scholarly journals Polymorphic organization in a planktonic graptoloid (Hemichordata: Pterobranchia) colony of Late Ordovician age

2012 ◽  
Vol 150 (1) ◽  
pp. 143-152 ◽  
Author(s):  
JAN A. ZALASIEWICZ ◽  
ALEX PAGE ◽  
R. BARRIE RICKARDS ◽  
MARK WILLIAMS ◽  
PHILIP R. WILBY ◽  
...  
Keyword(s):  
Soft Part ◽  
Division Of Labour ◽  
Alternative Interpretation ◽  
Late Ordovician ◽  
Evolutionary Success ◽  
Early Palaeozoic

AbstractGraptolites are common fossils in Early Palaeozoic strata, but little is known of their soft-part anatomy. However, we report a long-overlooked specimen ofDicranograptusaff.ramosusfrom Late Ordovician strata of southern Scotland that preserves a strongly polymorphic, recalcitrant, organic-walled network hitherto unseen in graptoloid graptolites. This network displays three morphologies: proximally, a strap-like pattern, likely of flattened tubes; these transform distally into isolated, hourglass-shaped structures; then, yet more distally, revert to a (simpler) strap-like pattern. The network most likely represents a stolon-like system, hitherto unknown in graptoloids, that connected individual zooids. Its alternative interpretation, as colonial xenobionts that infested a graptoloid colony and mimicked its architecture, is considered less likely on taphonomic and palaeobiological grounds. Such polymorphism is not known in non-graptolite pterobranchs, which are less diverse and morphologically more conservative: a division of labour between graptoloid zooids for such functions as feeding, breeding and rhabdosome construction may have been the key to their remarkable evolutionary success.

Soft-part preservation in a bivalved arthropod from the Late Ordovician of Wales

2009 ◽  
Vol 147 (2) ◽  
pp. 242-252 ◽  
Author(s):  
ALEX PAGE ◽  
PHILIP R. WILBY ◽  
MARK WILLIAMS ◽  
JEAN VANNIER ◽  
JEREMY R. DAVIES ◽  
...  
Keyword(s):  
Deep Water ◽  
Primary Productivity ◽  
Soft Part ◽  
Late Ordovician ◽  
Dorsal Margin ◽  
Post Mortem ◽  
Associated Fauna ◽  
Arthropod Fauna ◽  
Early Palaeozoic

AbstractA new component of the Early Palaeozoic arthropod fauna is described from a monospecific accumulate of carapaces in a Late Ordovician (Katian) hemipelagic mudstone from the Cardigan district of southwest Wales (UK). Its non-biomineralized carapace is preserved as a carbonaceous residue, as is more labile anatomy (soft-parts) including the inner lamella and sub-ovate structures near its antero-dorsal margin, which we interpret to be putative eyes. The depositional context and associated fauna indicate that the arthropods inhabited an area of deep water and high primary productivity above a pronounced submarine topography. The preserved density of carapaces suggests the arthropods may have congregated into shoals or been transported post-mortem into depressions which acted as detritus traps. The accumulate provides a rare example of soft-part preservation in hemipelagic mudstones and highlights the role of organic material as a locus for authigenic mineralization during metamorphism.

Age and Origin of Deep Crustal Meta-igneous Xenoliths from the Scottish Midland Valley: Vestiges of an Early Palaeozoic Arc and ‘Newer Granite’ Magmatism

2019 ◽  
Vol 60 (8) ◽  
pp. 1543-1574 ◽  
Author(s):  
Eszter Badenszki ◽  
J Stephen Daly ◽  
Martin J Whitehouse ◽  
Andreas Kronz ◽  
Brian G J Upton ◽  
...  
Keyword(s):  
Late Ordovician ◽  
Geochemical Data ◽  
Arc Magmatism ◽  
Precambrian Basement ◽  
Zircon Dating ◽  
Magmatic Arc ◽  
Granite Magmatism ◽  
Early Palaeozoic ◽  
First Time ◽  
Rule Out

