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Calibrating Rates of Early Cambrian Evolution
Abstract
An explosive episode of biological diversification occurred near the beginning of the Cambrian period. Evolutionary rates
in the Cambrian have been difficult to quantify accurately because of a lack of high-precision ages. Currently, uranium-lead
zircon geochronology is the most powerful method for dating rocks of Cambrian age. Uranium-lead zircon data from lower Cambrian
rocks located in northeast Siberia indicate that the Cambrian period began at approximately 544 million years ago and that
its oldest (Manykaian) stage lasted no less than 10 million years. Other data indicate that the Tommotian and Atdabanian stages
together lasted only 5 to 10 million years. The resulting compression of Early Cambrian time accentuates the rapidity of both
the faunal diversification and subsequent Cambrian turnover.
... However, in spite of a large amount of the age data obtained lately for this area's sections [Bowring et al., 1993;Rogov et al., 2015;Vishnevskaya et al., 2017;Grazhdankin et al., 2020a], a detailed correlation of the Vendian sections of the Olenek uplift with the sections of other areas of the Siberian Platform is hampered by significant differences in structure and composition. One of the nearest regions for which the Vendian deposits are described is the eastern Lena-Anabar trough wherein this stratigraphic section is entirely penetrated by the Burskaya-341-0 (B-341-0) and boreholes drilled in the late 20th century; the upper horizons of the Vendian are also penetrated by the Charchikskaya-1 (Ch-1) and Ust'-Olenekskaya-230-0 (UO-230-0) boreholes (Fig. 1). ...
... They are characterized by a relatively small (a few hundred meters) total thickness decreasing from margin to center of the craton, widespread occurrence of sedimentation gaps, and manifestation of bimodal rifting magmatism in the Vendian-Cambrian boundary interval [Shpunt et al., 1979;Rogov et al., 2015;Kiselev et al., 2016]. The age of deposits is based on the biostratigraphical [Grazhdankin et al., 2008;Rogov et al., 2012Rogov et al., , 2015, isotope-geochemical [Knoll et al., 1995;Vishnevskaya et al., 2017], and geochronological [Bowring et al., 1993;Grazhdankin et al., 2020а] data. Because of the lack of the core samples, stratification of the Vendian deposits in deep-borehole sections is mostly based on the logging data [Kontorovich et al., 2013]. ...
... The Turkut formation overlies the Khatyspyt formation with a short-term gap in sedimentation . In accordance with the small-shelly fossil finds and geochronological data on the overlying deposits, the formation can be constrained to between 550 and 544 Ma [Bowring et al., 1993;Rogov et al., 2015]. ...
Studies have been made of the carbon and strontium isotope composition in carbonate sediments of the Khorbusuonka group and Kessyusa formation corresponding to the Upper Vendian in deep Khastakhskaya-930 and Burskaya-341-0 boreholes at the northeastern margin of the Siberian Platform. The maximum δ ¹³ С values in carbonates of the Turkut and Kessyusa formations in the Khastakhskaya-930 borehole are +7.0...+7.4 %, while the minimum ⁸⁷ Sr/ ⁸⁶ Sr ratios are as low as 0.7079. Such isotope-geochemical characteristics suggest that these formations are younger (Tommotian) or older (early Vendian) than it was supposed and allow for the possibility of alternatives to their regional correlation with the Burskaya-341-0 borehole and Olenek uplift sections. The deposition of sediments of the age considered, more intensive than in the adjacent regions, may be indicative of rift-related extension settings.
... The Tas-Yuryakh Volcanic Complex corresponds to the lower Syhargalakh Formation , and comprises breccias, tuffs, tuffaceous sandstones . The tuff breccia has yielded a U-Pb zircon age of 543.9 ± 0.24 Ma (Bowring et al. 1993). ...
... Mafic intrusions with a U-Pb apatite age of 1386 ± 30 Ma are known in the Udzha Rift (Malyshev et al. 2018). The mafic volcanic eruptions on the Olenek Uplift are 543.9 ± 0.24 Ma (latest Ediacaran) in age (Bowring et al. 1993). ...
The Olenek-Anabar Composite Tectono-Sedimentary Element (OLA CTSE) is located in the northern part of the Siberian Craton. Its structure and depositional history is controlled by two main tectonic events: (i) Mesoproterozoic rifting; (ii) Ediacaran-Cambrian (to Middle Devonian?) post-rift thermal subsidence. The sedimentary succession is mainly represented by Mesoproterozoic to Cambrian strata including the black shales of lower-middle Kuonamka Formation (Fm). The hydrocarbon potential of the OLA CTSE is defined by the presence of large superficial bitumen fields, anticlinal structures, large reefal build-ups, and direct proximity to the potential source rock (Kuonamka Fm). A very scarce grid of seismic profiles and lack of exploration wells limits the knowledge of the petroleum potential. The lack of seals and broad occurrences of faults causing significant vertical permeability of sediment cover, abundant occurrences of mafic and kimberlite magmatism are limiting factors regarding the petroleum potential. Wells drilled during the Soviet-era did not recover any commercial discoveries.
... The Mattaia Formation is interpreted as a transition from open shelf to shoreface conditions . The Terreneuvian age of the Kessyusa Group is constrained by biostratigraphic (small skeletal fossils, trace fossils) (Khomentovsky and Karlova, 1993;Nagovitsin et al., 2015), chemostratigraphic , and U-Pb zircon age data (Bowring et al., 1993;Kaufman et al., 2012;Vishnevskaya et al., 2017) (Fig. 1.2). ...
