Late Ordovician scolecodonts from the Qusaiba-1 core hole, central Saudi Arabia, and their paleogeographical affinities

International audienceThis paper reports the discovery of Late Ordovician scolecodonts from the Qusaiba-1 core hole, central Saudi Arabia. The collection of about 100 relatively well preserved diagnostic jaws represents one of the richest jawed polychaete faunas from the Gondwanan realm and the first description of scolecodonts from the Arabian Peninsula. Scolecodonts were most diverse and abundant in the Quwarah Member and basal Sarah Formation, corresponding to the Ancyrochitina merga and Tanuchitina elongata chitinozoan zones (latest Katian to early Hirnantian). The polychaete assemblage contains up to 15 apparatus-based species and is dominated by the globally distributed genera Kettnerites, Oenonites and Atraktoprion, and a new genus of probable ramphoprionid affinity. Additionally, Skalenoprion and Kalloprion? are recorded, both for the first time from Gondwana. Characteristic of the assemblage is very low frequency of taxa with placognath-type jaws. Comparing the Qusaiba-1 assemblage with coeval faunas of Baltica and Laurentia revealed that northern Gondwana was more similar to mid-continent Laurentia than Baltica. High proportion of paulinitids, ramphoprionids and atraktoprionids, and scarcity of placognaths is typical of both northern Gondwana and Laurentia. The Baltic faunas, in contrast, had several endemic genera particularly among placognaths, whereas paulinitids and ramphoprionids were rare. This anomalous biogeographic pattern, diverging from that of most other fossil groups, cannot be fully explained without additional first-hand data from the Middle and early Late Ordovician of Gondwana and Laurentia

Discovery of Late Ordovician scolecodonts from the Qusaiba-1 drill core, Arabian Peninsula, and its palaeobiogeographical implications

International audienceScolecodonts, the jaws of eunicidan polychaetes, constitute one of the most common groups of acid-resistant microfossils in the Ordovician and Silurian deposits. They are rather well known from Baltica and Laurentia, but only few data are available from other palaeocontinents. The global picture of palaeobiogeography of scolecodonts is therefore biased and every new account from the hitherto poorly known regions deserves full attention. In the course of chitinozoan study of the Qusaiba-1 drill core, central Saudi Arabia, a rich assemblage of scolecodonts was recovered from the Late Ordovician Quwarah Formation. This interval belongs to the Armoricochitina nigerica chitinozoan zone, which is correlated with the upper part of the Katian Global Stage. The bulk of material was obtained by re-picking the sample residues but specimens scattered in the chitinozoan slides were also studied. The entire collection contains more than 150 well-preserved scolecodonts, including some fused jaws and a compound jaw apparatus. In sample from 497.8 ft 76 posterior maxillae were counted from 10 g making this one of the richest samples ever studied for scolecodonts. Taxonomically the assemblage is not particularly diverse containing two species of Oenonites, two species of "Atraktoprion", Kettnerites sp., possibly Kalloprion, an unidentified placognath and a new genus that cannot be confidently attributed to any known family. The latter taxon bears mixed characters of polychaetaspids, kalloprionids and ramphoprionids and accounts for the majority of the specimens in the collection. Similar forms have been reported from the Late Ordovician of North American Mid-continent region, but are unknown from Baltica so far. The low percentage of placognaths is also noteworthy and possibly indicates biogeographical relationships with Laurentia rather than Baltica. Moreover, Kettnerites, being also recorded in Bohemia, is a characteristic component of the Late Ordovician polychaete faunas of Laurentia. In Baltica, paulinitids are very rare in the Ordovician though becoming common in the Silurian. In consequence, this discovery of scolecodonts from the Arabian plate allows, for the first time, to assess biogeographical links of Ordovician jawed polychaetes of Gondwana. It shows that Gondwanan polychaetes were related to Laurentian rather than Baltic faunas. The high abundance of scolecodonts indicates also their potential in stratigraphy and biofacies analysis, but indeed more data is needed for tha