Abstract Deep crustal felsic xenoliths from classic Scottish Midland Valley localities, carried to the surface by Permo-Carboniferous magmatism, are shown for the first time to include metaigneous varieties with dioritic and tonalitic protoliths. Four hypotheses regarding their origin have been evaluated: (1) Precambrian basement; (2) Permo-Carboniferous underplating; (3) ‘Newer Granite’ magmatism; (4) Ordovician arc magmatism. U–Pb zircon dating results rule out the Precambrian basement and Permo-Carboniferous underplating hypotheses, but establish that the meta-igneous xenoliths represent both ‘Newer Granite’ and Ordovician (to possibly Silurian) arc magmatism. The metadiorite xenoliths are shown to have protolith ages of c. 415 Ma with εHft zircon values ranging from +0·1 to +11·1. These are interpreted to represent unexposed ‘Newer Granite’ plutons, based on age, mineralogical, isotopic and geochemical data. This shows that Devonian ‘Newer Granite’ magmatism had a greater impact on the Midland Valley and Southern Uplands crust than previously realized. Clinopyroxene–plagioclase–quartz barometry on the metadiorites from the east and west of the Midland Valley yielded a similar pressure range of c. 5–10 kbar, and a metadiorite from the east yielded a minimum two-feldspar temperature estimate of c. 793–816°C. These results indicate that the metadiorites once resided in the middle–lower crust. In contrast, two metatonalite xenoliths have a Late Ordovician protolith age (c. 453 Ma), with zircon εHft values of +7·8 to +9·0. These are interpreted as samples of a buried Late Ordovician magmatic arc situated within the Midland Valley. Inherited zircons with similar Late Ordovician ages and εHft=453 values (+1·6 to +10·8) are present in the metadiorites, suggesting that the Devonian ‘Newer Granites’ intruded within or through this Late Ordovician Midland Valley arc. A younger protolith age of c. 430 Ma from one of the metatonalites suggests that arc activity continued until Silurian times. This validates the long-standing ‘arc collision’ hypothesis for the development of the Caledonian Orogen. Based on U–Pb zircon dating, the metatonalite and metadiorite xenoliths have both experienced metamorphism between c. 400 and c. 391 Ma, probably linked to the Acadian Orogeny. An older phase of metamorphism at c. 411 Ma was possibly triggered by the combined effects of heating owing to the emplacement of the ‘Newer Granite’ plutons and the overthrusting of the Southern Uplands terrane onto the southern margin of the Midland Valley terrane.

Geochemistry and zircon U–Pb–Hf isotopes of the Mante Aobao granite porphyry at East Ujimqin Banner, Inner Mongolia: implications for petrogenesis and tectonic setting

2019 ◽  
Vol 157 (7) ◽  
pp. 1068-1086
Author(s):  
Fei Hu ◽  
Wei Huang ◽  
Zeli Yang ◽  
Simon A. Wilde ◽  
Harald Furnes ◽  
...  
Keyword(s):  
Inner Mongolia ◽  
Saturation Index ◽  
Tectonic Setting ◽  
Active Continental Margin ◽  
Late Ordovician ◽  
Granite Porphyry ◽  
Hf Isotopes ◽  
Margin Setting ◽  
Juvenile Crust ◽  
Early Palaeozoic

AbstractWe present detailed petrography, geochemistry and zircon U–Pb–Hf isotopes of the Mante Aobao granite porphyry in East Ujimqin Banner, Inner Mongolia, with the aim of determining its age and petrogenesis, important for understanding the early Palaeozoic tectonic evolution of the Xing’an–Mongolian Orogenic Belt. The Mante Aobao granite porphyry consists of plagioclase, quartz and minor biotite, but without amphibole. Zircon U–Pb analyses yield ages of 450 ± 1 Ma and 445 ± 2 Ma for the granite porphyry, indicating that it formed during Late Ordovician time. The granite porphyry is metaluminous to slightly peraluminous (aluminous saturation index A/CNK = 0.98–1.11) with high SiO2, K2O and Na2O concentrations and differentiation index (DI = 85–90). Chondrite-normalized rare earth element (REE) patterns display enrichment of light REEs (LREEs) with high ratios of (La/Yb)N and negative Eu anomalies. In the mantle-normalized multi-element variation diagrams, all samples are characterized by depletions of high-field-strength elements (HFSEs; Nb, Ta and Ti) and enrichments of large-ion lithophiles (LILEs; Rb, Th, U and K). These geochemical features indicate that the granite porphyry is a highly fractionated I-type granite and formed in a subduction-related setting. Zircon grains have positive εHf(t) values of +9.2 to +11.2, and TDM2(Hf) ages of 691–821 Ma, suggesting that the granite porphyry was generated by partial melting of Neoproterozoic juvenile crust with involvement of fractional crystallization during magmatic evolution. It is likely that underplating of mantle-derived magmas during Late Ordovician time provided the necessary heat to partially melt this juvenile crust. Combined with the regional geological data, we infer that the Mante Aobao granite porphyry was emplaced in an active continental margin setting that was probably related to the northwards subduction of the Paleo-Asian Plate beneath the South Mongolian Terrane along the Sonid Zuoqi–Xilinhot axis.

