Early Ordovician Research Articles

From oceanic subduction to continental collision: An overview of HP–UHP metamorphic rocks in the North Qaidam UHP belt, NW China

The North Qaidam UHPM belt is unique among “continental type” subduction zones in showing well-preserved subducted oceanic relics that predate continental subduction and collision. We review petrologic, thermobarometric, geochemical and geochronological studies for the Yuka, Luliang Shan, Xitieshan and Dulan terranes in this belt. UHP conditions are demonstrated by coesite inclusions in eclogite and country gneiss, and diamond inclusions from garnet peridotite. The relict subducted oceanic lithology crops out in the Shaliuhe cross-section in Dulan terrane; it originated as the floor of a Paleo-Qilian Ocean that existed between Qaidam and Qilian blocks before the early Ordovician. Whereas most eclogites of this belt are derived from mafic rocks of either a Neoproterozoic continental rift or incipient oceanic basin setting, which were subducted along with continental rocks in the early Paleozoic. The data, especially the two protolith sources for eclogites, demonstrate tectonic evolution of the North Qaidam UHPM belt from oceanic subduction to continental collision.

New insights into the paleobiogeography of the Early Ordovician graptolite fauna of northwestern Argentina

A multivariate analysis was used to determine the faunistic relationships of the northwestern Argentinean graptolite faunas of Floian age to faunas from Baltica, Avalonia, Laurentia and SW China. A statistical analysis at the species level of the five geographic regions for the Lower Floian was performed with the classic Jaccard's index. The resulting affinity dendrogram shows stronger relationships between Early Floian graptolite faunas of northwestern Argentina and those from Baltica, with less obvious similarities to Great Britain (Avalonia) faunas and only weak affinities to North American (Laurentia) and Chinese (SW China) graptolite faunas. The statistical analysis confirms the paleobiogeographic relationships previously observed in other areas of the Cordillera Oriental, and supports the hypothesis that during the Early Ordovician, northwestern Argentina was located at middle to high latitudes, corresponding to the Atlantic Faunal Realm of cold water graptolite biofacies. The studied graptolite material from the Quinilicán and Agua Chica sections is preserved in shales and siltstones interbedded with fine to medium-grained sandstones corresponding to the lower part of the Acoite Formation. The biostratigraphic implications of the associated graptolites are briefly discussed, and Trichograptus dilaceratus (Herrmann), Acrograptus gracilis (Törnquist), Expansograptus latus (T.S. Hall) and Corymbograptus v-fractus tullbergi (Monsen) are described from the Argentine Cordillera Oriental for the first time.

Early Ordovician mitrates and a possible solute (Echinodermata) from the western United States

Two new kirkocystid mitrate stylophorans (Echinodermata, Homalozoa) and a new possible solute (Echinodermata, Homalozoa) are described from the Early Ordovician of the western United States. The mitrates are among the earliest members of their clade to appear near the beginning of the Ordovician Radiation.Anatifopsis ninemilensisnew species comes from the Ninemile Shale in central Nevada and the McKelligon Canyon Formation in west Texas.Anatifopsis fillmorensisnew species comes from the middle Fillmore Formation in western Utah and a Ninemile Shale equivalent limestone bed in southern Nevada. The possible soluteDrepanocystis dubiusnew genus new species from the lower Wah Wah Limestone in western Utah, shows unusual morphology with an elongate theca and a long arm shaped like a sickle.

Evaporate deposits and salt diapirs of the Urvantsev trough on the North Kara Sea

The Evaporate Urvantsev Trough was recognized on the basis of new geophysical seismic data. The trough is filled with Lower Paleozoic deposits, with a regional unconformity located at the Devonian-Carboniferous boundary. Salt diapirs of various shapes are a typical peculiarity of the trough. By analogous wtih the sections of Severnaya Zemlya and the Timan-Pechora basin, the age of the salts is defined as Early Ordovician.

