Introduction

Ordovician trilobites of the Shirgesht Formation in the Derenjal Mountains of east Central Iran were revised recently by Bruton et al. (2004), but the Middle Ordovician fauna was mostly beyond the scope of that paper. There are some controversies about the upper age limits of the Shirgesht Formation. Early reports by Ruttner et al. (1968) gave evidence for the presence of Upper Ordovician rocks in the area. However, this was dismissed in later publications by Bassett et al. (1999) and Bruton et al. (2004), on the grounds that available data from studies of brachiopods and trilobites suggest that the age of the uppermost Shirgesht Formation cannot be younger than Arenig.

More recently, a Middle Ordovician age for the upper part of the Shirgesht Formation was supported by the discovery of a small ostracod fauna (Ghobadi Pour et al. 2006). The ostracod-bearing horizons also contain a moderately diverse trilobite assemblage, which includes Birmanites sp., Deanaspis sp., Liomegalaspides (= Megalaspides) winsnesi (Bruton, 2004), Neseuretinus birmanicus (Reed, 1906), Ovalocephalus aff. obsoletus (Zhou and Dean, 1986), Radnoria simplex (Kolobova, 1978) and Thaleops (Amphoriops) sp., among others. The occurrence of Neseuretinus and Ovalocephalus in this assemblage is of particular interest. Both taxa were relatively widespread within Gondwana and its associated terranes, but they have not been reported previously from Iran. Moreover, there are only a few reports of their co-occurrence in the same faunal assemblage, including the Upper Ordovician Şort Tepe Formation of the Turkish Taurides (Dean and Zhou 1988) and the Naungkangyi Group, northern Shan States, Myanmar (Burma) (Fortey and Cocks 1998). Areas occupied by these genera in the Mid and early Late Ordovician were otherwise separated. This peculiarity in the palaeogeographical distribution of Neseuretinus and Ovalocephalus may have important implications for the evaluation of the palaeogeographical position of Central Iran in the Middle Ordovician, which is discussed below.

Geological setting

The studied trilobites were sampled from the upper part of the Shirgesht Formation in the Derenjal Mountains, about 65 km north of Tabas. The geology of the area was outlined by Ruttner et al. (1968) and by Bruton et al. (2004), but no detailed faunal logs were given in these publications and the lists of fossils characteristic of the Shirgesht Formation are composite. Recently, Ghobadi Pour et al. (2006: 552, fig. 2) published a more detailed account of the faunal distribution through the upper part of the Shirgesht Formation exposed on the west side of Dahaneh Kolut (Section B). Units B5 and B6 of the section represent the main source of the trilobites studied and described in the present paper (Fig. 1). Geographical coordinates for the base and the top of Section B are 34°05′32″ N and 56°47′7.3″ E, and 34°05′21.7″ N and 56°47′06.7″ E respectively.

Fig. 1.

Geographical map (A) showing the position of fossil locality in the Derenjal Mountains north of Tabas and stratigraphical column (B) through Section B in the upper part of the Shirgesht Formation, showing stratigraphical ranges of selected trilobite, ostracod, and conodont species (after Ghobadi Pour et al. 2006). The dashed line for Neseuretinus birmanicus indicates the position of specimens collected lose on the surface and therefore only in approximate stratigraphical position.

Unit B5, about 70 m thick, comprises intercalating green calcareous argillites and argillaceous limestones with abundant brachiopods, bryozoans, gastropod and cephalopod molluscs, trilobites, ostracods, and echinoderms. Most trilobites were collected as loose specimens through the unit (sample F-35). Neseuretinus birmanicus is relatively abundant and is represented mostly by complete, enrolled exoskeletons.

Unit B6, about 19 m thick, comprises characteristic brownish-red sandy limestones with abundant gastropod and cephalopod molluscs, ostracods, echinoderm columnals and rhombiferan Heliocrinites sp. (Lebfebvre et al. 2005; sample F-36), and trilobites including Ovalocephalus aff. obsoletus and a few Neseuretinus birmanicus.

There is a barren interval of intercalating siltstones and argillites up to 150 m thick in the uppermost part of the Shirgesht Formation (Unit B7).

Systematic palaeontology

The terminology and systematic classification follow that of the revised edition of the Treatise on Invertebrate Palaeontology (Whittington and Kelly 1997).

Fig. 2.