A revised correlation of Silurian rocks in the Girvan district, SW Scotland

2002 ◽  
Vol 93 (4) ◽  
pp. 383-392 ◽  
Author(s):  
James D. Floyd ◽  
Mark Williams
Keyword(s):  
Depositional Environment ◽  
Late Ordovician ◽  
Important Research ◽  
British Geological Survey ◽  
Marine Fauna ◽  
Comprehensive Review ◽  
Iapetus Ocean ◽  
Northern Side ◽  
Lower Palaeozoic ◽  
Early Palaeozoic

ABSTRACTIn late Ordovician and early Silurian times, the Girvan district lay in a shelf marinesetting on the margin of Laurentia, on the northern side of the Iapetus Ocean. The Lower Palaeozoic rocks of the Girvan district, and their shelly and graptolitic fossil fauna, were systematically described by Lapworth in 1882 and have formed an important research resource ever since. They provide valuable evidence for the depositional environment and geological setting of Girvan during the early Palaeozoic, in both regional and wider contexts, and demonstrate the long-recognised close affinity with contemporaneous Laurentian faunas. However, by late Ordovician and into Silurian times, the earlier Iapetus oceanic barrier to faunal migration had largely gone and there is good correlation between contemporaneous marine fauna throughout the British Isles and Scandinavia. Despite much recent research in the area, including resurvey work by the British Geological Survey, no comprehensive review of Silurian lithostratigraphy at Girvan has been published since the revision by Cocks and Toghill in 1973. The present review of the Silurian rocks addresses this need and complements the recently published (Fortey et al. 2000) revision of the underlying Ordovician rocks, thus bringing the entire Girvan Lower Palaeozoic succession up to modern standards of nomenclature.

Late Ordovician to early Silurian calc-alkaline magmatism in the Xiemisitai Mountains, northern West Junggar: a response to the subduction of the Junggar-Balkhash Ocean

2019 ◽  
Vol 61 (16) ◽  
pp. 2000-2020 ◽  
Author(s):  
Chen ◽  
Ma ◽  
Simon ◽  
Du ◽  
Han ◽  
...  
Keyword(s):  
Magma Mixing ◽  
Crustal Contamination ◽  
Late Ordovician ◽  
Alkaline Magmatism ◽  
Calc Alkaline ◽  
Hydrous Minerals ◽  
Isotopic Signatures ◽  
Central Asian ◽  
West Junggar ◽  
Early Palaeozoic

The Early Palaeozoic tectonic evolution of northern West Junggar is essential for understanding the tectonic framework and accretionary processes in the southwestern segment of the Central Asian Orogenic Belt. However, its evolutionary history is controversial due to insufficient pre-late Silurian magmatic data. In order to clarify this issue, we conducted geochronological, geochemical, and Sr-Nd-Hf isotope analyses on samples from the Dengdeer and Maodun complexes in the Xiemisitai Mountains of Boshchekul-Chingiz (BC) arc, northern West Junggar. LA-ICP-MS zircon U-Pb dating yielded ages ranging from 445 Ma to 429 Ma, revealing the Late Ordovician to early Silurian magmatic events in northern West Junggar. The dioritic and metaluminous Dengdeer complex has high Mg# (49.5–52.9), moderate La/Yb (11.0–12.3), and low initial 87Sr/86Sr (0.70324–0.70410) and εNd(t) (+3.7 to +4.5). This complex exhibits variable zircon εHf(t) (+11.31 to +17.62), young Hf model ages (470–440 Ma), and appears to have been produced by differentiation of melts from metasomatized mantle. The intermediate to felsic, metaluminous to weakly peraluminous Maodun complex has abundant enclaves and inherited zircons. This complex seems to have fractionated from mantle- and crustal-derived magma mixing with some crustal contamination, based on its high Mg# (36.6–49.5), low Sr/Y (8.7–29.6), Nb/Ta (8.3–13.9), initial 87Sr/86Sr (0.70377) and εNd(t) (+4.0 to +4.2), and variable zircon εHf(t) (+11.59 to +15.21). The Dengdeer and Maodun samples are high-K calc-alkaline, rich in hydrous minerals, Rb, Th, and LREEs, depleted in Nb, Ta, and Ti, and show decoupling Hf-Nd isotopic signatures. These characteristics indicate that their source had been modified by melts from slab sediments. After combining our data with previous observations, we conclude that a trench-ward migration of an arc-back-arc system occurred during the Late Ordovician to early Silurian in northern West Junggar, while the Dengdeer and Maodun complexes were related to subduction-accretionary processes of the Junggar-Balkhash oceanic slab beneath the BC arc.

