Ordovician of North Iran: New lithostratigraphy, palaeogeography and biogeographical links with South China and the Mediterranean peri-Gondwana margin

The Ordovician litho- and biostratigraphic framework of Alborz, Kopet-Dagh and the East-Central Iranian blocks is outlined and significantly updated, and a broad summary of the current state of knowledge of the Ordovician deposits and faunas across Iran is documented. Four tectono-stratigraphical units (including the Alestan, Damghan, Saluk and Talesh domains) are distinguished in northern Iran. They differ considerably from one another in their lithology, facies, fossil record and completeness of their sedimentary record. A comprehensive revision of the Ordovician stratigraphy in the eastern Alborz and the Kopet-Dagh regions leads to the definition of the (i) Simeh-Kuh Formation (new), (ii) the Qumes Formation (new) and its subdivision into the Gerd-Kuh (new) and Raziabad (new) members, (iii) the Lashkarak Formation and its subdivision into the Cheshmeh-Ali (new) and Hajiabad (new) members, (iv) the Abarsaj Formation (formalised) in the Alestan Domain, (v) the Qyzlar (new) and Pelmis formations in the Saluk Domain, and (vi) the Tatavrud Formation (new) in the Talesh Domain. The Ordovician strata of the eastern Alborz and Kopet-Dagh Mountains comprise a rifting volcanism emplaced within an active horst-and-graben palaeotopography. Despite the strong Ordovician biogeographical affinities with South China and, to a lesser extent, with Mediterranean peri- Gondwana, zircon populations analysed from Cambro-Ordovician sandstones in the area point to the Arabian-Nubian Shield of the western Arabian Peninsula and northeastern Africa as the principal continental sources. Consequently, an open oceanic communication between the Mediterranean and the northern (Alborz) margins of Gondwana and the southern margin of South China favoured the establishment of strong biogeographical affinities between these mid-latitude basins

Oxygen as a Driver of Early Arthropod Micro-Benthos Evolution

BACKGROUND: We examine the physiological and lifestyle adaptations which facilitated the emergence of ostracods as the numerically dominant Phanerozoic bivalve arthropod micro-benthos. METHODOLOGY/PRINCIPAL FINDINGS: The PO(2) of modern normoxic seawater is 21 kPa (air-equilibrated water), a level that would cause cellular damage if found in the tissues of ostracods and much other marine fauna. The PO(2) of most aquatic breathers at the cellular level is much lower, between 1 and 3 kPa. Ostracods avoid oxygen toxicity by migrating to waters which are hypoxic, or by developing metabolisms which generate high consumption of O(2). Interrogation of the Cambrian record of bivalve arthropod micro-benthos suggests a strong control on ecosystem evolution exerted by changing seawater O(2) levels. The PO(2) of air-equilibrated Cambrian-seawater is predicted to have varied between 10 and 30 kPa. Three groups of marine shelf-dwelling bivalve arthropods adopted different responses to Cambrian seawater O(2). Bradoriida evolved cardiovascular systems that favoured colonization of oxygenated marine waters. Their biodiversity declined during intervals associated with black shale deposition and marine shelf anoxia and their diversity may also have been curtailed by elevated late Cambrian (Furongian) oxygen-levels that increased the PO(2) gradient between seawater and bradoriid tissues. Phosphatocopida responded to Cambrian anoxia differently, reaching their peak during widespread seabed dysoxia of the SPICE event. They lacked a cardiovascular system and appear to have been adapted to seawater hypoxia. As latest Cambrian marine shelf waters became well oxygenated, phosphatocopids went extinct. Changing seawater oxygen-levels and the demise of much of the seabed bradoriid micro-benthos favoured a third group of arthropod micro-benthos, the ostracods. These animals adopted lifestyles that made them tolerant of changes in seawater O(2). Ostracods became the numerically dominant arthropod micro-benthos of the Phanerozoic. CONCLUSIONS/SIGNIFICANCE: Our work has implications from an evolutionary context for understanding how oxygen-level in marine ecosystems drives behaviour

First report on the occurrence of Neseuretinus and Ovalocephalus trilobites in the Middle Ordovician of Iran

In the Derenjal Mountains of east Central Iran, the upper part of the Shirgesht Formation (uppermost Darriwilian) contains a distinct trilobite assemblage that includes Neseuretinus birmanicus and Ovalocephalus aff. obsoletus among others. Both genera were previously unknown in Iran. The occurrence of Ovalocephalus represents the earliest sign of westward taxon migration from China towards higher latitudes along the West Gondwanan margin, which may be related to global warming, after a short episode of cooler climate in the early to mid Darriwilian. Patterns of biogeographical distribution of Ovalocephalus and Neseuretinussuggest that Central Iran was part of an “overlap zone” where tropical and high latitude benthic taxa mingled

