Early Tremadocian Research Articles

Fryxellodontus inornatus (Conodonta) and associated conodonts from the Furongian (upper Cambrian) of the Cordillera Oriental, Argentina

The conodont biostratigraphy of the middle-upper Lampazar Formation exposed at El Angosto del Moreno area, Cordillera Oriental of Jujuy, Argentina is analyzed. This 150 m thick unit overlies the Padrioc Formation (late Cambrian) and underlies the Cardonal Formation (late Cambrian–early Tremadocian). The unit is composed of pelites interbedded with sandstones and calcarenites and occasional coquinas, which were deposited primarily in offshore environments. Two calcarenite samples (3200 g) yielded 599 conodont elements, corresponding to 35 species of the genera Coelocerodontus, Cordylodus, Eoconodontus, Eodentatus, Fryxellodontus, Furnishina, Granatodontus, Hirsutodontus, Phakelodus, Proconodontus, Prooneotodus, Prosagittodontus, Teridontus, and Anellodontus n. gen. The recovered specimens are well preserved, exhibiting a Color Alteration Index (CAI) value of 1.5–2, which represents overburden palaeotemperatures between 50°C and 140°C. The conodont association corresponds to the upper part of the Cordylodus proavus Zone, indicating a late Furongian age for the sample levels. This contribution reports the first conodont record of Fryxellodontus inornatus and F. lineatus species, the genus Eodentatus, and a new genus Anellodontus n. gen. in South America.

Ordovician plate tectonic and palaeogeographical maps

Abstract Plate tectonic and palaeogeographical maps are presented for nine time intervals during the Ordovician: early Tremadocian (485 Ma), late Tremadocian (480 Ma), early Floian (475 Ma), earliest Dapingian (470 Ma), early Darriwilian (465 Ma), late Darriwilian (460 Ma), middle Sandbian (455 Ma), middle Katian (450 Ma) and early Hirnantian (445 Ma). A plate model was used to predict the changing age of the ocean floor and, hence, the changing volume of the ocean basins and the resulting degree of continental flooding. The maps show the locations of active plate boundaries (i.e. subduction zones, mid-ocean rifts, transform faults, island arcs, back-arc basins and collision zones) and the age of the ocean floor. During the Ordovician there were six continental plates (Gondwana, Laurentia, Baltica, Cathaysia, Avalonia and Cuyania) and six oceanic plates (Iapetus, Ran, Kipchak, and Panthallasa 1, 2 and 3), as well as several poorly constrained back-arc basins (Zealandia, Alexander and Farwell). The palaeogeographical maps also illustrate the ancient distribution of mountains, land, shallow seas and deep ocean basins. Sea level was high during the Early Ordovician, fell during the middle Ordovician and rose again during the Late Ordovician. A precipitous fall in sea level occurred at the end of the Ordovician due to the short-lived Hirnantian Ice Age.

Using Constrained Optimization (CONOP) to examine Ordovician graptolite distribution and richness from the Central Andean Basin and their comparison with additional data from North America and Baltoscandia

Ordovician graptolite fossils represent one of the first abundant, well-documented records of marine zooplankton. An accurate assessment of their stratigraphic ranges, biogeography, and taxon richness is crucial to studies on the Great Ordovician Biodiversity Event. Unfortunately, our understanding of graptolite richness from high paleo-latitudes is poor and recent work has demonstrated that many high paleo-latitude taxa are often misidentified endemic species. Hence, high paleo-latitude localities likely contain a hidden graptolite richness.This work uses CONOP9 to construct a range chart and correlation model from the graptolite occurrences at thirty-eight Ordovician stratigraphic sections from the Central Andean Basin of South America, and parts of Laurentia and Baltoscandia. Thirteen graptolite biozones spanning the early Tremadocian (Tr1)–middle Dapingian interval (Dp2) are identified. Some taxa in the Central Andean Basin composite exhibit unusual ranges and occur in biostratigraphically unexpected positions in the composite range chart. In order to examine the possible causes of these unusual ranges, sections from Baltica and Laurentia were added to the analyses. These additional data facilitated results that were more consistent with the generally accepted positions of certain taxon.The first raw taxon richness curve of graptolites from the Central Andean Basin obtained with CONOP9 shows a diversification phase in the early Floian that continues to a peak in the late early Floian. A second peak occurs at the beginning of the middle Floian. A decreasing trend is observed from the top of the Didymograptellus bifidus Biozone to the “Isograptus victoriae” Biozone. These results generally agree with the graptolite richness curves published for the same interval in Baltica, Australasia, and the Yangtze region of SW China, but differs from the patterns observed for the Jiagnan and Yiyang regions of SE China. The rarified regional taxon richness curve, obtained with CONOP64, is also in agreement with global curves previously proposed by different authors.