... (3) A sketch map of the Olenek Uplift with the studied section (0705). U-Pb ages: 545 ± 3 Ma, youngest detrital zircons (YDZ) (Vishnevskaya et al., 2017); 543.9 ± 0.24 Ma, zircons from the Tas-Yurakh volcanic complex (Bowring et al., 1993); 529.7 ± 0.3 Ma, zircons from tuff interbed (Kaufman et al., 2012). CSe2 = Cambrian Series 2; CS3 = Cambrian Stage 3; Tr. = Turkut; Syh. ...
In the early Cambrian fossil record, triradial symmetry is typical for anabaritids and occurs among carinachitids. The former are an extinct group of minute benthic cnidarians covered with a calcareous tubular exoskeleton. The origin of the anabaritids is poorly understood, but previously reported triradial pyramid-shaped steinkerns and molds of the oldest conulariids, Vendoconularia , from the upper Ediacaran of the White Sea region suggested the anabaritids were closely related to conulariids. However, triradial symmetry could originate independently in different lineages in the late Ediacaran and early Cambrian. Herein we describe a new taxon, Ilankirus kessyusensis new genus new species, from the base of the Cambrian Stage 2 of the Olenek Uplift (Siberian Platform). These fossils occur as ornamented steinkerns in the shape of trilateral pyramids and lack any relics of a mineralized exoskeleton. Abundant plastic deformations and fractures of the casts suggest the organism was weakly if at all mineralized. The steinkerns are encrusted with a thin patina of iron-rich chlorite (chamosite) formed because of a multistage diagenetic replacement of authigenic glauconite (glauconite–berthierine–chamosite) under reducing conditions of oxygen-depauperate pore- and seawater. Both lacking two major autapomorphies of the crown-group conulariids (mineralized periderm and quadrate cross section of the oral region of the periderm), the late Ediacaran triradial Vendoconularia and Terreneuvian Ilankirus represent stem-group conulariids.
UUID: http://zoobank.org/a2ce04fa-36a5-485a-806c-f13f4749fc7f
... In fact, the only radiometric constraint available is a maximum age for intrusion of the volcanic breccia of the Tas-Yuryakh volcanic complex within the lower part of the Syhargalakh Formation (lower Kessyusa Group), which unconformably overlies the Khatyspyt and overlying Turkut formations. The maximum age for intrusion of this unit is 542.8 ± 1.30 Ma, provided by zircon U-Pb air abrasion ID-TIMS (Table S1) (Bowring et al., 1993;Maloof et al., 2010;Rogov et al., 2015). Notwithstanding uncertainties in this age (Table S1), the Turkut Formation, which overlies the Khatyspyt Formation, contains the local FAD of the anabaritid Cambrotubulus decurvatus and the onset of a negative excursion which may be equivalent either to the A4 anomaly or the BACE (depending on the preferred model, Figs. 3 and 4). ...
... A maximum age for intrusion of the Tas-Yuryakh volcanic breccia within the lower Syhargalakh Formation (lower Kessyusa Group) along the Khorbusuonka River is suggested by a zircon U-Pb air abrasion ID-TIMS age of 542.8 ± 1.30 Ma (Table S1) (Bowring et al., 1993;Maloof et al., 2010;Rogov et al., 2015). The intrusive Tas-Yuryakh volcanic breccia unconformably overlies the Turkut Formation. ...
The Ediacaran-Cambrian transition, which incorporates the radiation of animals, lacks a robust global temporal and spatial framework, resulting in major uncertainty in the evolutionary dynamics of this critical radiation and its relationship to changes in palaeoenvironmental geochemistry. We first present a new δ¹³Ccarb composite reference curve for the Ediacaran Nama Group of southern Namibia, and we then outline four new possible global age models (A to D) for the interval 551–517 million years ago (Ma). These models comprise composite carbonate‑carbon isotope (δ¹³Ccarb) curves, which are anchored to radiometric ages and consistent with strontium isotope chemostratigraphy, and are used to calibrate metazoan distribution in space and time. These models differ most prominently in the temporal position of the basal Cambrian negative δ¹³Ccarb excursion (BACE). Regions that host the most complete records show that the BACE nadir always predates the Ediacaran-Cambrian boundary as defined by the first appearance datum (FAD) of the ichnospecies Treptichnus pedum. Whilst treptichnid traces are present in the late Ediacaran fossil record, the FAD of the ichnospecies T. pedum appears to post-date the LAD of in situ Cloudina and Namacalathus in all environments with high-resolution δ¹³Ccarb data. Two age models (A and B) place the BACE within the Ediacaran, and yield an age of ~538.8 Ma for the Ediacaran-Cambrian boundary; however models C and D appear to be the most parsimonious and may support a recalibration of the boundary age by up to 3 Myr younger. All age models reveal a previously underappreciated degree of variability in the terminal Ediacaran, incorporating notable positive and negative excursions that precede the BACE. Nothwithstanding remaining uncertainties in chemostratigraphic correlation, all models support a pre-BACE first appearance of Cambrian-type shelly fossils in Siberia and possibly South China, and show that the Ediacaran-Cambrian transition was a protracted interval represented by a series of successive radiations.
The Ediacaran-Cambrian radiation occurred over a protracted interval without global mass extinctions and with generally diachronous metazoan appearances.
... large impact events which affected the lunar surface through time as crater formation and their absolute ages got determined by radiometric dating of obtained lunar samples. The lunar surface is generally modified by impact cratering and volcanism, so, it is possible to define the lunar geological events in time basis on stratigraphic principles [8][9][10]. ...
... 8 It began with the formation of the Earth about 4.6 billion years ago and ended at 4 billion years ago. 9 It began from the appearance of oxygen in Earth's atmosphere (2.5 billion years ago) to just before the proliferation of complex life at 500 million years ago. [16]. ...