Phacopide trilobites from Middle Ordovician Shirgesht Formation of Iran. A-E. Reedocalymenid trilobite Neseuretinus birmanicus (Reed, 1906). A. NMW2004.22G.570, complete enrolled exoskeleton, sample F-35; A₁, dorsal view of the cephalon; A₂, anterior view of cephalon and pygidium; A₃, side view. B. NMW2004.22G.567, complete enrolled exoskeleton, sample F-35, side view. C. NMW2004.22G.568, complete enrolled exoskeleton, sample F-35; C₁, side view; C₂, dorsal view of cephalon; C₃, anterior view of cephalon and pygidium. D. NMW2004.22G.571, complete enrolled exoskeleton, sample F-35; D₁, dorsal view of cephalon; D₂, dorsal view of pygidium. E. NMW2004.22G.574, incomplete enrolled pygidium and part of thorax, sample F-35, dorsal view of pygidium. F–H. Pliomerid trilobite Ovalocephalus aff. obsoletus (Zhou and Dean, 1986) from sample F-36. F. NMW2004.22G.575, complete enrolled slightly crushed exoskeleton, sample F-36; F₁, dorsal view of cephalon and part of thorax; F₂, side view; F₃, dorsal view of pygidium and part of thorax. G. NMW2004.22G.577, dorsal view of pygidium. H. NMW2004.22G.576, dorsal view of incomplete cranidium.

Age and biogeographical affinities of the fauna

According to Turvey (2005), N. birmanicus (Reed, 1906) is a senior synonym of Neseuretus (Neseuretinus) turcicus Dean, 1967 and this view is accepted here. Turvey also gave an account of the geographical distribution of the species, which is confined mostly to high and medium latitude peri-Gondwana, including Annamia, Sibumasu, Afghanistan, the Zerafshan Range in Uzbekistan (which was situated on the margin of the Turan microcontinent (Biske 1996)), Turkish Taurides and Armorica (Sardinia) (Fig. 3). With the exception of the Portixeddu and Tuviois formations of Sardinia, which are considered as lower Ashgill (Hammann and Leone 1997), all listed localities are usually dated as Caradoc (Upper Ordovician, probably, Sandbian). At the same time, the occurrence of N. birmanicus in South China was not confirmed in the revision of existing records (Turvey 2005: 564). There is also no record of the occurrence of Neseuretinus in Tarim, and Reedocalymeninae are remarkably absent in Kazakhstanian terranes, which were mostly located in low latitudes through the Middle and Late Ordovician (Fortey and Cocks 2003).

By contrast, during the Middle Ordovician Ovalocephalus was endemic to South China, North China and Tarim (Lu 1975; Lu and Zhou 1979; Zhou and Dean 1986), and only spread towards terranes of Kazakhstan and Central Asia (Koroleva 1959; Abdullaev 1972; Apollonov 1974) during the Late Ordovician, and then to Armorica (Hammann 1992) and Baltica (Kielan 1960). Remarkably, Ovalocephalus almost became a lazarus taxon in the late Darriwilian, prior to its geographical expansion. Ovalocephalus aff. obsoletus from the upper Shirgesht Formation probably fills this gap. The upper Darriwilian age of the locality was suggested earlier from the ostracod studies (Ghobadi Pour et al. 2006), whereas the occurrence of M. birmanicus supports a latest Middle Ordovician or younger age (for details see Turvey 2005). Iranian specimens show distinct similarity to Ovalocephalus intermedius (Lu and Zhou, 1979) from the lower Darriwilian of Inner Mongolia and to Ovalocephalus primitivus extraneus (Lu and Zhou, 1979), and can therefore be referred to the “Species group 2 of Ovalocephalus” after Dean and Zhou (1988), together with Ovalocephalus globosus (Abdullaev, 1972) from the Late Ordovician (probably early Katian) of the central Karakum desert (Turan Plate after Biske 1996); Ovalocephalus kanlingensis Zhang, 1981 from the Upper Ordovician of Tarim, North China (Zhou and Dean 1986, see also discussion on probable synonymy), Alai Range (Petrunina in Repina et al. 1975) and north-eastern Central Kazakhstan (Apollonov 1974); Ovalocephalus plewesae Fortey, 1997 from the Upper Ordovician, Sandbian of southern Tailand (Sibumasu), and Ovalocephalus tetrasulcatus (Kielan, 1960), which was widespread in Baltica, Armorica (Spain) and Turkish Taurides (Hammann 1992; Dean and Zhou 1988). The occurrence of Ovalocephalus aff. obsoletus in Central Iran also represents the earliest sign of migration of Ovalocephalus outside South China and Tarim, and precedes subsequent westward migration of the lineage towards the Turkish Taurides and Armorica.

Fig. 3.

Palaeogeographical reconstruction for the Middle Ordovician showing geographical distribution of biogeographically informative trilobite genera and species that occur in the upper Shirgesht Formation.

Table 1.

Measurements and basic statistics for cephala and pygidia of Neseuretinus birmanicus (Reed, 1906) from the sample F-35. Abbreviations: Cl, Cw, length and width of cephalon; Gl, Gw, length and width of the glabella; PGl, PGw, length and width of preglabellar field; ORl, ORw, length (sag.) and width of occipital ring; Pl, Pw, length and width of pygidium; Al, Aw, length and width of pygidial axis; X, average; S, standard deviation; N, number of measured specimens.