Biodiversity changes of the Ordovician trilobites in the middle Yangtze region of South China

The biodiversity changes of trilobites in the middle Yangtze region of South China in the Great Ordovician Biodiversification Event and the end-Ordovician mass extinction have been analysed. The Ordovician trilobites in this region originated in the early Tremadocian and radiated from the late Floian to the early Katian. Two peaks of diversity appeared in the late Dapingian to the early Darriwilian and the early Katian. The extinction occurred in the late Katian, followed by a survival period in the Hirnantian. The Whiterock fauna began to surpass the Ibex fauna during the early Katian and were dominant by the late Katian to the Hirnantian, indicating that Cambrian Evolutionary Fauna was gradually replaced. The study once again confirms the view that the effects of factors such as sea level, climate, oxygen content, nutrient supply, and tectogenesis differ temporally and to different degrees in different regions, resulting in differences in the vertical sequence and horizontal combination of ecosystems, which is the reason for the different evolution patterns of Ordovician trilobites in different regions.

Early–Middle Ordovician brachiopod diversification in the middle Yangtze region of South China

The evolution of the Early–Middle Ordovician brachiopod diversity in the middle Yangtze region of South China has been analyzed. The brachiopods in this region originated in the early Tremadocian and radiated from the late Floian to the late Dapingian. The composition of rhynchonelliformean brachiopods underwent major changes in the late Floian, and endemic genera migrated from the upper Yangtze region to replace the cosmopolitan genera. A global comparison of the brachiopod diversity curves of South China, Laurentia, and Baltica reveals that (1) the processes and scales of brachiopod radiation in different paleocontinents were distinct; (2) unlike the global evolution of graptolites and chitinozoans, the correlation between brachiopod diversification and regional habitat change was more obvious, and the impact of habitat heterogeneity was reflected not only between plates but also in the interior of the plates, such as the closely connected middle and upper Yangtze regions of South China; and (3) geographical isolation was a key factor in the differential radiation on different paleocontinents.

Tectonic controls on late Cambrian-Early Ordovician deposition in Cordillera oriental (Northwest Argentina)

The western Gondwana margin underwent a complex geodynamic history during the early Paleozoic, and major uncertainties remain as to the role of tectonism in sedimentary dynamics. This study focuses on the lower part Santa Rosita Formation and the coeval Guayoc Chico Group (Cordillera Oriental; Northwest Argentina), ranging from the late Cambrian (Furongian; Age 10) to Early Ordovician (early Tremadocian; Tr1). This stratigraphic interval has been previously interpreted as deposited in an extensional basin to a retro-arc basin without major regional tectonic-induced deformation during its deposition, only recording long-term relative sea-level fluctuations. Four areas (Sierra de Cajas, Angosto del Moreno, Quebrada de Trancas, and Quebrada de Moya) were chosen because they host the most complete and temporally well-constrained stratigraphic sections of the Cordillera Oriental. Throughout the stratigraphic sections, four main facies zones are described and attributed to deposition in estuarine, foreshore-shoreface, delta-front, and offshore environments. Trilobite biozones are used as the biostratigraphic framework. By integrating sedimentary facies analysis, biostratigraphy, and sequence stratigraphy from the four selected sections, a new scenario showcasing the evolution of the basin is proposed. This scenario interprets a tectonically induced deformation during the deposition of the Santa Rosita Formation and the coeval Guayoc Chico Group. The newly acquired sedimentological data show that physiographical changes took place during the Cambrian-Ordovician transition and are expressed in various localities. This major change is recorded in the stratigraphic architecture, where extensive wave-ravinement surfaces and sedimentary hiatus are the result of local, syn-depositional basement uplifts. The initiation of the Puna-Famatinian volcanic arc during the Early Ordovician on the western margin was likely responsible for deformation in the retro-arc basin and the proposed scenario is consistent with the stratigraphic evolution in other areas of the Cordillera Oriental (e.g., Sierra de Mojotoro) and the Sierra de Famatina. Therefore, this study helps to constrain the evolution of the western Gondwana margin during the early Paleozoic, showing changes in the stratigraphic architecture and basin evolution from an extensional to a retro-arc style.