Early Silurian chitinozoans from the Qusaiba type area, North Central Saudi Arabia

International audienceSixty one core samples from three shallow core holes (Qusaiba-1, Baq'a-3 and Baq'a-4) penetrating the Qalibah Formation (Qusaiba Member) in the North Central Saudi Arabia were investigated. These core slabs corresponding to grey–greenish hemipelagic shale or silty-shale yield well-preserved and very abundant early Silurian chitinozoans. Most of the recorded chitinozoan assemblages are diverse and include several new species occurring with well-known species. An informal early Rhuddanian chitinozoan assemblage dominated by Cyathochitina caputoi is documented in the lowermost part of the Qusaiba Member. The six chitinozoan biozones defined above this basal assemblage range from Aeronian to Telychian. They are considered of regional value for northern Gondwana. From the lowest to the highest they are the Angochitina qusaibaensis (pro parte), Conochitina alargada, Angochitina hemeri, Angochitina macclurei, Tanuchitina obtusa and Euconochitina silurica biozones. The older regional biozones have been previously documented in Saudi Arabia whereas the two youngest ones, the eponymous index species of which were described from the subsurface of the Algerian Sahara, are documented for the first time in the Arabian Peninsula. Four subbiozones, of at least of regional application, are also defined in order to improve the precision of biostratigraphic correlation between Silurian lithostratigraphic units cropping out along the Arabian Shield and their counterparts in the subsurface of Central Saudi Arabia. Thanks to the closely spaced and to the regular sampling, the total range of several highly diagnostic forms as well as the interval of uncertainty between the successive chitinozoan biozones are better constrained. A hiatus with a duration of late Rhuddanian to early Aeronian separates the lowest informal chitinozoan assemblage from the qusaibaensis Biozone. A recent detailed study of the graptolites in the three cored holes provides independent chronostratigraphical control calibrating the previous chitinozoan-based age assignments. The qusaibaensis (pro parte), the alargada, and the hemeri chitinozoan biozones occur with the mid-Aeronian convolutus graptolite Biozone whereas the macclurei chitinozoan Biozone extends through the early Telychian guerichi and turriculatus graptolite biozones. The obtusa and the silurica chitinozoan biozones occur in an interval devoid of usable graptolites. They are referred to the mid–late Telychian as they do not contain typical Sheinwoodian species.Besides the abundant chitinozoans and acritarchs, sporadic occurrences of scolecodonts and of eurypterid cuticle fragments are also noted in the organic residues. The presence of eurypterids reflects a shallowing trend in the sedimentary record, which is consistent with the distribution of the graptolite remains (siculae and/or rhabdosomes). Indeed, graptolites are common throughout the Qusaiba Member, except in the youngest processed samples presaging the shallower environments prevalent in the Sharawra Member. The taphonomy of the chitinozoans (isolated vesicles, chain-like structures, clusters, coprolites) and their environmental context is briefly discussed.Fifteen new species are described and illustrated: Ancyrochitina alhajrii sp. nov., Ancyrochitina camilleae sp. nov., Armoricochitina crassicarinata sp. nov., Armoricochitina gengi sp. nov., Bursachitina baqaensis sp. nov., Conochitina viiuae sp. nov., Cutichitina minivelata sp. nov., Cyathochitina neolatipatagium sp. nov., Fungochitina merrelli sp. nov., Muscochitina olivieri sp. nov., Plectochitina alisawyiahensis sp. nov., Plectochitina alnaimi sp. nov., Plectochitina jaquelineae sp. nov., Plectochitina lucasi sp. nov., and Spinachitina geerti sp. nov.Keyword

Aeronian and Telychian chitinozoans from Central Saudi Arabia.