It seems anorthosites are by far interested by geologists because they give us great information about Earth history and how it was evolved in planetary geology. Planetary geology is subject the geology of the celestial bodies such as the planets and their moons, asteroids, comets, and meteorites. It is nearly abundant in the moon. So, it seems studying of these rocks give us good information about planetary evolution and the own early time conditions. Anorthosites can be divided into few types on earth such as: Archean-age (between 4,000 to 2,500 million years ago) anorthosites, Proterozoic (2.5 billion years ago) anorthosite (also known as massif or massif-type anorthosite) – the most abundant type of anorthosite on Earth, Anorthosite xenoliths in other rocks (often granites, kimberlites, or basalts). Furthermore, Lunar anorthosites constitute the light-colored areas of the Moon’s surface and have been the subject of much research. According to the Giant-impact hypothesis the moon and earth were both originated from ejecta of a collision between the proto-Earth and a Mars-sized planetesimal, approximately 4.5 billion years ago. The geology of the Moon (lunar science) is different from Earth. The Moon has a lower gravity and it got cooled faster due to its small size. Also, it has no plate tectonics and due to lack of a true atmosphere it has no erosion and weathering alike the earth. However, Eric A.K. Middlemost believed the astrogeology will help petrologist to make better petrogenic models to understand the magma changing process despite some terms geological differences among the Earth and other extraterrestrial bodies like the Moon. So, it seems that these future studies will clarify new facts about planet formation in planetary and earth, too.
... Cambrian strata comprise a 1000 m thick carbonate succession, with latest Ediacaran-earliest Cambrian rhyolites and basalts reported from north-eastern Siberia [2,26,27]. Devonian deposits crop out in a few localities and mainly consist of interbedded carbonates, calcarenites, and litharenites, with subordinate shale beds. A >400 m thick unit comprising basalt flows has also been described from the northernmost part of the anticlinorium (along the Bykov channel of the Lena Delta) [20,28]. ...
... However, detrital zircons of these ages have also been found within Ediacaran-Cambrian sedimentary strata located in close proximity to the studied Devonian succession [1]. Latest Neoproterozoic-earliest Cambrian zircons could be sourced from coeval volcanics located in close proximity to the studied outcrops [26,27,43]. ...
We present new data on the tectonic evolution of north-eastern Siberia using an integrated provenance analysis based on U–Pb detrital zircon dating and sandstone petrography of Devonian sedimentary strata. Our petrographic data suggest that Upper Devonian sandstones of north-eastern Siberia were derived from a local provenance, supported by the widespread distribution of ca. 1900–2000 Ma magmatic events in the basement of the neighboring Ust’-Lena and Olenek uplifts. Devonian detrital zircon age distributions of the Devonian sandstones are similar to ages of Middle Paleozoic magmatic rocks of Yakutsk-Vilyui large igneous province (LIP). Therefore, we suggest that the studied sandstones were derived from proximally-located uplifted blocks composed of Proterozoic–Devonian rocks and Middle–Late Devonian volcanics. Moreover, the abundance of Middle–Late Devonian zircons is suggestive of a wider distribution of coeval magmatism across north-eastern Siberia than previously supposed. We propose that widespread Devonian magmatism associated with the Yakutsk-Vilyui LIP also occurred to the east of our study area and is now buried beneath thick Carboniferous–Jurassic sedimentary rocks of the eastern Siberian passive margin, subsequently deformed into the Late Jurassic–Cretaceous Verkhoyansk fold-and-thrust belt. We also propose that the major pulse of the Yakutsk-Vilyui LIP occurred in north-eastern Siberia during the Middle Devonian at ca. 390 Ma, some 15 million years earlier than within the Vilyui rift basin in eastern Siberia (ca. 375 Ma).
... This event may be part of Ediacaran-Early Cambrian tectonic and magmatic activity in the Olenek uplift that produced mafic breccia pipes, small basaltic lava flows, as well as dolerite dikes and sills in Late Ediacaran carbonates (Kiselev et al., 2015). The 543.9 ± 0.24 Ma U-Pb zircon ages of the breccias (Bowring et al., 1993) agree well with the 542.5 ± 15 Ma age obtained for sample Zv1-17. Potassic breccia pipes have been previously reported in the Olenek uplift (Shpunt & Shamshina, 1989), which were the source of Cr spinels found in alluvium along rivers within the uplift and in Carnian (T 3 ) sediments apparently formed during the same event . ...