Ziyang Fauna: A slope facies fossil lagerstätte of Early Ordovician in South China

“Great Ordovician Biodiversification Event” (GOBE) is one of the major biological innovation in Phanerozoic. Trilobite is one of the major taxa in Cambrian and Ordovician. In 1998, Adrain divided the global Ordovician trilobites into two groups, i.e. Ibex Fauna and Whiterock Fauna, and subsequently, the former was subdivided into Ibex I and Ibex II faunas in 2004. The Ordovician radiation of the trilobites was associated with temporal and spatial expansion of the Whiterock Fauna and contraction of the Ibex Fauna. Ibex Fauna is also known as the Cambrian Evolutionary Fauna, which originated at Cambrian, had high diversity and differentiation soon, decline rapidly in Early Ordovician, and went extinct in Late Ordovician. The Ibex I Fauna raised and flourished only in the Early Tremadocian, while Ibex II fauna declined after the Early Ordovician radiation. Whiterock Fauna is also called the Paleozoic Evolutionary Fauna, which appears in Early Ordovician and shows rapidly radiation and high diversification in Middle Ordovician, and then survived the Ordovician Mass Extinction, evolved into Silurian Fauna, some of them might develop and extend to Devonian, Carboniferous and Permian. In 2006, Zhiyi Zhou dubbed the Ordovician trilobite radiation in the South China Block, suggested that the time of onset and the environmental pattern of the Ordovician trilobite radiation may have varied geographically, and the major radiation was likely a long-sustained event that extended through about 25 Ma of the late Early-early Late Ordovician interval, coinciding with the worldwide expansion and domination of the Whiterock Fauna. So the taxa of Zhou’s Ibex Fauna were proportionally dominant over those of Whiterock Fauna. Ziyang fauna, an Early Ordovician fossil lagerstatte is discovered and reported from Ziyang County, Shaanxi Province. It is Floian (the late Early Ordovician) in age based on the occurrences of Tetragraptus approximatus Nicholson. The fossiliferous rocks are blackish-grey mud limestone and calcareous mudstone of possible slope facies in the Yangzte platform. The fauna is dominated by trilobites, accompanied with graptolites, brachiopods, arthropods and some trace fossils. With relatively high taxonomic diversity, the trilobites in this fauna mainly consist of Seleneceme , Niobe , Nileus , Ampyx , Degamella , Borogothus , Geragnostus and Dividuagnostus , etc assgigned to seven families, of them, Asaphidae, Nileidae, Agnostoidea, Peragnostoidea and Alsataspidiae belong to the Ibex Fauna, and Cyclopygidae and Raphiophoridae belong to Whiterock Fauna. Asaphidae and Nileidae are main components of Ibex I Fauna, and Agnostoidea, Peragnostoidea and Alsataspidiae are components of Ibex II Fauna respectively. Quantitatively, Alsataspidiae and Asaphidae account for more than 75% of the total fauna, Ibex I Fauna were predominent over Ibex II Fauna. Other associated fossils include graptolites of Tetragraptus approximatus , Tetragraptus bigsbyi , Tetragraptus quadribrachiatus , Tetragraptus sp., and Paratemnograptus sp., and brachiopods of Tritoechia , Lingulella , etc. The distinctive composition of Ziyang Fauna holds promise for providing insights into the evolution between Ibex and Whiterock faunas in South China.

Latest Cambrian–earliest Ordovician conodonts and microbrachiopods from northern Victoria Land, Antarctica: Handler Ridge revisited

Conodonts and microbrachiopods of latest Cambrian–earliest Ordovician age are documented from the Handler Formation of northern Victoria Land, Antarctica. Five spot samples from allochthonous limestone clasts collected during expeditions conducted by the Korean Polar Research Institute in 2013–2014 yielded the conodonts Cordylodus proavus, C. lindstromi, Hirsutodontus simplex, and Teridontus nakamurai, associated with four species of brachiopods (two indeterminate acrotretides, a zhanatellid lingulide, and a siphonotretide tentatively referred to Schizambon reticulatus). The four conodont species are distinctive taxa occurring in the widely distributed Cordylodus lindstromi Biozone of the latest Cambrian and the Iapetognathus Biozone at the base of the Ordovician. The allochthonous limestone clasts in the Handler Formation were likely derived from now-lost carbonate shelves developed locally (in the Bowers Arc) or transported from further north. Fossil evidence from the Handler Formation indicates that the final episode of the Ross Orogeny that uplifted and deformed the Robertson Bay Group was no older than early Tremadocian (earliest Ordovician). Striking similarities in fossil content, ages and depositional settings between the limestone olistoliths in the Handler Formation and latest Cambrian–earliest Ordovician autochthonous and allochthonous carbonates of the post-Delamerian Orogen recognized in the Koonenberry Belt (Gnalta Shelf) in far western New South Wales support the depositional model considering the Robertson Bay Group as the synorogenic deposits of the Ross Orogeny.