전기고생대 태백산분지 영월층군의 순차층서 연구를 통한 고지리적 추론

전기 고생대 태백산분지 영월층군은 탄산염-규산쇄설성 퇴적암 복합체로서 하부로부터 삼방산층, 마차리층, 와곡층, 문곡층, 영흥층으로 이루어져있다. 영월층군에 대한 순차층서학적 분석에 따르면 중기 캠브리아기에 일어난 범람에 의해 최하부의 규산쇄설성 사질 퇴적암이 우세한 삼방산층이 퇴적되었다. 이어지는 후중기 캠브리아기 ~ 전후기 캠브리아기에 지속적으로 발생한 빠른 해수면 상승으로 마차리층 하부에는 셰일, 입자암, 각력암층을 협재한 사면 혹은 심부 램프 시퀀스가 형성되었다. 후기 캠브리아기 동안 지속된 해수면 상승은 실질적인 퇴적가능공간을 창출하였고, 조하대 환경에 탄산염 퇴적물 공장이 만들어졌으며, 탄산염 대지에는 마차리층을 구성하는 탄산염암이 우세한 조하대 시퀀스가 형성되었다. 마차리층 상부의 와곡층은 후후기 캠브리아기의 완만한 해수면 상승국면에서 만들어진 탄산염 램프 시퀀스로 해석되며, 퇴적 당시에는 리본 탄산염암과 탄산염 역암을 포함하는 이회암으로 구성되었던 것으로 보인다. 와곡층은 퇴적직후에 일차적으로 캠브리아기와 오르도비스기 사이의 해수면 하강국면에서 불안전 백운암화 과정을 거치고, 후에 심부 매몰 속성환경에서 광범위한 백운암화 작용을 받은 것으로 해석된다. 전기 오르도비스기에도 세계적인 해수면 상승과 해침은 지속되었으며, 영월층군의 조하대 램프 퇴적환경은 그대로 유지되어 탄산염 역암층을 협재하는 석회이암과 이회암이 교호하는 전형적인 램프 시퀀스인 문곡층이 형성되었다. 문곡층은 중기 오르도비스기에 퇴적된 것으로 알려진 영흥층에 덮여 있다. 영흥층은 주로 윤회층리를 보이는 조석대지 탄산염암으로 이루어져 있으며, 문곡층의 최상부에서 조하대 퇴적환경이 영흥층의 조석대지 퇴적환경으로 변화한다. 세계적 1차 규모 순차 경계면인 소크(Sauk)와 티피카누(Tippecanoe) 시퀀스의 경계는 영흥층 중부에서 관찰되는 최소퇴적가능공간 부근에서 인지된다. 중기 오르도비스기 초기의 세계적 해수면 하강과 이어지는 해수면의 급격한 상승은 영흥층의 전반적인 상향 천해화 윤회층의 전진퇴적체를 형성하였다. 영월층군이 퇴적된 영월 탄산염 대지의 상대적 해수면 변동곡선을 복원해 보면 같은 태백산 분지의 태백층군이 퇴적된 태백 탄산염 대지의 해수면 변동 곡선과 유사함을 확인할 수 있다. 이것은 두 개의 탄산염 대지가 유사한 조 구조적 운동 역사를 갖는다는 것을 의미하며, 이러한 유사성은 영월층군이 형성된 영월 탄산염 대지가 비록 태백층군이 퇴적된 태백 탄산염 대지와 상이한 퇴적시스템을 갖기는 하지만 상대적으로 가까운 지역에 속해 있었음을 암시한다. 퇴적층서 분석결과에 따르면 영월 탄산염 대지는 태백 탄산염 대지에 비해 상대적으로 열린 천해 환경이었을 것으로 추측된다. 고생대 후기와 중생대 전기에 걸쳐 발생한 북중국지괴와 남중국지괴의 충돌 시기에 영월 탄산염 대지와 태백 탄산염 대지가 복잡한 이동과정을 거쳐 현재의 태백산 분지에 모이게 된 것으로 해석된다. The Yeongweol Group is a Lower Paleozoic mixed carbonate-siliciclastic sequence in the Taebaeksan Basin of Korea, and consists of five lithologic formations: Sambangsan, Machari, Wagok, Mungok, and Yeongheung in ascending order. Sequence stratigraphic interpretation of the group indicates that initial flooding in the Yeongweol area of the Taebaeksan Basin resulted in basal siliciclastic-dominated sequences of the Sambangsan Formation during the Middle Cambrian. The accelerated sea-level rise in the late Middle to early Late Cambrian generated a mixed carbonate-siliciclastic slope or deep ramp sequence of shale, grainstone and breccia intercalations, representing the lower part of the Machari Formation. The continued rise of sea level in the Late Cambrian made substantial accommodation space and activated subtidal carbonate factory, forming carbonate-dominated subtidal platform sequence in the middle and upper parts of the Machari Formation. The overlying Wagok Formation might originally be a ramp carbonate sequence of subtidal ribbon carbonates and marls with conglomerates, deposited during the normal rise of relative sea level in the late Late Cambrian. The formation was affected by unstable dolomitization shortly after the deposition during the relative sea-level fall in the latest Cambrian or earliest Ordovician. Subsequently, it was extensively dolomitized under the deep burial diagenetic condition. During the Early Ordovician (Tremadocian), global transgression (viz. Sauk) was continued, and subtidal ramp deposition was sustained in the Yeongweol platform, forming the Mungok Formation. The formation is overlain by the peritidal carbonates of the Yeongheung Formation, and is stacked by cyclic sedimentation during the Early to Middle Ordovician (Arenigian to Caradocian). The lithologic change from subtidal ramp to peritidal facies is preserved at the uppermost part of the Mungok Formation. The transition between Sauk and Tippecanoe sequences is recognized within the middle part of the Yeongheung Formation as a minimum accommodation zone. The global eustatic fall in the earliest Middle Ordovician and the ensuing rise of relative sea level during the Darrwillian to Caradocian produced broadly-prograding peritidal carbonates of shallowing-upward cyclic successions within the Yeongheung Formation. The reconstructed relative sea-level curve of the Yeongweol platform is very similar to that of the Taebaek platform. This reveals that the Yeongweol platform experienced same tectonic movements with the Taebaek platform, and consequently that both platform sequences might be located in a body or somewhere separately in the margin of the North China platform. The significant differences in lithologic and stratigraphic successions imply that the Yeongweol platform was much far from the Taebaek platform and not associated with the Taebaek platform as a single depositional system. The Yeongweol platform was probably located in relatively open shallow marine environments, whereas the Taebaek platform was a part of the restricted embayments. During the late Paleozoic to early Mesozoic amalgamations of the Korean massifs, the Yeongweol platform was probably pushed against the Taebaek platform by the complex movement, forming fragmented platform sequences of the Taebaeksan Basin.