Ordovician ostracods from East Central Iran

Ordovician ostracods are described for the first time from Iran, enhancing the record of this group from the Lower Palaeozoic of the Middle East. The ostracods occur in the Shirgesht Formation, in the east central part of the country, and comprise Ogmoopsis achaemenid sp. nov., Cerninella aryana sp. nov., Ordovizona amyitisae sp. nov., cf. Aechmina? ventadorni, Vogdesella sp., and podocope taxa tentatively identified as species of Longiscula, Pullvillites, and Rectella. These taxa were hitherto unknown from this region. The ostracod−bearing interval is associated with trilobites (Neseuretinus) and brachiopods (Nicolella) that suggest a late Middle Ordovician age. Although the ostracod fauna is small, it demonstrates biogeographical links at genus−level, and possibly at species−level, with the fauna of the late Middle Ordovician Travesout Formation of western France, which also lay in a peri−Gondwanan palaeogeographical setting. It also suggests the potential use of some ostracods as stratigraphical tools for correlating Ordovician rock successions between Europe and the Middle East

Silurian stratigraphy of Central Iran - an update

The Silurian biostratigraphy, lithostratigraphy, and facies of Central Iran including the Kashmar (Boghu Mountains), Tabas (Derenjal Mountains, Ozbak-Kuh), Anarak (Pol-e Khavand) and Kerman regions is reviewed and updated. The current state of knowledge of the Silurian in the Zagros Basin, Alborz, Kopet-Dagh and Talysh regions, as well as in a few areas scattered across the Sabzevar Zone, and the Sanandaj-Sirjan terranes is also reviewed. Silurian volcanism in various parts of Iran is briefly discussed. The end of the Ordovician coincided with a widespread regression across Iran synchronous with the Hirnantian glaciation, and only in the Zagros Basin is there a continuous Ordovician–Silurian transition represented by graptolitic black shales of the Sarchahan Formation. In the Central-East Iranian Platform marine sedimentation re-commenced in the early to mid Aeronian. By the Sheinwoodian, carbonate platform depositional environments were established along its north-eastern margin. In other parts of Iran (e.g., Kopet-Dagh and the Sabzevar Zone), siliciclastic sedimentation continued probably into the late Silurian. The Silurian conodont and brachiopod biostratigraphy of Central Iran is significantly updated facilitating a precise correlation with the Standard Global Chronostratigraphic Scale, as well as with key Silurian sections in other parts of Iran. The Silurian lithostratigraphy is considerably revised and two new lithostratigraphical units, namely the Boghu and Dahaneh-Kalut formations, are introduced

Long-period orbital climate forcing in the early Palaeozoic?

Facies indicators and geochemical proxies of early Palaeozoic global climate cooling suggest episodes of fluctuating glacio-eustasy and severe cold or glaciation from the Mid–Late Cambrian to Silurian (c. 85 myr), with a mean frequency of 2.6 myr. Long-period orbital time series predicted through the Phanerozoic are used to generate sine waves to test against these data; the null hypothesis of no orbital influence is rejected with a high confidence level. Cooling episodes appear most frequent through the Late Ordovician leading up to the Hirnantian glacial maximum, but even ‘greenhouse’ intervals of the Early–Mid-Ordovician and early Silurian provide evidence for periodic cooling episodes

Early Ordovician (Tremadocian) faunas and biostratigraphy of the Gerd-Kuh section, eastern Alborz, Iran

The Tremadocian of the East Alborz Region is dominated by condensed fine clasticsediments. These beds have yielded low to medium diversity trilobite associations, which belong to theolenid, nileid and raphiophorid biofacies, characteristic of an outer shelf environment. Five successivetrilobite biozones can be recognised in the Tremadocian succession of Alborz. The lower TremadocianAsaphellus inflatus–Dactylocephalus and Psilocephalina lubrica zones are characterised by mediumdiversity trilobite associations with strong links to contemporaneous faunas of South China. Three upperzones are documented in the section at Gerd-Kuh, the successive Vachikaspis insueta and Kayseraspiszones represent a low diversity interval during a time of rapid changes in the sea level changes; themedium diversity fauna of the Asaphellus fecundus–Taihungshania miqueli zone shows strong links tothe faunas of Mediterranean segment of Gondwana. Brachiopods in Gerd-Kuh are represented by themonotaxic Tarfaya Association and the low diversity Paralenorthis–Xinanorthis Association. Therecurrent oligotaxic Protambonites Association invaded the area in the late Tremadocian during shortterm regressive episodes

Early Ordovician (Tremadocian) faunas and biostratigraphy of the Gerd-Kuh section, eastern Alborz, Iran

The Tremadocian of the East Alborz Region is dominated by condensed fine clasticsediments. These beds have yielded low to medium diversity trilobite associations, which belong to theolenid, nileid and raphiophorid biofacies, characteristic of an outer shelf environment. Five successivetrilobite biozones can be recognised in the Tremadocian succession of Alborz. The lower TremadocianAsaphellus inflatus–Dactylocephalus and Psilocephalina lubrica zones are characterised by mediumdiversity trilobite associations with strong links to contemporaneous faunas of South China. Three upperzones are documented in the section at Gerd-Kuh, the successive Vachikaspis insueta and Kayseraspiszones represent a low diversity interval during a time of rapid changes in the sea level changes; themedium diversity fauna of the Asaphellus fecundus–Taihungshania miqueli zone shows strong links tothe faunas of Mediterranean segment of Gondwana. Brachiopods in Gerd-Kuh are represented by themonotaxic Tarfaya Association and the low diversity Paralenorthis–Xinanorthis Association. Therecurrent oligotaxic Protambonites Association invaded the area in the late Tremadocian during shortterm regressive episodes