
Diagnosis. See McLean (2010: p. 65). 
Type material see Rohart and Semenoff-Tian- Chansky (1981: p. 4-12, pl. 1-3). 
Material.Two coralites with 5 thin sections (Fig­ure 3).

Figure 3. (A, C) External view of colonies. (A) DKH01. (B) DKH02. (B, D, E, G) Transverse sections. (B) DKH 1. (D) DKH 2. (E) DKH 3. (G) DKH 4. (F) Longitudinal section DKH 5. DKH is abbreviation of The Devonian Khoshyeilagh Formation.

Description.The Corallum is cerioid. Coralla have a diameter ranging from about 50 mm to 200 mm. Calices are bell-shaped and deep. Within transveral sections, large corallites have a 15-17 mm diameter and small corallites have a 7-13 mm diameter. Large corallites contain 18-20 x 2 septa. 

The septa are clearly differentiated into major and minor. Major septa meet each other in the axis of the corallite. Minor septa have ½ to ¾ of the length of the major septa. Septa are thin at the margin of the corallite and in the center of the corallite. Between the margin and the center of the corallite, septa are slightly thickened. Swol­len septal trabeculae are developed occasionally resulting in septa that show dispersed carinae-like structures. Within transveral sections, the width of the tabu­larium reaches from 3 mm to 55 mm. 
Within longitudinal sections, the tabularium dis­plays both a flat-topped axial row of tabulae and periaxial sagging tabulae. The dissepimentarium is filled with small dissepiments. Within a corallite of about 14 mm in diameter, the width of the tabularium is about 4-5 mm. 
The microstructure of the skeletal elements shows diagenetic alterations as is usual for Devonian corals; thickened skeletal elements often are shine brighter in transmitted light and sometimes the delimitation of skeletal elements is a bit vague. However, these alterations are not very strong, because a dark middle line is observable in many places. 
Discussion.Our material is very similar to Hexag­onaria hexagona (Goldfuss, 1826), the type species of the genus Hexagonaria Gürich, 1896. Descriptions and illustrations of typical specimens of Hexago­naria hexagona are given by Pickett (1967, p. 58-60, pl. 4 fig. 15), Birenheide (1969, p. 40-41, pl. 1 fig. 1, pl. 4 fig. 11), Birenheide (1978, p. 95-96, pl. 13 fig. 3) and Sorauf (1994, p. 323-327, pl.1 fig. A-D). Differences with typical specimens of Hexagonaria hexagona are the thinner septa and fewer carinae in our material. Furthermore, in typical hexagona the major septa extend only nearly up to the axis and do not meet each other. Our material fits very well with the holotype and the additional material of Hexagonaria bassleri bassleri (Webster and Fenton, in Fenton and Fenton, 1924) from the Frasnian of Iowa described and illustrated by Sorauf (1998, p. 64-67, pl. 3 fig. 7, pl. 30 fig. 1-6, pl. 31 fig 1-6; pl. 32 fig. 1). There is no doubt that Hexagonaria bassleri and our material are conspecific. McLean (2010, p. 66) considers Hexagonaria bassleri as synonymous with Hexagonaria davidsoni (Milne-Edwards and Haime, 1851). The material of Hexagonaria david­soni (Milne-Edwards and Haime, 1851) from the mid-late Frasnian of western Canada described and illustrated by McLean (2010: p. 63-66, pl. 27 fig.3-4, pl.28-30) is very similar to our mate­rial. Nevertheless, there are some transversal sections figured by McLean (2010), which show a remarkable open space at the axis of the corallite. However, in other corallites figured by McLean (2010), the major septa meet each other in the axis. This demonstrates, that the open space at the axis should not be overestimated. Consequently, we agree with McLean (2010) in synonymizing Hexagonaria bassleri (Webster and Fenton, in Fen­ton and Fenton, 1924) with Hexagonaria davidsoni (Milne-Edwards and Haime, 1851). Rohart and Semenoff-Tian-Chansky (1981, p. 4-12, pl. 1-3) described and figured the types of Hexagonaria davidsoni (Milne-Edwards and Haime, 1851) from the Frasnian of northern France. Based on this description, we have no reason to doubt, that our material is conspecific with Hexagonaria davidsoni (Milne-Edwards and Haime, 1851). Furthermore, the Hexagonaria davidsoni (Milne-Edwards and Haime, 1851) described by Coen-Aubert (1994, p. 33-34, pl. 2 fig. 1) from the middle Frasnian of Belgium is conspecific with our material. 
The material of Wrzolek (1993, p. 240-243, fig. 16-17) described as Hexagonaria davidsoni from the Devonian Kowala Formation of Poland, is conspecific too. Following McLean (2010, p.66), Polish material of Hexagonaria davidsoni is of latest Givetian to early Frasnian age. 
Occurrence. Hexagonaria davidsoni is a cosmopol­itan species, which is known from the Frasnian of western Canada, Iowa, northern France, Belgium, Poland and China (McLean, 2010: p. 66). Our material is the first evidence of this species from Iran.

6. Palaeobiogeography

Hexagonaria davidsoni is a cosmopolitan species, known from the Old World Realm as well as from the Eastern Americas Realm (see Pedder and Oliver, 1990). This is a clear confirmation for the observation, that within the marine benthos, the level of provincialism decreased to zero in the Frasnian (May, 1996: p. 41). Taking into account that within the Frasnian it was no longer possible to distinguish between an Old World Realm and an Eastern Americas Realm, it can be stated only, that within the Frasnian, the Alborz Mountains belonged to a very broad tropical to subtropical belt with corals and reef development.

7. Conclusions

The finding of Hexagonaria davidsoni (Milne-Ed­wards and Haime, 1851) in the Khoshyeilagh Formation allows for some interesting conclusions because of the fact that the earliest occurrences of Hexagonaria davidsoni are of latest Givetian to early Frasnian age, and the vast majority of its occur­rences are definitely of Frasnian age. These find­ings confirm a Frasnian age for the middle part of the Khoshyeilagh Formation. Furthermore, these findings document that during the Frasnian, the Alborz Mountains belonged to a very broad tropical to subtropical belt with corals and reef development.

Acknowledgements

We would like to thank Dr. Sergio Rodríguez and Dr. Ross A. McLean for their constructive reviews of the manuscript, as well as Dr. Victor Ogar and Dr. Robert Blodgett for their useful comments and suggestions which significantly improved our manuscript.

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Manuscript received: May 7, 2017.
Corrected manuscript received: October 14, 2017.
Manuscript accepted: October 23, 2017.