Biofacies and palaeoenvironments of conodonts in Cambro‐Ordovician sequences of the Quebrada de Humahuaca, Cordillera Oriental of Jujuy, Argentina

The taxa frequencies and cluster analysis of 10 837 conodonts recovered from the Santa Rosita Formation on the eastern flank of the Quebrada de Humahuaca, Cordillera Oriental of Argentina, were carried out for palaeoenvironmental interpretations. The first type of analysis allowed us to identify three conodont biofacies: Variabiloconus‐Teridontus, Utahconus‐Acanthodus and Tilcarodus‐Drepanoistodus, and the cluster analysis helps define respective sub‐biofacies. The first biofacies is not constrained to a particular environment, the second biofacies, which is characterized by typical Laurentian genera, is related to sandstones from shallow‐water environments, while the third one is better represented in deeper water siliciclastic lithofacies. A nektobenthic mode of life is suggested for Utahconus and Acanthodus, but a pelagic behaviour is apparent for the rest of the taxa, well adapted to off‐shore biotopes in particular cases (proto‐ and paraconodonts). The faunal composition reveals a mixture of Baltic and Laurentian taxa, as well as endemic forms that define the Southwestern Gondwana Province from the Cold Domain in the Shallow‐Sea Realm. The presence of typical species from low latitudes (e.g. Australia) confirms the installation of an oceanic perigondwanian corridor, which was open to faunal migration during the late Cambrian–early Ordovician. Copyright © 2012 John Wiley & Sons, Ltd.