Early Ordovician (Tremadocian and Floian) graptolites from the Than Sa Formation, northeast Vietnam

AbstractThe lower Palaeozoic marine succession of NE Vietnam accumulated on the South China plate. Despite historical works dating to French colonial times, the stratigraphy and palaeontology of the succession is poorly constrained. Chief amongst the lower Palaeozoic lithostratigraphical divisions is the Than Sa Formation, a c. 1200 m thick succession of clastic rocks of Cambrian and early Ordovician age. Newly collected graptolites (including Rhabdinopora? sp. and Tetragraptus approximatus) from the upper part of the formation identify strata assignable to the Tremadocian and Floian stages of the Lower Ordovician. The same succession also probably records the Cambrian–Ordovician boundary. Our analysis identifies one or more intervals of graptolite-bearing laminated mudstones in the upper part of the Than Sa Formation that may be widespread in NE Vietnam during the early Tremadocian.

Broeggeria Walcott and other upper Cambrian and Tremadocian linguloid brachiopods from NW Argentina

The Broeggeria assemblage is a peculiar brachiopod association that has been recorded mainly from Tremadocian black shales of Baltoscandia, the Anglo‐Welsh Basin, Nova Scotia, South Urals and Kazakhstan. Here, we report a low diversity brachiopod association dominated by Broggeria omaguaca n. sp. from upper Tremadocian open‐shelf mudstones of northwestern Argentina. The dark‐grey transgressive muddy interval bearing the Broeggeria assemblage starts with an erosive flooding surface marked by a glauconite‐rich bed containing reworked bioclasts, which overlies storm‐dominated shoreface sandstones. Lithologic and taphonomic evidence indicates that the fauna from the mudstones represents a palaeocommunity inhabiting oxygen‐depleted bottoms. Besides Broeggeria omaguaca, which by far is the more abundant linguloid, the assemblage includes the obolids Palaeoglossa elongata (Harrington) and Libecoviella aff. tilcarensis Benedetto and Muñoz, and the punctate orthide Lipanorthis andinus Benedetto. Associated trilobites are abundant and low diversity, the more common being the large asaphid Asaphellus jujuanus Harrington. Coeval strata exposed in the Santa Victoria area contain rare specimens of Broeggeria omaguaca associated with Leptembolon argentinum Benedetto and Muñoz and Palaeoglossa elongata. Comparable dysoxic black shales of Furongian age bearing the olenid trilobite Parabolina (Neoparabolina) frequens have yielded a monospecific association of Lingulella n. sp. The records of Broeggeria at global scale support its late Cambrian origin in the Anglo‐Welsh Basin (Avalonia) and its rapid migration into the Karatau–Naryn terrane (a part of the Kazakhstanian collage). By the early Tremadocian, Broeggeria spread to Belgium and to the neighbouring Megumia terrane, migrating later (mid‐late Tremadocian) to Baltica, Southern Urals, Kazakhstanian terranes, Perunica and the Central Andean Basin. By the late Floian/early Dapingian, Broeggeria colonized palaeoequatorial deep shelves (Spitsbergen). Copyright © 2017 John Wiley & Sons, Ltd.

Taxonomic and biostratigraphic reappraisal of some early Tremadocian (Ordovician) graptolites from Changde, South China

A diverse fauna of dendroid graptolites and graptoloids occurs in the Tremadocian Madaoyu Formation of the Panjiazui section, near Changde, Hunan Province, South China. The dendroids are Callograptus cylindricus, C. ellipsoideus, C. isolateralis, C. salteri, C. suxianensis, C. taitzehoensis, and Dendrograptus yini; the graptoloids are Anisograptus delicatulus, Anisograptus sp., Adelograptus sp., Staurograptus cf. mojotorensis, Rhabdinopora flabelliformis anglica, R. umbellata, Rhabdinopora sp. 1, and Rhabdinopora sp. 2. All taxa are figured and the selected graptoloid taxa are described. Based on this taxonomic revision, the Tremadocian Rhabdinopora flabelliformis anglica and Adelograptus biozones are recognized in the Panjiazui section, allowing correlation with other graptolitic successions worldwide.