The northeastern Siberian craton stores numerous placers of diamonds that differ in morphology and texture, and originate from different types of primary deposits, primarily kimberlites. Analyses of mantle zircons selected from samples of alluvial and coarse clastic sediments in the watersheds of the Olenek and Kyutyungde rivers in the Anabar-Olenek subprovince of the Yakutian diamond province provide constraints on kimberlite magmatism events in the area. The new U-Pb ages and typical kimberlitic signatures of the Kyutyungde zircons trace-element patterns reveal three main magmatic events that produced kimberlites: Middle Paleozoic (Late Devonian to Early Carboniferous), Jurassic and Triassic. The Jurassic zircons (177-149 Ma) are mainly related to kimberlites in the proximal Kuoika-Molodo and Khorbusuon fields. Triassic zircons (240-221 Ma) were probably released during large-scale erosion of the eastern Anabar shield, however, the presence of such zircons in the Kyutyungde area indicates that kimberlites of the respective magmatic stage may exist within the immediate vicinity of the area. The only Middle Paleozoic zircon sample (358.6 Ma, D 3-C 1 boundary) may originate from a proximal kimberlite within the Kyutyungde area. The presence of Middle Paleozoic kimberlitic zircons, along with harzburgite-dunitic pyropes, indicates that the primary deposit of diamonds and related minerals may exist near or within the Kyutyungde area. Late Ediacaran-Early Cambrian (573-473 Ma) mantle zircons found in Early Cambrian conglomerates of the area, along with other mantle minerals including Cr spinel of a diamond assemblage, record another Precambrian event of potentially diamondiferous magma-tism in the Kyutyungde basin. K E Y W O R D S detrital zircon, diamond, kimberlite, pyrope, Siberian craton, U-Pb dating
... Improved radioisotopic age constraints are steadily enhancing the stratigraphic resolution of evolutionary and geochemical changes across the Ediacaran-Cambrian boundary. Over the last four decades, calibration of the base of the Cambrian Period has shifted progressively younger, from c. 570 Ma (50) to c. 544 Ma (51), to c. 541 Ma (28,44), to c. 539 Ma (23,34), and now to c. 533 Ma. While paleontological studies reveal an increasingly apparent Ediacaran root to animal evolution (e.g., ref. 52), this revision to the geologic time scale lengthens the fuse and demonstrates that the early Cambrian radiation of modern animal phyla was even more explosive than formerly realized. ...
The geologically rapid appearance of fossils of modern animal phyla within Cambrian
strata is a defining characteristic of the history of life on Earth. However, temporal
calibration of the base of the Cambrian Period remains uncertain within millions of
years, which has resulted in mounting challenges to the concept of a discrete Cambrian
explosion. We present precise zircon U–Pb dates for the lower Wood Canyon Formation,
Nevada. These data demonstrate the base of the Cambrian Period, as defined by both
ichnofossil biostratigraphy and carbon isotope chemostratigraphy, was younger than
533 Mya, at least 6 My later than currently recognized. This new geochronology condenses
previous age models for the Nemakit–Daldynian (early Cambrian) and, integrated
with global records, demonstrates an explosive tempo to the early radiation of modern
animal phyla.
... Integrated biostratigraphic Rogov et al., 2015), chemostratigraphic, and geochronological data (Grazhdankin et al., 2020) suggest the Fortunian age of the fossil-bearing strata (Fig. 1c). The U-Pb age estimates are 543.9 ± 0.24 Ma for zircons from tuff-breccia of the Tas-Yuryakh volcanic complex (Bowring et al., 1993) interfin- (CSGM 2054-191); c -subhorizontal bilobate Didymaulichnus (dy) and poorly preserved cleaved plug-shaped structure nominally assigned to Bergaueria (Bg?), positive hyporelief (CSGM 2054-192); d -subhorizontal probing Treptichnus, positive hyporelief (CSGM 2054-121); e -short parallel scratch imprints Monomorphichnus, positive hyporelief (CSGM 2054-83); f -pyritized olenichnus networks, epirelief (CSGM 2054-171); g -thin horizontal Helminthoidichnites, positive hyporelief (CSGM 2054-193); h -densely-coiled helicoidal Protospiralichnus with rare tangential probing segments, positive hyporelief (CSGM 2054-214). gering with the middle-upper part of the Syhargalakh Formation and 545 ± 3 Ma for the youngest detrital zircons from the basal Syhargalakh sandstones (Vishnevskaya et al., 2017). ...
—Preservation of soft-bodied organisms as casts is common in the Ediacaran fossil record and extremely rare in Cambrian rocks. Among the factors, limiting fossilization of soft tissues, there is bioturbation–sediment disturbance by burrowing organisms. It is the emergence of burrowing metazoans and associated sediment bioturbation that is thought to be one of the major causes for the disappearance of Ediacaran soft-bodied organisms from the fossil record around ~540 Ma. Here, we study an assemblage of fossils preserved as casts in the Fortunian carbonates of the Olenek Uplift (northeastern Siberian Platform) in association with a typical Fortunian ichnoassemblage. The overall morphology and preservation of the fossils reveal that they comprise microbially induced sedimentary structures and soft-bodied holdfasts. The latter are vaguely reminiscent of some Ediacaran holdfasts, though it is unlikely that they include their phylogenetic descendants. Three-dimensional preservation of the studied fossils was caused by authigenic crystallization of calcite and its further early-diagenetic dolomitization. Our study confirms the critical importance of specific environmental conditions ensuring preservation of soft-bodied organisms as three-dimensional molds and casts. This unique interplay of environmental factors became rare in the Cambrian Period, which was caused by intensification and expansion of bioturbation in marine basins.
... Cambrian is characterized by the emergence and radiation of marine skeletal organisms which diversi ed rapidly at the beginning of the Early Cambrian and reached a peak by the middle Early Cambrian (Sepkoski 1992; Braga et al. 1995). 'Cambrian explosion' (Cloud 1948;Bowring et al. 1993) (Zhuravlev 1996; Rowland and Shapiro 2002). Therefore, the Middle to Late Cambrian (Miaolingian to Furongian) is regarded as the longest geological range of metazoan 'reef interval' during Phanerozoic (Zhuravlev 1996; Rowland and Shapiro 2002;Kiessling 2009). ...