In the Late Ordovician there was considerable biogeographical differentiation in the distribution of Ovalocephalus as already recognised by Lu and Zhou (1979), and Zhou and Dean (1986). The biogeographical area of “Species group 2 of Ovalocephalus” closely followed the West Gondwanan margin, whereas “Species group 1”, typified by such species as Ovalocephalus kelleri Koroleva, 1959, O. ovatus (Sheng, 1964) and O. yangtzeensis (Lu, 1974), was confined to north-eastern Central Kazakhstan, Tarim, South China, and Sibumasu, mainly in low southern latitudes (Fig. 3). In Kazakhstanian terranes, North China, Sibumasu and Central Asia (Alai and Turan microplates) species belonging to two groups co-occur. It is probable that differentiation of Ovalocephalus was climatically controlled.

The co-occurrence of Ovalocephalus and Neseuretinus in the Naungkangyi Group of northern Shan States requires special attention. As pointed out by Fortey (in Fortey and Cocks 1998), some specimens illustrated by Reed (1915: pl. 8: 8–10) as Calymene lilluensis Reed, 1915 and Pliomera (Encrinurella) lingshangensis Reed, 1906, must be reassigned to Ovalocephalus. The morphology of the illustrated cranidium shows distinct similarity to Ovalocephalus yangtzeensis in general outline of the glabella, characters of glabellar furrows (excluding S4 which was not shown on the figure) and small preoccipital lobes. Two pygidial internal moulds illustrated by Reed (1915) are strongly transverse, have a subtrapezoidal outline with slightly concave posterior margin and transverse pleural ribs only slightly inclined backwards, which also suggests affinity to O. yangtzeensis. Precise taxonomic attribution of the specimens from the Naungkangyi Group is impossible without direct observation, because of insufficient description and illustrations, but there is little doubt that they lack a complete preoccipital ring and must therefore be assigned to the “Species group 1” after Dean and Zhou (1988).

It is probable that the appearance of Ovalocephalus in the late Darriwilian of Central Iran and somewhat later appearance of the genus in the early Sandbian of Sibumasu may represent a sign of a warm water faunal invasion towards temperate latitudes, which related to global warming following an episode of cooler climate in the early to mid Darriwilian. The lower Darriwilian deposits in the Shirgesht section are barren. However, the upper Darriwilian part of the Shirgesht Formation can be correlated with the uppermost part (Lenodus pseudoplanus Conodont Subzone and higher) of the Lashkarak Formation in Simeh-Kuh, Eastern Alborz Mountains (Ghobadi Pour et al. 2007). The lower Darriwilian of Simeh-Kuh coincides with invasion of the Neseuretus biofacies, which was a sign of a cooler climate, and a period of a sea-level low-stand followed by a sea-level rise and invasion of a new, diverse benthic fauna (Ghobadi Pour et al. 2007; Ghobadi Pour 2008). Both in Central Iran and the Eastern Alborz the late Darriwilian faunal assemblages contain abundant plectambonitoidean and strophomenoidean brachiopods not characteristic of earlier faunas. In the Simeh-Kuh section the brachiopods include Ishimia (Ghobadi Pour 2008: fig. 3A, B), which is otherwise characteristic of the low latitude faunas of “East” Gondwana (Australia, Sibumasu, Tibet) (for detailed review see Cocks and Zhan 1998; Percival et al. 2001). Another new element typical elsewhere for the Middle Ordovician low latitude “East” Gondwanan faunas is the trilobite Birmanites, which occurs both in the upper Darriwilian part of the Lashkarak and Shirgesht formations (Ghobadi Pour 2008: fig. 3E, F). Diachronous biogeographical expansion of Birmanites towards “West” peri-Gondwana and Baltica during the Late Ordovician (Zhou and Dean 1986; Romano and Owen 1993; Fortey and Cocks 2003) mirrors that of Ovalocephalus.

Characters of sea level fluctuations during the Darriwilian documented for the Iranian sequence (Ghobadi Pour et al. 2007; Ghobadi Pour 2008) correspond closely with those of Baltica, where a regressive episode is documented for the lower to middle Darriwilian (Kundian Regional Stage), whilst the uppermost Darriwilian (Uhaku Regional Stage) represents a sea level high-stand (Nielsen 2004). According to Meidla et al. (2004) and Kaljo et al. (2007) a period of low-stand in the Kundian-Aserian coincides with low magnitude (1–3‰) positive stable carbon and oxygen isotope excursions suggesting a cooling event in Baltica.

The observed biogeographical patterns of distribution of Ovalocephalus and Neseuretinus suggest that Central Iran, together with Sibumasu and the Turkish Taurides, was part of an “overlap zone” (see also Fortey and Cocks 2003), where tropical and high latitude benthic taxa mingled, and which can be considered in the Ordovician as major gateways for latitudinal faunal migrations and exchange along the Gondwanan margin.