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.

U–Pb ages, geochemistry, and tectonomagmatic history of the Cambro-Ordovician Annidale Group: a remnant of the Penobscot arc system in southern New Brunswick?1This article is one of a series of papers published in this CJES Special Issue: In honour of Ward Neale on the theme of Appalachian and Grenvillian geology.

The Penobscot arc system of the northeastern Appalachians is an Early Cambrian to early Tremadocian (ca. 514–485 Ma) ensialic to ensimatic arc–back-arc complex that developed along the margin of the peri-Gondwanan microcontinent Ganderia. Remnants of this Paleozoic arc system are best preserved in the Exploits Subzone of central Newfoundland. Correlative rocks in southern New Brunswick are thought to occur in the ca. 514 Ma Mosquito Lake Road Formation of the Ellsworth Group and ca. 497–493 Ma Annidale Group; however in the past, the work that has been conducted on the latter has been of a preliminary nature. New data bearing on the age and tectonic setting of the Annidale Group provides more conclusive evidence for this correlation. The Annidale Group contains subalkaline, tholeiitic to transitional, basalts to basaltic andesites, picritic tuffs and calc-alkaline to tholeiitic felsic dome complexes that have geochemical signatures consistent with suprasubduction zone magmatism that was likely generated in a back-arc basin. New U–Pb ages establish that the Late Cambrian to Early Tremadocian Annidale Group and adjacent ca. 541 Ma volcanic rocks of the Belleisle Bay Group in the New River belt were affected by a period of younger magmatism ranging in age from ca. 479–467 Ma. This provides important constraints on the timing of tectonism in the area. A ca. 479 Ma age for the Stewarton Gabbro that stitches the faulted contact between the Annidale and Belleisle Bay groups, demonstrates that structural interleaving and juxtaposition occurred during early Tremadocian time, which closely coincides with the timing of obduction of Penobscottian back-arc ophiolites onto the Ganderian margin in Newfoundland.

Tribes Hill–Rochdale formations in east Laurentia: proxies for Early Ordovician (Tremadocian) eustasy on a tropical passive margin (New York and west Vermont)

AbstractSlow subsidence and tectonic quiescence along the New York Promontory margin of Laurentia mean that the carbonate-dominated Tribes Hill and overlying Rochdale formations serve as proxies for the magnitude and timing of Tremadocian eustatic changes. Both formations are unconformity-bound, deepening–shoaling, depositional sequences that double in thickness from the craton into the parautochthonous, western Appalachian Mountains. A consistent, ‘layer cake’ succession of member-level units of the formations persists through this region. The Tribes Hill Formation (late early Tremadocian, late Skullrockian, late Fauna B–Rossodus manitouensisChron) unconformably overlies the terminal Cambrian Little Falls Formation as the lowest Ordovician unit on the New York Promontory. It was deposited during the strong early Tremadocian, or Stonehenge, transgression that inundated Laurentia, brought dysoxic/anoxic (d/a) slope water onto the shelf and led to deposition of the Schaghticoke d/a interval (black mudstone and ‘ribbon limestone’) on the Laurentian continental slope. The uniform lithofacies succession of the Tribes Hill includes a lower sand-rich member; a middle, dark grey to black mudstone that records d/a in eastern exposures; and an upper, shoaling-up carbonate highstand facies. A widespread (12000+ km2) thrombolitic interval in the highstand carbonate suggests the New York Promontory was rimmed by thrombolites during deposition of the Tribes Hill. Offlap and erosion of the Tribes Hill was followed by the relatively feeble sea-level rise of the Rochdale transgression (new) in Laurentia, and deposition of the Rochdale Formation. The Rochdale transgression, correlated with theKierograptusDrowning Interval in Baltica, marks a eustatic rise. The Rochdale Formation represents a short Early Ordovician interval (early late Tremadocian, middle–late Stairsian,Macerodus dianaeChron). It correlates with a depositional sequence that forms the middle Boat Harbour Formation in west Newfoundland and with the Rte 299 d/a interval on the east Laurentian slope. The Rochdale has a lower carbonate with abundant quartz silt (Comstock Member, new) and an upper, thrombolitic (Hawk Member, new) high-stand facies. Tribes Hill and Rochdale faunas are mollusc-rich, generally trilobite-poor, and have low diversity, Laurentian faunal province conodonts.Ulrichodina rutnikaLanding n. sp. is rare in Rochdale conodont assemblages. Trilobites are also low in diversity, but locally form coquinas in the middle Tribes Hill. The poorly preserved Rochdale trilobites include the bathyuridRandaynia, at least two hystricurid species andLeiostegium.