Leiolites exposed in the Jinzhouwan section of Cambrian Miaolingian in Dalian City, Liaoning Province, North China Platform, were developed in the highstand systems tract of third-order sequence. The macroscopic and microscopic characteristics of leiolites are studied via field and indoor polarizing microscope observations. Macroscopically, leiolites mainly consist of micrites without any special internal structure. Microscopically, in addition to micrite matrix, calcimicrobes (including Girvanella , Subtifloria , Angulocellularia , Renalcis , Tarthina , and Bacinella-like fabric), microclots, benthic ooids, bioclasts, dolomite, and terrigenous debris were observed. Girvanella shows four types of growth habits. Differences in microfabrics and microbial communities within the Miaolingan leiolites may be caused by the different local environmental conditions. The calcimicrobes dominated by calcified sheaths of filamentous cyanobacteria, together with amounts of pyrites and poorly preserved calcified microbes, indicate that the formation of leiolites is related to the calcification induced by microbial metabolic activities and the degradation of organic matter by heterotrophic sulfate-reducing bacteria. The complex calcification in the microbial mats dominated by filamentous cyanobacteria in the study area provides an example for studying the first cyanobacterial calcification episode during Phanerozoic.
... See geographic location of the sections in Fig. 1a. Uranium-lead ages in the Olenek Uplift section originate from the following studies: 543.9 ± 0.24 Ma - (Bowring et al., 1993), 529.7 ± 0.24 Ma - (Kaufman et al., 2012). Indices of the δ 13 C excursions are given from works referred to below the sections. ...
—The Precambrian–Cambrian transition is one of the most fundamental evolutionary turnovers in the Earth’s history. However, in many cases precise identification and correlation of this crucial boundary in lithologically contrasting sections is complicated, even if the whole spectrum of biostratigraphic and chemostratigraphic methods is applied. The Precambrian–Cambrian transitional strata of the Igarka Uplift (Sukharikha Formation) perfectly illustrate this problem. This unit has arguably one of the most detailed carbonate carbon isotope curves for this time interval (at least within the Siberian Platform). However, an extremely poor paleontological record previously reported from the Sukharikha Formation makes identification of major stratigraphic boundaries highly debatable. We present a detailed study of the Sukharikha and overlying Krasnyi Porog formations in three sections (Sukharikha River, Kulyumbe River, and Khantaisko-Sukhotungusskaya-1 well). Our chemostratigraphic and biostratigraphic data provide a correlation basis for these sections and identify the Cambrian boundary in terms of both the International Chronostratigraphic Chart and General Stratigraphic Scale of Russia. We show that lithologic boundaries and levels of the local first appearance of Tommotian small skeletal fossils are diachronous within the Igarka paleobasin. The latter is putatively caused by paleoecology of the early Cambrian biota and by taphonomic factors. Our data specify the location of the Tommotian Stage (sensu lato)/Cambrian Stage 2 boundary in other key Precambrian–Cambrian transitional sections of the Siberian Platform.
... The Early Cambrian zircons most likely indicate their transport from the northeast of the Siberian Craton, where the riftogenic bimodal complex and tuffs with isotope ages of 525.6 ± 3.9 to 546.0 ± 7.7 Ma are exposed [24,54]. ...
... Olenek L e n a 30 . While this date was originally thought to provide a constraint on the Ediacaran-Cambrian boundary, a U-Pb zircon age of ca. ...
The Ediacaran biota were soft-bodied organisms, many with enigmatic phylogenetic placement and ecology, living in marine environments between 574 and 539 million years ago. Some studies hypothesize a metazoan affinity and aerobic metabolism for these taxa, whereas others propose a fundamentally separate taxonomic grouping and a reliance on chemoautotrophy. To distinguish between these hypotheses and test the redox-sensitivity of Ediacaran organisms, here we present a high-resolution local and global redox dataset from carbonates that contain in situ Ediacaran fossils from Siberia. Cerium anomalies are consistently >1, indicating that local environments, where a diverse Ediacaran assemblage is preserved in situ as nodules and carbonaceous compressions, were pervasively anoxic. Additionally, δ ²³⁸ U values match other terminal Ediacaran sections, indicating widespread marine euxinia. These data suggest that some Ediacaran biotas were tolerant of at least intermittent anoxia, and thus had the capacity for a facultatively anaerobic lifestyle. Alternatively, these soft-bodied Ediacara organisms may have colonized the seafloor during brief oxygenation events not recorded by redox proxy data. Broad temporal correlations between carbon, sulfur, and uranium isotopes further highlight the dynamic redox landscape of Ediacaran-Cambrian evolutionary events.
... A maximum age for intrusion of the Tas-Yuryakh volcanic breccia of the lower Syhargalakh Formation along the Khorbusuonka River is suggested by a zircon U-Pb air-abrasion isotope dilution-thermal ionization mass spectrometry age of 542.8 ± 1.30 Ma (Bowring et al., 1993;Maloof et al., 2010a;Rogov et al., 2015). The Tas-Yuryakh volcanic breccia unconformably overlies the Turkut Formation (Fig. 10), thereby providing an estimated maximum age for the top of the Turkut Formation. ...