International audienceA total of 58 core samples from three shallow core holes (Qusaiba-1, Baq'a-3 and Baq'a-4) penetrating the Qalibah Formation in Central Saudi Arabia have been investigated. The core samples are grey-greenish hemipelagic shale or silty-shale and yield very well-preserved and abundant Early Silurian chitinozoans. Most of the recorded chitinozoan assemblages are highly diverse and include several new taxa coexisting with well-known species. Five chitinozoan biozones ranging from the Aeronian to the Telychian, and considered of regional value at the scale of Northern Gondwana regions, are identified. From the oldest to the youngest they are the: Conochitina alargada, Angochitina hemeri, Angochitina maclurei, Tanuchitina obtusa and Euconochitina silurica biozones. The three older regional biozones have been already reported in Saudi Arabia (Paris et al. 1995) whereas the two youngest ones are documented for the first time in the Arabian Peninsula. Their eponymous index species were described from the subsurface of the Algerian Sahara. Several sub-biozones of at least local application are also defined. The fairly closely spaced and regular sampling permits the total range of several highly diagnostic forms to be better constrained and thus the interval of uncertainty between biozones is reduced. Beside abundant chitinozoans and acritarchs, sporadic occurrences of scolecodonts and of eurypterid cuticle fragments are also noted in the organic residues. The presence of eurypterids suggests a shallowing trend in the sedimentary record, which is consistent with the distribution of the graptolites sicula and/or rhabdosomes. Indeed, the graptolites are frequent through the Qusaiba Member, except in the youngest examined samples where shallower environments like those in the Sharawra Member begin. Recently, Zalasiewicz et al. (2007) studied in detail the range of the graptolites in the three shallow cores. These graptolite data provide independent chronostratigraphical control refining previous age assignments. The alargada and the hemeri chitinozoan biozones correspond to the mid-Aeronian convolutus graptolite biozone whereas the maclurei chitinozoan biozone extends through the early Telychian guerichi and turriculatus graptolite biozones. The obtusa and the silurica chitinozoan biozones occur in an interval devoid of graptolites are referred to the mid-late Telychian as they do not contain yet typical Sheinwoodian species

Biostratigraphy of Late Ordovician chitinozoans from the QUSAIBA shallow core (NW Saudi Arabia). In : IPC, Bonn, September 2008

International audienceThe QUSAIBA-1 shallow core drilled in NW Saudi Arabia penetrated the Qalibah, Sarah and Qasim formations. The Silurian graptolites and chitinozoans from the Qusaiba Member of the Qalibah Formation have been investigated previously. The present study focuses on the Upper Ordovician part of the core hole. A total of 39 core samples from the glacio-marine Sarah Formation and from the upper part of the Qasim Formation were processed for chitinozoans. Part of the sample set yielded abundant and well-preserved specimens associated with graptolite remains, scolecodonts, acritarchs and cryptospores. Other samples from glacially related sediments contained only a few fragmented chitinozoan vesicles of early Late Ordovician species suggesting reworking. Four different chitinozoan assemblages are identified. The youngest one is restricted to two samples located very close to the gamma ray peak and to the highest evidence of glacial sediments. This assemblage is dominated by Cyathochitina species including C. caputoi, associated with subordinate numbers of Ancyrochitina, Euconochitina and Belonechitina species. This assemblage does not contain the classical Late Ordovician chitinozoan taxa and an early Rhuddanian age is considered possible. The underlying interval is less productive and sometimes virtually barren. This interval yields a second assemblage mainly composed of broken specimens of Tanuchitina, Cyathochitina, and Euconochitina. Most of these forms seem reworked, probably through glacial erosion. The composition changes drastically in the older core samples where abundant Armoricochitina nigerica and Ancyrochitina species are associated with e.g., Euconochitina lepta, Calpichitina lenticularis, Acanthochitina barbata and Desmochitina minor. This constitutes the third recovered chitinozoan assemblage. In addition to the classical components, new species have been observed. This assemblage occurs in pre-glacial strata and is assigned to the late Katian (Ashgill). The oldest processed sample contains Belonechitina, Hercochitina, Conochitina and Euconochitina species. This fourth assemblage, however, lacks A. nigerica and suggests an older age assignment (e.g., late Caradoc of the British chronostratigraphy)