First discovery of Early PalaeozoicBathysiphon(Foraminifera) – test structure and habitat of a ‘living fossil’

AbstractThe giant, agglutinated foraminiferanBathysiphonSars, previously Triassic–Recent, occurs in much older sedimentary rock (Early Ordovician, late early Tremadocian) of Avalonia. The genus extends back toc. 485 Ma based on its discovery in platform mudstone of the Chesley Drive Group in Cape Breton Island, Nova Scotia. Elongate (up to 60 mm), epibenthicBathysiphontubes occur in wave-rippled, green-grey mudstone with a low diversity, probably dysoxic fauna. The mudstone is coeval with and lithologically similar to the Shineton Formation in Shropshire and the Welsh Borderlands. Scanning microscopy of theBathysiphonwalls shows imbricated mica grains that parallel the long axis of the tests. The lumen has a mélange of packed sediment grains, some of which are spherical structures of siliciclastic mud studded with tetrahedral pyrite crystals. A felt-like, agglutinated test, a lumen packed with spherical structures (probable stercomata) and the domal ends of some specimens are consistent with modernBathysiphon. This report is the first time that cytoplasmic activity and stercomata formation have been used to refer fossil protists to a modern group.Bathysiphondiffers from the Cambrian foraminiferanPlatysolenitesPander, which has an open lumen without stercomata, but support a comparable, sediment deposit-feeding niche.Bathysiphonis truly a ‘living fossil’, with a mode of test construction, cytoplasmic activity that formed stercomata and a niche unchanged for almost 500 million years. Foraminiferans have not been found prior to the Cambrian Period, and the Early Cambrian appearance of agglutinated foraminiferans is part of the radiation of Phanerozoic communities.

The Haraa Gol terrane in the western Hentiyn Mountains (northern Mongolia): geochemistry, geochronology, and geodynamics

According to geological, petrological, geochemical, and geochronological studies, the Haraa Gol terrane in the western Hentiyn Mts. is dominated by two rock assemblages of different ages, associated with the initiation and development of the island arcs and marginal spreading seas of the Mongol–Okhotsk Ocean. The Late Cambrian, Early Ordovician, and Middle Ordovician were marked by the effusion of basalt and basaltic andesite and the formation of gabbro and gabbro-dolerite in back-arc spreading basins. In the Late Silurian–Devonian, after a short pause, tectonomagmatic processes were activated, with the formation of differentiated island-arc volcanics, gabbro, and granitoids. Their absolute 40Ar–39Ar age is given in the paper. The model age of the TNd(DM) protolith of the Haraa Gol igneous rocks corresponds to the composition of the Mesoproterozoic juvenile crust.

Peduncular attached secondary tiering acrotretoid brachiopods from the Chengjiang fauna: Implications for the ecological expansion of brachiopods during the Cambrian explosion

Brachiopods are usually thought to contribute little to the tiering complexity from the Paleozoic to the Recent, mainly due to the fact that their recent representatives live primarily in lower tiers directly above or below the water–sediment interface. Here we present the first and oldest record of varied levels of secondary tiering in minute brachiopods attached by exceptionally preserved thread-like pedicles around the branched fronds of the algae-like Malongitubus kuangshanensis Hu, 2005. The specimens illustrated herein were recovered from the Chengjiang fauna (Series 2, Stage 3) in the Lower Cambrian Heilinpu Formation at the Kuangshan section in Malong County, Yunnan Province, southern China. The micro-morphology and oval outline of the attached brachiopods demonstrate that they can be assigned to acrotretoid brachiopods (Linguliformea, Lingulata, Acrotretoidea), described here as Kuangshanotreta malungensis gen. et sp. nov. This is the first report on the occurrence of acrotretoid brachiopods in the Lower Cambrian muddy deposits from southern China. The posterior margins of the Kuangshanotreta shells are invariably either in direct contact with, or directed towards, and then in situ attached to the algal frond of M. kuangshanensis, indicating a secondary tiering in the ecological structure of Early Cambrian brachiopods. The acrotretoid Kuangshanotreta/algae malongitubus association represents both the first and oldest evidence into the enigmatic paleoecology of the diverse acrotretoid linguliformean stock that comprises an important component of the Cambrian evolutionary fauna, and sheds light on medium-high levels of secondary tiering (+5 to +10cm) Cambrian soft substrate suspension-feeding communities. When compared to other Chengjiang brachiopods, the miniature morphology and concomitant weight reduction of the shell of K. malungensis may be a good adaption to a suspended epifaunal tiering life style. We infer that the diversification of micromorphic and miniaturized acrotretoids that occurred from the Middle Cambrian to Early Ordovician may be the result of the increased availability of habitable surfaces provided by high levels of tiering in new types of ecological spaces.