The transition from the terminal Ediacaran to early Cambrian (ca. 550−530 Ma) witnessed both the decline of Ediacaran-type soft-bodied and skeletal biota and the rapid diversification of Cambrian-type skeletal biota, which dominate the Terreneuvian (ca. 538.8−521 Ma) fossil record. This interval hosts globally widespread positive and negative δ13Ccarb excursions, including a negative δ13Ccarb excursion near the Ediacaran-Cambrian boundary termed the 1n/BACE. Efforts to produce a global composite chemostratigraphic and biostratigraphic correlation through this interval are complicated by stratigraphic incompleteness and a dearth of radiometric ages with which to constrain δ13Ccarb chemostratigraphy. Extensive and richly fossiliferous open-marine carbonates of the Siberian Platform were deposited from the terminal Ediacaran to beyond Cambrian Series 2, and they offer a unique archive to refine this chemostratigraphic and biostratigraphic framework. Here, we present new δ13Ccarb data from two sections of the southeastern Siberian Platform, and we synthesize these with published δ13Ccarb data from multiple sections throughout the Siberian Platform that record near-continuous carbonate deposition from the latest Ediacaran to Cambrian Series 2. This compilation allowed the construction of two possible chemostratigraphic age models that conform to a coherent framework of lithostratigraphic correlation and platformwide stratal stacking patterns. These age models were then used to test alternative calibrations of fossil first appearances and the spatiotemporal evolution of carbonate deposition on the Siberian Platform. Both models support a pre-1n/BACE appearance of anabaritids in the most distal open-marine sections, and they confirm a transitional Ediacaran-Cambrian biotic assemblage that consisted of co-occurring cloudinids and anabaritids. Sedimentologic and sequence stratigraphic analysis on the Siberian Platform also provides strong evidence to indicate that the 1n/BACE marks the onset of a gradual, pulsed rise in relative sea level that was sustained throughout the Terreneuvian and Series 2 of the Cambrian.
... Cambrotubulus is known from early Cambrian deposits in the Siberian Platform (Karlova and Vodanyuk 1985;Karlova 1987), where it defines the base of the Anabarites trisulcatus Assemblage Zone (Rogov et al. 2015); China (Rozanov et al. 1969), particularly on the Yangtze Platform (Qian 1999;Li et al. 2007); Western Mongolia (Vasil'eva 1986;Esakova and Zhegallo 1996); France (Kerber 1988); Germany (Elicki 1994(Elicki , 2005; and Iran (Hamdi et al. 1989;Hamdi 1995;Devaere et al. 2021). The oldest-known occurrence of Cambrotubulus is 1.4 m above the lower boundary of the Turkut Fm. in the Siberian Platform (Bowring et al. 1993;Rogov et al. 2015), which correlates chemostratigraphically with strata immediately underlying the lower Cambrian of the Anabar Uplift of northeastern Siberia-plausibly representing terminal Ediacaran rocks. ...
The fossil record spanning the latest Ediacaran and earliest Cambrian is characterized by the proliferation of small, mineralized organisms that comprise the well-known and abundant deposits of small shelly fauna. Many of these fossils are tubular or conical forms with simple morphologies, and thus present difficulties in both taxonomic and phylogenetic interpretation. This study investigates a community of poorly preserved shelly tubicolous organisms in two fossiliferous slabs from the Soltanieh Formation, northern Iran. Analysis of the taphonomy of this fossil assemblage using thin-section petrography, scanning electron microscopy, and energy dispersive X-ray spectroscopy, suggests a two-part preservational pathway involving phosphatic replacement of the shell wall and separate, diagenetically later infillings of void space with either phosphatic or calcium carbonate cements. In parallel with the taphonomic study and given the difficulty in assigning the observed fossils taxonomically, morphometrics of the shelly organisms were also explored. Biometric measurements were collected from high-resolution photomosaic images of the slab-surface fossils, as well as from a three-dimensional volume of the interior of one of the slabs generated via X-ray tomographic microscopy. Statistical analysis of these measurements revealed a separation of the fossils into two morphologically distinct groups of conical and tubular forms, which we characterize respectively as ‘conomorphs' and ‘tubomorphs'. Based on previous studies of fossils from the Soltanieh Fm., we can offer tentative generic-level assignment to Anabarites and Cambrotubulus to at least some of the fossils present, though these are dependent on views in thin section rather than morphometric distinction. Cumulatively, we provide a conservative, taxonomy-free approach for detailing the morphology and preservation of poorly preserved fossils from the Ediacaran–Cambrian transition.
... The uppermost magmatic body is located just above the contact between Neoproterozoic and Cambrian rocks and is often interpreted as a volcanic surface flow that formed contemporaneously with sills . U-Pb zircon dating yields a 540-530 Ma age for this mafic magmatic event (Bowring et al. 1993;Prokopiev et al. 2016). ...
A combined structural and geochronological study was carried out to identify the tectonic evolution of the northern Verkhoyansk Fold-and-Thrust Belt, formed on the east margin of the Siberian Craton during late Mesozoic collision. Fault and fold geometries and kinematics were used for palaeostress reconstruction along the Danil and Neleger rivers cross-cutting the central and western parts of the Kharaulakh segment of the northern Verkhoyansk. Three different stress fields (thrust, normal and strike-slip faulting) were identified after separation from heterogeneous fault-slip data. Thrust and normal faulting stress fields were found in both areas, whereas a strike-slip faulting stress field was only found in Neoproterozoic rocks of the Neleger River area. U-Pb laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) dating of calcite slickenside samples reveals the following succession of major deformation events across the northern Verkhoyansk: (i) The oldest tectonic event corresponding to the strike-slip faulting stress field with NE-SW-trending compression axis is Early Permian (Cisuralian, 284 ± 7 Ma) and likely represents a far-field response to the late Palaeozoic collision of the Kara terrane with the northern margin of the Siberian Craton. (ii) A slickenfibrous calcite age of 125 ± 4 Ma is attributed to the Early Cretaceous compression event, when the fold-and-thrust structure was formed. (iii) U-Pb slickenfibre calcite ages of 76-60 Ma estimate the age of a Late Cretaceous-Palaeocene compression event, when thrusts were reactivated. Slickensides related to both (ii) and (iii) compressional tectonic events formed by similar stress fields with WE -trending compression axes. (iv) From the Palaeocene onwards, extensional tectonics with approximately WE extension predominated.