Carbon-isotope stratigraphy of the Lower Ordovician succession in Northeast Greenland: Implications for correlations with St. George Group in western Newfoundland (Canada) and beyond

The Lower Ordovician sequence on the Ella Ø Island in Northeast (N-E) Greenland is a thick shallow marine platform carbonate sequence (∼ 1415 m thick) and constitutes the major part of the Kong Oscar Fjord Group. It consists, from bottom to top, of the Antiklinalbugt, Septembersø, and Cape Weber formations, which are believed to be respectively coeval with the Watts Bight, Boat Harbour, and Catoche formations of the St. George Group in western Newfoundland, Canada. Samples were collected from outcrops at high-resolution intervals and micritic materials were extracted by microdrilling after screening their petrographic and geochemical criteria to evaluate the degree of preservation. The δ 13C and δ 18O values of well preserved micrite microsamples range from −5.2‰ to 0.5‰ (VPDB) and from −10.3‰ to−6.5‰ (VPDB), respectively. The δ 13C carb profile of the sequence reveals few negative shifts, which vary between ∼ 2 and 4.7‰ and are associated with unconformities/disconformities, thus reflecting the effect of significant sea-level changes. The δ 13C shifts can be correlated with counterparts on the St. George Group and also on the global Lower Ordovician δ 13C profiles around the early Tremadoc (∼ 2.3‰) and late Tremadoc–early Arenig (∼ 4.7‰).

Solving the mystery of crinoid ancestry: new fossil evidence of arm origin and development

Apektocrinus ubaghsinew genus and species is a monospecific taxon assigned to the new family Apektocrinidae based on additional preparation of a single previously studied specimen.Apektocrinusis among the oldest known crinoids (Early Tremadoc, Early Ordovician). Although expressing crinoid apomorphies, it is interpreted as retaining plesiomorphies in its arms reflecting early edrioasteroid rather than blastozoan (eocrinoid) ancestry. Apomorphies represent basal crinoid and cladid (crownward) levels of phylogeny.Restudy fortifies previous reports of the presence of a basal echinoderm plesiomorphy; floor plates above brachials in the arms ofApektocrinus, as well as in other approximately contemporary crinoids.Apektocrinusfurnishes the first record of podial basins in crinoid arms. Arms and calyx ofApektocrinusmerge gradually, facilitated by continuations of interbrachials (extraxial body plates) extending onto the arms and separating floor plates from brachials. These arm interbrachials, which diminish and pinch out distally as floor plates nestle into the brachial (adoral) groove, have not been recognized as such in crinoids.

New U–Pb and Rb–Sr constraints on pre-Acadian tectonism in North Wales

A new U–Pb date of 615.2 ± 1.3 (2σ) Ma for the Twt Hill Granite, North Wales, contrasts with an Rb–Sr isochron age of 491 ± 12 (2σ) Ma from the same body. The latter age is thought to result from isotope resetting during regional low-grade metamorphism or fault reactivation. The Rb–Sr age also coincides with the onset of latest Cambrian to Early Tremadoc regression and is taken to reflect tectonic uplift prior to the Arenig overstep at around 478 Ma. It is proposed that this in turn reflects plate-scale processes along the contemporary peri-Gondwanan continental margin.