Discovery of an albite gneiss from the Ile de Groix (Armorican Massif, France): geochemistry and LA-ICP-MS U–Pb geochronology of its Ordovician protolith

For the first time, an albite orthogneiss has been recognised and dated within the HP–LT blueschist facies metabasites and metapelites of the Ile de Groix. It is characterised by a peraluminous composition, high LILE, Th and U contents, MORB-like HREE abundances and moderate Nb and Y values. A U–Pb age of 480.8 ± 4.8 Ma was obtained by LA-ICP-MS dating of zircon and titanite. It is interpreted as the age of the magmatic emplacement during the Early Ordovician. Morphologically different zircon grains yield late Neoproterozoic ages of 546.6–647.4 Ma. Zircon and titanite U–Pb ages indicate that the felsic magmatism from the Ile de Groix is contemporaneous with the acid, pre-orogenic magmatism widely recognised in the internal zones of the Variscan belt, related to the Cambro-Ordovician continental rifting. The magmatic protolith probably inherited a specific chemical composition from a combination of orogenic, back-arc and anorogenic signatures because of partial melting of the Cadomian basement during magma emplacement. Besides, the late Devonian U–Pb age of 366 ± 33 Ma obtained for titanite from a blueschist facies metapelite corresponds to the age of the HP–LT peak metamorphism.

Stenian granitoids of the west Kyrgyz Ridge (North Tien Shan): Position, structure, and age determination

In the structure of west Kyrgyz Ridge (North Tien Shan), a great role is played by complexly dis� located Upper Precambrian-Cambrian terrigenouscarbona te and shale strata, as well as by granitoids that comprise several coupled WNWstriking synforms and anti forms, the largest of which is the Makbal antiform. Southeast of the core of this antiform, granitoids comprise the large Kara Dzhilga massif and several massifs that are of lesser size and have tectonic correlations with the hosting terrigenouscarbonate strata. In the Kara Dzhilga massif, the rocks of three penetration phases are distinguished; contacts between rocks are often of tectonic character. The early phase is presented by monzonite and monzodiorite; the main one, by largepor� phyric biotitic granites; and the additional one, by aplitic granites and pegmatites. By the chemical compo� sition, granites of Kara Dzhilga massif of the main phase correspond to subalkaline granites of highpotas� sium calcalkali series. The age of their crysta llization (zirconbased U-Pb method) is 1131 ± 4 Ma (Ste� nian). The formation of Stenian granitoids in the North Tien Shan may be related to development of Grenville fold belts, whose fragments were identified in the units of the Central Asian Belt. Tectonic correla� tions between these granitoids and hosting terrigenouscarbonate strata appeared as a result of immersion to significant depths and subsequent exhumation into the upper crustal horizons in the Early Ordovician.

A critical graptolite correlation into the Lower Ordovician of Gondwana

SUMMARY The early Ordovician was a time of maximum faunal provinciality. Correlation between strata deposited at low palaeolatitudes and those deposited close to the Ordovician pole in Gondwana is accordingly particularly difficult. The graptolite used to define the base of the internationally accepted Floian Stage, Tetragraptus approximatus , is essentially a lower latitude species. The discovery of a closely related and coeval species, T. akzharensis , in the early Ordovician of the Montagne Noire is reported. It occurs at the same level as the acme of the trilobite Taihungshania miqueli . The new record permits a direct tie with the earliest part of the Floian Stage and the shelly faunas typical of much of western Gondwana.