... This event was characterized by the appearance of many of the major phyla that make up modern animal life. [28][29][30] Newly developed species included (a) animals with defined heads and tails for directional movement; (b) animals with hard body parts like shells and spines; (c) chordates, animals with a dorsal nerve chord; (d) brachiopods resembling clams; (e) arthropods, the ancestors of spiders, insects, and crustaceans; (f) mollusks; (g) segmented worms; and (h) species that burrow into the sediments of the seafloor rather than lying on top of it. During the Cambrian Explosion, the proliferation of new species far outstripped the ability for them to have been created by natural selection. ...
The objective of this study is to test the hypothesis that comparing DNA encoding to binary computer programming may explain historical evolutionary bursts that go far beyond anything that could have been anticipated by Darwinian natural selection theory. Statistical analysis of biological sequences suggests that randomness may have a negligible effect on evolution. Every organism is preprogrammed with a binary encoded genetic template for what it could evolve to as a species plus endless possibilities for the evolution of new species. Each DNA molecule consists of a base pair of nucleotides, either guanine (G) coupled with cytosine (C), or adenine (A) coupled with thymine (T). GC and AT are base molecules linked together in long chains. This is analogous to binary computer coding in which each molecule is either a "GC" or an "AT" (rather than a "1" or a "0"). Advanced species have significantly less DNA encoding than primitive species. The amphibian that evolved from a fish no longer needs those parts of its DNA that were exclusive to fish and so loses them. Similarly, the lizard loses those parts of its DNA that were required by amphibians, and so on up the evolutionary scale. Every species carries with it disproportionately huge amounts of inactive DNA that they themselves cannot possibly use. This is for the apparent purpose of keeping biological codes in reserve as a backup contingency plan in case of mass extinctions.
... Apart from South China, Nenoxites has been reported from carbonates in the basal Khatyspyt Formation in Siberia (Rogov et al., 2012(Rogov et al., , 2013 and from clastic rocks of the upper Zhengmuguan Formation in North China (Shen et al., 2007). In Siberia, the Khatyspyt Formation is overlain by the Turkut Formation containing the shelly fossils Cambrotubulus decurvatus of early Cambrian age and yielding a U -Pb zircon age of 543.9 ± 0.3 Ma (Bowring et al., 1993;Knoll et al., 1995), where the age of Nenoxites is further constrained by older fossils (i.e., the bilaterian Kimberella, dated to 555 Ma; Martin et al., 2000). In North China, the Zhengmuguan Formation comprises siltstones and sandstones that are overlain directly by carbonates yielding Cambrian SSFs (Shen et al., 2007), indicating that occurrences of Nenoxites in the upper Zhengmuguan Formation are of late Ediacaran age. ...
The Ediacaran-Cambrian (E-C) transition was a critical stage for the origin and evolution of early complex life on Earth. Because studies of E-C biotas and environments are relatively fewer for deep-water than for shallow-water facies, we undertook a systematic paleontological analysis of the E-C transition in the deep-water Liuchapo Formation at Longbizui, South China. Two sequential but distinct assemblages were recovered, namely the Lower Liuchapo Assemblage (LLA) and the Upper Liuchapo Assemblage (ULA). The LLA consists mainly of soft-bodied fossils, including Longbizuiella hunanensis gen. et sp. nov., Nenoxites jishouensis sp. nov., Nenoxites irregularis sp. nov., Nenoxites sp., Helanoichnus sp., Shaanxilithes? sp. and tiny-sized spherical fossils. The ULA is dominated by skeletal organisms comprising the small shelly faunas (SSFs, mainly protoconodonts, hyoliths and Cambroscleritida), sponge spicules, and the first reported Poratusiramus xiangxiensis gen. et sp. nov. Biostratigraphic correlation support a latest Ediacaran age for the LLA, whereas the majority of the ULA belongs to the early Cambrian—thus the change from the LLA to the ULA coincided with the E-C transition. Our results suggest that, during the E-C transition, a biological revolution may have occurred in deep-water settings synchronously with the well-studied faunal turnover in shallow-water environments. At Longbizui, fossil abundance and diversity decreased gradually during the latest Ediacaran and recovered stepwise after the E-C transition. The deep-water fauna, including both benthic and pelagic members, developed a biomineralization ability during the E-C transition. As calcareous and possibly phosphatic skeletal organisms began to flourish in shallow-water environments, siliceous skeleton-forming organisms, represented by the spicule-bearing sponges, colonized the deep seafloor in great abundance. In addition, the siliceous spicules documented in this study record fast evolutionary development during the early history of sponges, one of the earliest-branching metazoan phyla. The evolution of spicules from monaxon to diaxon-triaxon and then to tetraxon and polyactinal forms, as well as their considerable increase in diameter and length, was achieved during the first SSF biozone.
... Keller and Chumakov (1983), based on the experience of their predecessors, did not recommend using K-Ar dates for glauconite until they are confirmed by other radioisotope methods. Bowring et al. (1993) proved that Rb-Sr dating of rocks is not highly reliable. Thus, the available dates for glauconite of the Karatau Group cannot be used as a reference. ...