Late Ordovician–earliest Silurian chitinozoans from the Qusaiba-1 core hole (North Central Saudi Arabia) and their relation to the Hirnantian glaciation

International audienceThe continuously cored Qusaiba-1 drilled in North Central Saudi Arabia penetrated successively the Qalibah, Sarah and Qasim formations. Silurian graptolites and chitinozoans from the Qusaiba Member of the Qalibah Formation were previously investigated. The present study focuses on the Upper Ordovician and lowermost Silurian parts of the core hole. Part of the sample set yielded abundant and well-preserved specimens associated with eurypterid remains, scolecodonts, acritarchs and cryptospores. Other samples from glacially derived shaly sediments contain only a few fragmented chitinozoan vesicles of Middle and Late Ordovician species indicating reworking through glacial processes. Four different chitinozoan assemblages are identified. The first recorded chitinozoan assemblage is restricted to the deepest processed sample (core 56). It contains Belonechitina cf. robusta, Hercochitina sp. A, and Spinachitina cf. kourneidaensis, and taxa also represented in the second assemblage, e.g., Fungochitina spinifera and Euconochitina species. This first assemblage, however, lacks some of the diagnostic species of the second one (e.g., Acanthochitina barbata, Tanuchitina cf. elongata) and suggests a slightly older late Katian age. The second recovered chitinozoan assemblage is documented in the upper part of the Qasim Formation (cores 49–56) and in the deeper core samples referred to the lower part of the Sarah Formation (cores 45–47 from the disrupted facies of the Sarah Sandstone Member). It contains abundant and well-preserved Armoricochitina nigerica and Ancyrochitina merga associated with e.g., Euconochitina lepta, Calpichitina lenticularis, A. barbata and Desmochitina typica. In addition to these classical components, new species have been observed. This assemblage occurring in pre-glacial as well as in glacially related strata is assigned to the late Katian–earliest Hirnantian (Ashgill). The overlying productive interval, corresponding to the Baq'a Shale Member, is less productive and sometimes virtually barren of chitinozoans. This interval yields a third assemblage mainly composed of broken specimens of Angochitina cf. curvata, C. lenticularis, Cyathochitina sp., D. typica, F. spinifera, Tanuchitina fistulosa, Euconochitina gr. lepta, and even Siphonochitina formosa. Most of these forms are interpreted as glacially reworked. The youngest assemblage is restricted to three samples (core 27), which are located very close to a Silurian gamma ray peak and above the highest evidence of glacial sediments represented by the Sarah Formation. This forth assemblage contains extremely abundant chitinozoans coexisting with a few graptolite remains and inarticulate brachiopods. This chitinozoan assemblage is dominated by Cyathochitina caputoi, which is associated with Belonechitina pseudarabiensis and subordinate numbers of Ancyrochitina and Euconochitina species. The classical Late Ordovician chitinozoan taxa are no longer present and an earliest Rhuddanian age is proposed.Carbon stable isotope (δ13Cchit.) values have been measured on picked chitinozoan vesicles from the upper part of the Quwarah Member (upper Qasim Formation) and from the basal part of the Qusaiba Member (Qalibah Formation) where they register a shift towards more negative values close to the Gamma ray peak. The timing of the Late Ordovician glaciation and correlation with Hirnantian glacial events recorded in Northern Gondwana regions are briefly discussed based on the recovered chitinozoans.Four new species, i.e., Conochitina rotundata sp. nov., Belonechitina tenuicomata sp. nov., Hercochitina multiansata sp. nov., and Calpichitina bernardae sp. nov. are described, discussed and illustrated. Biometric data are provided for Acanthochitina barbata and for Armoricochitina nigerica