Sedimentary facies and trilobite and conodont faunas of the Ordovician Rann Formation, Ras Al Khaimah, United Arab Emirates

ABSTRACTThe Rann Formation occurs as unique ‘exotic’ rafts in front of the Semail Ophiolite in the northern Oman Mountains. Its Ordovician age has been poorly constrained and it is often associated with the Ayim rock unit, which has been considered Devonian, Carboniferous or Ordovician by different workers. Here we present new trilobite and conodont evidence for the Ordovician ages of the three members of the Rann Formation, which includes the Ayim. The members are readily distinguishable on sedimentological and faunal grounds.The Lower Member comprises shales, quartzitic sandstones and thin fossiliferous shell beds. Large Cruziana are common, as is lingulacean debris and, at several horizons, possible hyolithids. Assemblages of graptolites, acritarchs, trilobites (Neseuretus cf. arenosus and Taihungshania cf. miqueli) and conodonts (Baltoniodus sp., Drepanodus arcuatus, Drepanoistodus sp. and Protopanderodus sp., Scolopodus sp.) are considered to range in age from Floian to early Dapingian, late Early Ordovician. The Ayim Member (previously formation) consists of fossiliferous shales and griotte-like nodular bioclastic limestones. The member is distinguished by its red colour and by numerous orthoconic nautiloids. Conodont faunas (Complexodus cf. originalis, Eoplacognathus protoramosus, Dapsilodus sp., Cornuodus sp. and Panderodus sp.) imply a late Darriwilian, Middle Ordovician age. The Upper Member consists of siltstones and sandstones generally lacking bioturbation and with rare shell beds and faunas. Trilobites (Deanaspis goldfussii seftenbergi, Vietnamia teichmulleri and Dreyfussina taouzensis) and chitinozoans are interpreted to indicate an early-middle Katian, Late Ordovician age.The three members represent shallow-marine deposits on a continental shelf subject to changing sand supply, storm-wave activity and sea-bottom oxygenation. The three periods of deposition, Floian – early Dapingian, late Darriwilian and early – middle Katian, correspond to highstands of Paleo-Tethys that also flooded interior Oman and Arabia. The limited burial and lack of metamorphism of the Rann is remarkable given its proximity to the Semail Ophiolite and to subduction related metamorphic rocks occurring nearby.

Early Ordovician (Floian) ostracods from the Cordillera Oriental, Northwest Argentina

The ostracod fauna from the Floian (Lower Ordovician) strata of the Cordillera Oriental, Argentina (Acoite Formation) are documented. One new genus Acoitella and four species are recognized, two of which are new (Nanopsis victoria sp. nov. and Conchoprimitia frequens sp. nov.). The diversity of the Early Ordovician ostracod faunas of the Central Andean Basin is evaluated. Compared with other regions, the ostracod diversity, at generic and specific level, of the Cordillera Oriental was comparatively high during the Tremadocian (nine species recorded) and comparatively low during the Floian (four species recorded). The taxonomic composition of the fauna is similar during the entire Early Ordovician of the Cordillera Oriental, where it displays a high percentage of soanellids. The Floian assemblages are characterized by the dominance of the eridostracan C. frequens sp. nov. with a high percentage of nondimorphic palaeocopids: Nanopsis and two genera of the Soanellidae family (Saltite and Acoitella gen nov.). An ostracod shell bed is described and interpreted to be a parautochthonous assemblage, as a result of transport within the same habitat. The composition of the fauna, with the presence of Saltite and Nanopsis, shows significant affinities with East Gondwana. Copyright © 2011 John Wiley & Sons, Ltd.

Long‐lived orogenic construction along the paleo‐Pacific margin of Gondwana (Deep Freeze Range, North Victoria Land, Antarctica)