Early Palaeozoic metamorphic history of the Midland Valley, Southern Uplands–Longford-Down massif and the Lake District, British Isles

1984 ◽  
Vol 75 (2) ◽  
pp. 245-258 ◽  
Author(s):  
G. J. H. Oliver ◽  
J. L. Smellie ◽  
L. J. Thomas ◽  
D. M. Casey ◽  
A. E. S. Kemp ◽  
...  
Keyword(s):  
Accretionary Prism ◽  
Late Ordovician ◽  
Ocean Floor ◽  
Lower Grade ◽  
Lake District ◽  
Reflectance Measurement ◽  
Early Ordovician ◽  
Metamorphic History ◽  
History Of ◽  
Early Palaeozoic

ABSTRACTA model for the early Palaeozoic metamorphic history of the Midland Valley and adjacent areas to the S in Scotland, England and Ireland is based on the results of new field mapping, thin section petrography, electron probe microanalysis, X-ray diffractometry, conodont and palynomorph colouration and graptolite reflectance measurement.The oldest metamorphic rocks of the Midland Valley of Scotland, excluding xenoliths in post-Silurian lavas, are possibly the blueschist occurrences in the melange unit of the Ballantrae complex. These may be tectonised remnants of (?)pre-Arenig ocean-floor subducted during closure of the Iapetus Ocean. In the early Ordovician, the melange terrane was dynamothermally metamorphosed during obduction of newly-formed ocean crust. The obduction process piled up a thick sequence of various ocean-floor types such that burial metamorphism in parts reached pumpellyite-actinolite facies; elsewhere prehnite-pumpellyite and zeolite facies was attained.Whilst the Midland Valley acted as an inter- or fore-arc basin during the Late Ordovician and Silurian and experienced burial metamorphism, an accretionary prism was formed to the S. Accretion, tectonic burial and metamorphism of ocean-floor and trench sediment was continuous in the Southern Uplands and the Longford-Down massif of Ireland through Late Ordovician to Late Silurian times. Rocks at the present-day surface vary from zeolite facies to prehnitepumpellyite facies. Silurian trench-slope basin sediments can be recognised in part by their lower grade of burial metamorphism. Greenschist facies rocks of the prism probably lie close to the surface.The Lake District island-arc terrane of Northern England has an early Ordovician history of burial metamorphism up to prehnite-pumpellyite facies. The Late Ordovician and Silurian metamorphic history is one of sedimentary burial complicated by tectonism and intrusion of granite plutons to a relatively high level. The Iapetus suture is marked by a weak contrast in metamorphic grade.

scholarly journals Chapter 24 Late Ordovician zooplankton maps and the climate of the Early Palaeozoic Icehouse

2013 ◽  
Vol 38 (1) ◽  
pp. 399-405 ◽  
Author(s):  
Thijs R. A. Vandenbroucke ◽  
Howard A. Armstrong ◽  
Mark Williams ◽  
Florentin Paris ◽  
Koen Sabbe ◽  
...  
Keyword(s):  
Late Ordovician ◽  
Early Palaeozoic

Late Neoproterozoic to Early Palaeozoic palaeogeography of Avalonia: some palaeomagnetic constraints from Nuneaton, central England

2003 ◽  
Vol 140 (6) ◽  
pp. 685-705 ◽  
Author(s):  
HAROLDO VIZAN ◽  
JOHN N. CARNEY ◽  
PETER TURNER ◽  
ROBERT A. IXER ◽  
MARK TOMASSO ◽  
...  
Keyword(s):  
Lower Cambrian ◽  
Late Ordovician ◽  
Magnetic Components ◽  
Intrusive Rocks ◽  
History Of ◽  
Sandstone Formation ◽  
Late Neoproterozoic ◽  
Tectonic Rotation ◽  
Early Palaeozoic ◽  
Good Agreement