Onset of the Ordovician cephalopod radiation – evidence from the Rochdale Formation (middle Early Ordovician, Stairsian) in eastern New York

AbstractThe Rochdale Formation of eastern New York (= Fort Ann and lower Bascom formations, designations abandoned) is now recognized to record the earliest stages of the Great Ordovician Radiation of cephalopods. The earliest Bassleroceratidae, Tarphyceratidae and endoceridans on the east Laurentian shallow carbonate platform occur in the upper, thrombolite-bearing member of the Rochdale. This fauna demonstrates that the earliest radiation of Ordovician nautiloids took place in the late Tremadocian and is best recorded in tropical platform facies. Revision of this cephalopod fauna based on approximately 190 specimens collected along a 200 km, N–S belt in easternmost New York has provided new information on inter- and intraspecific variation of earlier described species. The ellesmeroceridVassarocerasand the endoceridsMcqueenocerasandParaendocerasare emended. New taxa includeBassleroceras champlainensesp. nov. andB. triangulumsp. nov.,Mccluskiceras comstockensegen. et sp. nov.,Exoclitendoceras rochdalensegen. et sp. nov. andParaendoceras depressumsp. nov. A rank abundance plot of 146 specimens from a locality in the Lake Champlain lowlands provides information on the community structure of a nautiloid fauna in which the longiconic cyrtoconicBasslerocerasis shown to dominate strongly. The nautiloid community structure of the Rochdale Formation is similar to that of the underlying Tribes Hill Formation (late early Tremadocian) with respect to the depositional setting, diversity and evenness but displays a remarkably increased taxonomic distinctness.

New specimens of Lingulocystis Thoral, 1935 (Eocrinoidea, Blastozoa) from the Arenig (Lower Ordovician) of Montagne noire (southern France): Intraspecific morphological variability, stratigraphic, and palaeoecological implications

The genus Lingulocystis Thoral, 1935 is a preponderant component of the echinoderm fauna of the Lower Ordovician of Montagne noire (France), as suggested by its abundant remains. Its morphology is characterized by a flattened theca, composed of a frame enclosing numerous small polygonal central plates. The new specimens have been determined partly as Lingulocystis elongata Thoral, 1935 and partly as Lingulocystis aff. deani Ubaghs, 1994. This study is the first report of Lingulocystis in the Foulon Formation. New stratigraphic ranges have been defined for both species: L. elongata from the early Tremadoc (Saint-Chinian Formation) to the middle Arenig (lower part of the Landeyran Formation), L. aff. deani from the early Arenig (Cluse de l’Orb Formation and Foulon Formation), and L. deani from the middle Arenig (upper part of the Landeyran Formation). The new specimens of L. elongata highlight its intraspecific variability, mainly concerning the size and arrangement of the central plates, the size of the theca, and the type of stem columnal articulations. Sedimentological studies in Montagne noire currently use the presence of Lingulocystis remains as taphonomic and palaeoecological indicators. The presence of two types of columnal articulations enhances the palaeoecological use. The development of the synostosial articulation seems to be correlated with a proximal platform palaeoenvironment, whereas development of the symplexial type seems to be related to a deeper palaeoenvironment or relatively high hydrodynamic conditions.

Graptolite biostratigraphy and its correlation of the Mungok and Yeonghung formations at Yeongwol area, Korea

Based on graptolites from the Mungok and Yeonghung formations at Yeongwol, Korea and recent taxonomy on graptolites, the previous three graptolite zones are refined as theAnisograptus matanensis, Adelograptus cf.tenellus, Callograptus spp. zones, and two zones are firstly proposed for the Tremadocian rocks in the area: theParadelograptus antiquus andAorograptus victoriae zones, in ascending order: TheAnisograptus matanensis Zone yieldsAnisograptus matanensis indicating an early Tremadoc age. TheAdelograptus cf.tenellus Zone is erected by the occurrence ofAdelograptus cf.tenellus andPsigraptus jacksoni which represent lowermost Late Tremadocian in age. TheCallograptus spp. Zone yielding plenty of dendroids includingCallograptus andDendrograptus, is tentative but useful to correlate with other rocks in the Sino-Korean Block. TheParadelograptus antiquus Zone is recognized by the occurrence ofParadelograptus antiquus andClonograptus aureus and is correlated with the middle Late Tremadoc of Yukon, Canada. TheAorograptus victoriae Zone consists mainly ofAorograptus victoriae and subordinately ofKiaerograptus andClonograptus and corresponds to the eponymous zones elsewhere. The boundary between the Mungok and the Yeonghung formations consists of ribbon rock and grainstone to packstone facies of the Mungok Formation and laminated dolomite, argillaceous limestone, and wackestone to grainstone facies of the Yeonghung Formation and is drawn within theAorograptus victoriae Zone which indicates middle Late Tremadocian.