The paleo‐Pacific margin of Gondwana records a prolonged history of convergence during the Cambrian–Early Ordovician Ross‐Delamerian and the Middle Ordovician–Early Silurian Lachlan orogenies. This study describes structure, petrology, and geochronology of a set of NW‐SE striking ductile shear zones that crosscut the Ross age, Early Cambrian granitoid rocks of the Wilson Terrane in the Deep Freeze Range (North Victoria Land, Antarctica). The shear zones developed under amphibolite facies metamorphic conditions (650–700°C and 0.5–0.7 GPa) and show a systematic top‐to‐the‐NE sense of shear. The shear zone activity interferes with emplacement of late, subhorizontal leucocratic dikes and combined U‐(Th)‐Pb (zircon and monazite) and 40Ar‐39Ar (biotite and phengite) geochronology constrains the shearing event at ∼470 Ma, with the sheared granite yielding U‐Pb zircon crystallization ages of ∼508 Ma. The reconstructed P‐T path followed by the granite protoliths indicates an anticlockwise trajectory, suggesting the synshearing amphibolite metamorphism was associated with the burial of an early formed, Ross continental crustal section. These new findings are interpreted as evidence of a renewed, Ordovician episode of orogenic construction at the paleo‐Pacific margin of Gondwana that predated the onset of the Lachlan orogeny in the region. A polycyclic reactivation of the Ross age Wilson Terrane of North Victoria Land is suggested, which is used to propose a unitary framework for the space‐time transition from the Ross‐Delamerian to the Lachlan orogeny along the proto‐Pacific active margin of Gondwana.

Phylogenetic and Biogeographic Analysis of Sphaerexochine Trilobites

BackgroundSphaerexochinae is a speciose and widely distributed group of cheirurid trilobites. Their temporal range extends from the earliest Ordovician through the Silurian, and they survived the end Ordovician mass extinction event (the second largest mass extinction in Earth history). Prior to this study, the individual evolutionary relationships within the group had yet to be determined utilizing rigorous phylogenetic methods. Understanding these evolutionary relationships is important for producing a stable classification of the group, and will be useful in elucidating the effects the end Ordovician mass extinction had on the evolutionary and biogeographic history of the group.Methodology/Principal FindingsCladistic parsimony analysis of cheirurid trilobites assigned to the subfamily Sphaerexochinae was conducted to evaluate phylogenetic patterns and produce a hypothesis of relationship for the group. This study utilized the program TNT, and the analysis included thirty-one taxa and thirty-nine characters. The results of this analysis were then used in a Lieberman-modified Brooks Parsimony Analysis to analyze biogeographic patterns during the Ordovician-Silurian.Conclusions/SignificanceThe genus Sphaerexochus was found to be monophyletic, consisting of two smaller clades (one composed entirely of Ordovician species and another composed of Silurian and Ordovician species). By contrast, the genus Kawina was found to be paraphyletic. It is a basal grade that also contains taxa formerly assigned to Cydonocephalus. Phylogenetic patterns suggest Sphaerexochinae is a relatively distinctive trilobite clade because it appears to have been largely unaffected by the end Ordovician mass extinction. Finally, the biogeographic analysis yields two major conclusions about Sphaerexochus biogeography: Bohemia and Avalonia were close enough during the Silurian to exchange taxa; and during the Ordovician there was dispersal between Eastern Laurentia and the Yangtze block (South China) and between Eastern Laurentia and Avalonia.

Emplacement and exhumation of the Kuznetsk‐Alatau basement (Siberia): implications for the tectonic evolution of the Central Asian Orogenic Belt and sediment supply to the Kuznetsk, Minusa and West Siberian Basins

Terra Nova, 23, 248–256, 2011AbstractNew geochronological data [zircon U/Pb, titanite fission‐track (TFT) and apatite fission‐track (AFT) dating and apatite (U‐Th‐Sm)/He thermochronology] and thermal history modelling yield constraints on the development of the granitoid basement of the Kuznetsk‐Alatau Mountains, southern Siberia. The final stages of magmatism in the Kuznetsk‐Alatau palaeo‐island‐arc are Late Cambrian, and collision of the arc with Siberia occurred in the Early Ordovician. The basement was exhumed by the Early Devonian. Continuous Devonian–Early Triassic sedimentation filled the adjoining Kuznetsk and Minusa basins and buried (and re‐heated) the Kuznetsk‐Alatau basement. After initial Pangaea break‐up and Siberian flood‐basalt magmatism, the basement reached TFT and AFT retention‐temperatures in the Middle Triassic and Early Cretaceous, respectively, during denudation‐induced cooling.Terra Nova, 00, 1–9, 2011

The oldest bryozoan reefs: a unique Early Ordovician skeletal framework construction