Palaeomagnetic studies have been carried out on Neoproterozoic, Cambrian and Ordovician rocks in the Nuneaton inlier, England (52.5° N, 1.5° W). Three magnetic components were recognized, which provide a consistent structural and magnetic history of the inlier. Neoproterozoic volcaniclastic and intrusive rocks acquired a characteristic remanent magnetization (ChRM) dated at 603Ma. Late Ordovician rocks are represented by lamprophyre and diorite intrusions and their ChRMs were probably imprinted during their emplacement, at about 442 Ma. The Lower Cambrian sedimentary sequence of the Hartshill Sandstone Formation, which unconformably overlies the Neoproterozoic rocks and hosts the Ordovician intrusions, does not preserve a primary magnetization but shows the imprints of the Late Ordovician (442 Ma) remagnetization, as well as a probable end-Carboniferous remagnetization. Palaeolatitudes calculated for the late Neoproterozoic rocks and Ordovician intrusions are in good agreement with other palaeolatitudes calculated for Avalonia during those times. Both the late Neoproterozoic and Late Ordovician rocks additionally show ChRMs with declination anomalies indicating a large tectonic rotation of the Nuneaton area, possibly during one of the Caledonian phases of deformation affecting southern Britain.

scholarly journals Mandibular morphology, task specialization and bite mechanics in Pheidole ants (Hymenoptera: Formicidae)

2021 ◽  
Vol 18 (179) ◽  
pp. 20210318
Author(s):  
Cristian L. Klunk ◽  
Marco A. Argenta ◽  
Alexandre Casadei-Ferreira ◽  
Evan P. Economo ◽  
Marcio R. Pie
Keyword(s):  
Social Life ◽  
Division Of Labour ◽  
Ecological Significance ◽  
Task Specialization ◽  
Element Analysis ◽  
Differential Performance ◽  
Biomechanical Responses ◽  
Evolutionary Success ◽  
Morphological Differences ◽  
Apical Tooth

Ants show remarkable ecological and evolutionary success due to their social life history and division of labour among colony members. In some lineages, the worker force became subdivided into morphologically distinct individuals (i.e. minor versus major workers), allowing for the differential performance of particular roles in the colony. However, the functional and ecological significance of these morphological differences are not well understood. Here, we applied finite element analysis (FEA) to explore the biomechanical differences between major and minor ant worker mandibles. Analyses were carried out on mandibles of two Pheidole species, a dimorphic ant genus. We tested whether major mandibles evolved to minimize stress when compared to minors using combinations of the apical tooth and masticatory margin bites under strike and pressure conditions. Majors performed better in pressure conditions yet, contrary to our expectations, minors performed better in strike bite scenarios. Moreover, we demonstrated that even small morphological differences in ant mandibles might lead to substantial differences in biomechanical responses to bite loading. These results also underscore the potential of FEA to uncover biomechanical consequences of morphological differences within and between ant workers.

Late Ordovician trilobites from northwest Iran and their biogeographical affinities

2008 ◽  
Vol 99 (2) ◽  
pp. 101-124 ◽  
Author(s):  
Talia S. Karim
Keyword(s):  
Northwest China ◽  
Late Ordovician ◽  
Northern Iran ◽  
Eastern Margin ◽  
Close Proximity ◽  
History Of ◽  
Northwest Iran ◽  
Early Palaeozoic ◽  
Alborz Region ◽  
The Village

ABSTRACTA diverse, previously undescribed trilobite fauna from northwest Iran sheds new light on the Early Palaeozoic geography of the Alborz terrane. The structural history of the Alborz region is well known, though faunal data for the area are sparse, especially for the Ordovician. Trilobites, brachiopods, conodonts, bryozoans and echinoderms occur in a red grainstone/packstone near the village of Tatavrud, 35 km southwest of Bandar-e-Anzali. The fossils indicate a Late Ordovician age, although lithologically similar outcrops in northern Iran have previously been considered Silurian. Trilobite genera occurring include Trinodus, Geragnostus, Illaenus, Panderia, Phorocephala, Ovalocephalus, Dicranopeltis, Symphysops, Cyclopyge, and Sphaerexochus. Similar assemblages have been reported from the Late Ordovician of Ireland, Spain, Poland, Norway, northwest China, Kazakhstan and the Turkistan–Alai Ridge (Uzbekistan–Kyrgyzstan border). Reconstructions of Late Ordovician geography place the Alborz terrane near the eastern margin of Gondwana, but there is doubt as to its precise location. Some workers have considered it part of the margin while others have considered it a separate terrane. Several species present, including Mezzaluna tatavrudensis n. sp. and Phorocephala cf. ulugtana (Petrunina, 1975) are very similar to taxa described from the Turkistan–Alai Ridge. In addition, the isocolid genus Paratiresias has only been reported from Iran, the Turkistan–Alai Ridge and northwest China. The presence of these taxa indicates that the Alborz terrane was in close proximity to the eastern margin of Gondwana at this time.

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