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Lower Ordovician (lower + ordovician)

Distribution by Scientific Domains

Earth and Environmental Science	75%

Selected Abstracts

Fossilized embryos are widespread but the record is temporally and taxonomically biased

EVOLUTION AND DEVELOPMENT, Issue 2 2006
Philip C. J. Donoghue
SUMMARY We report new discoveries of embryos and egg capsules from the Lower Cambrian of Siberia, Middle Cambrian of Australia and Lower Ordovician of North America. Together with existing records, embryos have now been recorded from four of the seven continents. However, the new discoveries highlight secular and systematic biases in the fossil record of embryonic stages. The temporal window within which the embryos and egg capsules are found is of relatively short duration; it ends in the Early Ordovician and is roughly coincident with that of typical "Orsten"-type faunas. The reduced occurrence of such fossils has been attributed to reducing levels of phosphate in marine waters during the early Paleozoic, but may also be owing to the increasing depth of sediment mixing by infaunal metazoans. Furthermore, most records younger than the earliest Cambrian are of a single kind,large eggs and embryos of the priapulid-like scalidophoran Markuelia. We explore alternative explanations for the low taxonomic diversity of embryos recovered thus far, including sampling, size, anatomy, ecology, and environment, concluding that the preponderance of Markuelia embryos is due to its precocious development of cuticle at an embryonic stage, predisposing it to preservation through action as a substrate on which microbially mediated precipitation of authigenic calcium phosphate may occur. The fossil record of embryos may be limited to a late Neoproterozoic to early Ordovician snapshot that is subject to dramatic systematic bias. Together, these biases must be considered seriously in attempts to use the fossil record to arbitrate between hypotheses of developmental and life history evolution implicated in the origin of metazoan clades. [source]

Parablastoid holdfasts from the Lower Ordovician of South Wales

GEOLOGICAL JOURNAL, Issue 3 2005
John C. W. Cope
Abstract Problematical discoid fossils from the Lower Arenig (Moridunian Stage) of the Llangynog Inlier, South Wales, are interpreted as parablastoid holdfasts. They constitute the first record of holdfasts in this rare echinoderm class. The lower surface has a radial ribbed structure, in contrast to the concentric, reticulate structure of the upper surface. The substrate for attachment was not lithified and may instead have been sediment stabilized by microbial mats. An associated pluricolumnal indicates that the parablastoid column was homeomorphic. Copyright © 2005 John Wiley & Sons, Ltd. [source]

Magnetostratigraphic constraints on the Gondwanan origin of North China: Cambrian/Ordovician boundary results

GEOPHYSICAL JOURNAL INTERNATIONAL, Issue 1 2002
Zhenyu Yang
Summary A significant gap in the middle Palaeozoic apparent polar wander (APW) path precludes polarity definition of the early Palaeozoic palaeopole for North China. This problem can in part be resolved by the intercontinental correlation of magnetic polarity patterns across small time intervals. A magnetostratigraphic study was carried out on upper Cambrian to lower Ordovician sediments near Zhaogezhuang (long. 118.5°E, lat. 39.7°N), North China. After stepwise thermal or thermal and alternating field demagnetizations, a characteristic magnetic component with normal and antipodal reversed directions was identified. These data, drawn from 49 samples, yield a north palaeopole at long. 294.6°E, lat. 32.9°N (dp = 3.0°, dm = 5.3°). A concordant magnetic polarity pattern around the Cambrian,Ordovician boundary and lowest Ordovician obtained from different continents favours a Southern Hemisphere origin (,17°) formation site. Using the Cambrian,Ordovician APW paths between North China and Gondwana, we suggest that the North China block (NCB) was part of Gondwana during the Cambro,lowest Ordovician, and started breaking away from Gondwana in the lower Ordovician. This finding is contrary to some palaeomagnetic models where the NCB was quite separate from Gondwana in the late Proterozoic, and was attached to the ,Pacific' side of Antarctica. [source]

Thermochemical Sulfate Reduction in the Tazhong District, Tarim Basin, Northeast China: Evidence from Formation Water and Natural Gas Geochemistry

ACTA GEOLOGICA SINICA (ENGLISH EDITION), Issue 2 2010
Caifu XIANG
Abstract: Systematic analyses of the formation water and natural gas geochemistry in the Central Uplift of the Tarim Basin (CUTB) show that gas invasion at the late stage is accompanied by an increase of the contents of H2S and CO2 in natural gas, by the forming of the high total dissolved solids formation water, by an increase of the content of HCO,3, relative to Cl,, by an increase of the 2nd family ions (Ca2+, Mg2+, Sr2+ and Ba2+) and by a decrease of the content of SO2,4, relative to Cl,. The above phenomena can be explained only by way of thermochemical sulfate reduction (TSR). TSR often occurs in the transition zone of oil and water and is often described in the following reaction formula: ,CH+CaSO4+H- 2O,H2S+CO2+CaCO3. (1) Dissolved SO2,4 in the formation water is consumed in the above reaction, when H2S and CO2 are generated, resulting in a decrease of SO2,4 in the formation water and an increase of both H2S and CO2 in the natural gas. If formation water exists, the generated CO2 will go on reacting with the carbonate to form bicarbonate, which can be dissolved in the formation water, thus resulting in the enrichment of Ca2+ and HCO,3. The above reaction can be described by the following equation: CO2+H2O+CaCO3,Ca2++2HCO,3. The stratigraphic temperatures of the Cambrian and lower Ordovician in CUTB exceeded 120°C, which is the minimum for TSR to occur. At the same time, dolomitization, which might be a direct result of TSR, has been found in both the Cambrian and the lower Ordovician. The above evidence indicates that TSR is in an active reaction, providing a novel way to reevaluate the exploration potentials of natural gas in this district. [source]