Adachi, N., Ezaki, Y. & Liu, J. 2011: The oldest bryozoan reefs: a unique Early Ordovician skeletal framework construction. Lethaia, Vol. 45, pp. 14–23. The oldest bryozoan reefs occur in the Lower Ordovician (late Tremadocian) Fenhsiang Formation of the Three Gorges area, South China. These reefs show a unique type of bryozoan (Nekhorosheviella) framework, and were constructed as follows: the first stage involved colonization by lithistid sponges, which acted as a baffler to trap sediments, providing bryozoans with a stable substrate for attachment. The bryozoans then grew as an encruser on the surfaces of sponges, showing a preferential downwards and lateral growth within the sponge scaffolding to avoid biological and physical disturbance. Finally, these biotic combinations among skeletal organisms formed a rigid, three-dimensional skeletal framework. This mode of bryozoan growth in association with lithistid sponges is remarkable and unique in its growth direction, and the appearance of such reefs, just prior to the widespread development of skeletal-dominated reefs as part of the Great Ordovician Biodiversification Event, provides an excellent example of the earliest attempts by skeletal organisms to form frameworks by themselves. This find significantly enhances our understanding of the initial stages of skeletal-dominated reef evolution and the ensuing development of reefs during the Middle–Late Ordovician. □Bryozoa, Early Ordovician, lithistid sponge, Ordovician radiation, reef.

Assembly of Pampia to the SW Gondwana margin: A case of strike-slip docking?

Different hypotheses have been proposed to account for the geologic evolution of the southwestern margin of Gondwana in the Early Paleozoic, involving accretion and displacement of different terranes in a protracted convergent margin. In order to constrain and understand the kinematic and paleogeographic evolution of the Pampia terrane a paleomagnetic study was carried out in different Cambrian to Devonian units of the Eastern Cordillera (Cordillera Oriental) and the Interandean Zone (Interandino) of NW Argentina. Paleomagnetic poles from the Campanario Formation (Middle to Upper Cambrian): 1.5°N 1.9°E A95=9.2° K=37.46N=8; and Santa Rosita Formation (Lower Ordovician): 8.6°N 355.3°E A95=10.1° K=26.78 n=9, representative of the Pampia terrane, are interpreted to indicate a Late Cambrian significant displacement with respect to the Río de la Plata and other Gondwana cratons. A model, compatible with several geological evidences that explains this displacement in the framework of the final stages of Gondwana assembly is presented. We propose a simple dextral strike-slip kinematic model in which Pampia and Antofalla (−Arequipa?) blocks moved during Late Cambrian times from a position at the present southern border of the Kalahari craton into its final position next to the Rio de la Plata craton by the Early Ordovician.

EARLY ORDOVICIAN SHIFT IN REEF CONSTRUCTION FROM MICROBIAL TO METAZOAN REEFS

The Ordovician is a period when novel reef ecosystems appeared along with new reef constructors and skeletal-dominated reefs. The Lower Ordovician (late Tremadocian) Fenhsiang Formation of the Three Gorges area in South China contains the oldest known bryozoan reefs (lithistid sponge–bryozoan and bryozoan–pelmatozoan reefs) alongside lithistid sponge–microbial reefs. The latter are characterized by the dominance of microbialites that encrusted and bound the frame-building sponges and inter-sponge sediments. In contrast, the lithistid sponge–bryozoan and bryozoan–pelmatozoan reefs are generally characterized by bryozoans that encrusted the frame-building sponges or pelmatozoans and grew to fill the inter-frameworks. These sponges and pelmatozoans did not construct the rigid frameworks unaided; their association with bryozoans enabled the development of small skeletal-dominated reefs with rigid frameworks. Skeletal-dominated reefs, for which frame-constructing and encrusting roles are conspicuous, were largely unknown before the Early Ordovician. The appearance of skeletal organisms (specifically colonial, encrusting bryozoans) enabled the development of skeletal-dominated reefs, which were pioneers in the rise of Middle–Late Ordovician reefs. The Early Ordovician establishment of skeletal-dominated reefs at the earliest stages of the Great Ordovician Biodiversification Event would have created novel niches and biological interactions that further promoted the evolution of reef-building and -dwelling organisms, as well as ensuing reef ecosystems.