—This paper presents an alternative variant of stratigraphic subdivision of the upper Riphean–Vendian deposits (approximately corresponding to the Neoproterozoic Erathem of the International Chronostratigraphic Chart (ICC)) of the Bashkir Mega-Anticlinorium (BMA), based on the modern concepts of changes in the biosphere, atmosphere, and hydrosphere (and, accordingly, in the climate and the course of sedimentary processes) in the period ~1000–540 Ma, with regard to the results of recent studies and the proposals for improving the Russian General Stratigraphic Scale (GSS) of the Precambrian. The strict necessity of subdividing chronostratigraphic and lithostratigraphic units is shown. The presented data give grounds to regard the Zil’merdak Formation as a Group and assign three of its lower formations to the Middle Riphean (Yurmatinian). It is shown that the period of accumulation of the Karatau Group (comprising the Katav, Inzer, Min’yar, and Uk formations) was significantly shorter than the duration of the Late Riphean (Karatavian). Special attention is focused on the age of the Uk Formation (analysis of the existing data made it possible to refine its stratigraphic position): It formed, most likely, in the period 780–740 Ma. A high uncertainty of the stratigraphic position of the overlying geologic bodies is demonstrated. The Bakeevo, Tolparovo, Suirovo, and Kurgashla formations must be regarded as part of the Terminal Riphean (Arshinian). The Arsha Group, comprising the Bainas, Makhmutovo, Igonino, and Shum formations in the Tirlyan trough and represented by the Krivaya Luka Formation in the Krivaya Luka syncline, should not be totally assigned to the Terminal Riphean. It is proposed to exclude the Bakeevo Formation and the Tolparovo–Suirovo sequence from the Asha Group, because these deposits are, most likely, a modification of the Arsha Group located on the western flank of the BMA. The Uryuk Formation is probably of pre-Vendian age. The current contradictory data on the stratigraphy of the supra-Uryuk unit of the Asha Group are interpreted. The ages of the geologic bodies composing the Group and its stratigraphic interval still call for a thorough study.
... Накопление свиты происходило в обстановках, удаленных от береговой линии, приближенных к области глубокого шельфа, на что указывает преобладание глинистых известняков и характерные осадочные текстуры [1]. Возраст трубок взрыва на основе U-Pb датирования цирконов из туфобрекчий одной из них моложе 543.9 ± 0.24 млн лет [5]. Обстановку формирования трубок взрыва связывают с плюмовым событием, сопряженном с рифтогенезом на северо-востоке Сибирского кратона [3]. ...
... В поле распространения кесюсинской свиты находятся трубки взрыва, которые, по имеющимся данным, прорывают туркутскую свиту и нижнюю часть кесюсинской свиты [2]. Для одной из таких трубок получены U-Pb определения 543.9 ± 0.24 млн лет [3], что ограничивает верхний возраст туркутской свиты. В средней части кесюсинской свиты U-Pb датировки 529.6 ± 0.24 млн лет [4] сделаны по цирконам из слоя вулканического пепла. ...
... Plant phylogeny showed that the higher plants possessed more MYB genes than the lower plants, such as green algae (e.g., Ostreococcus lucimarinus, Volvox carteri, and Chlamydomonas reinhardtii). A significant expansion of MYB genes was observed after the Cambrian (about 5404 80MYA), demonstrating an explosive biological diversification episode near the early period [24]. Most of the phylogenetic nodes of plant species were observed in the Cretaceous, a geological period when a typical global warming climate contributed to the terrestrial species diversity [25]. ...
Background
Sugarcane (Saccharum) is the most critical sugar crop worldwide. As one of the most enriched transcription factor families in plants, MYB genes display a great potential to contribute to sugarcane improvement by trait modification. We have identified the sugarcane MYB gene family at a whole-genome level through systematic evolution analyses and expression profiling. R2R3-MYB is a large subfamily involved in many plant-specific processes.
Results
A total of 202 R2R3-MYB genes (356 alleles) were identified in the polyploid Saccharum spontaneum genomic sequence and classified into 15 subgroups by phylogenetic analysis. The sugarcane MYB family had more members by a comparative analysis in sorghum and significant advantages among most plants, especially grasses. Collinearity analysis revealed that 70% of the SsR2R3-MYB genes had experienced duplication events, logically suggesting the contributors to the MYB gene family expansion. Functional characterization was performed to identify 56 SsR2R3-MYB genes involved in various plant bioprocesses with expression profiling analysis on 60 RNA-seq databases. We identified 22 MYB genes specifically expressed in the stem, of which RT-qPCR validated MYB43, MYB53, MYB65, MYB78, and MYB99. Allelic expression dominance analysis implied the differential expression of alleles might be responsible for the high expression of MYB in the stem. MYB169, MYB181, MYB192 were identified as candidate C4 photosynthetic regulators by C4 expression pattern and robust circadian oscillations. Furthermore, stress expression analysis showed that MYB36, MYB48, MYB54, MYB61 actively responded to drought treatment; 19 and 10 MYB genes were involved in response to the sugarcane pokkah boeng and mosaic disease, respectively.
Conclusions
This is the first report on genome-wide analysis of the MYB gene family in sugarcane. SsMYBs probably played an essential role in stem development and the adaptation of various stress conditions. The results will provide detailed insights and rich resources to understand the functional diversity of MYB transcription factors and facilitate the breeding of essential traits in sugarcane.
... In only the next 11 million years evolution increased dramatically during what is called the Cambrian Explosion. This event was characterized by the appearance of many of the major phyla that make up modern animal life [28][29][30]. Newly developed species included (a) animals with defined heads and tails for directional movement; (b) animals with hard body parts like shells and spines; (c) chordates, animals with a dorsal nerve chord; (d) brachiopods resembling clams; (e) arthropods, the ancestors of spiders, insects, and crustaceans; (f) mollusks; (g) segmented worms; and (h) species that burrow into the sediments of the seafloor rather than lying on top of it. ...
... A horizontal gene transfer to green algae and possibly other clades might have occurred after the appearance of the echinodermata. After the cambrian explosion (Bowring et al., 1993), several whole genome doubling events as well as doubling of individual genes may have led to the observed broad spectrum of Trpv5-and Trpv6-like genes (ohnologous genes, (Ohno, 1999)). An N-terminal extension of the TRPV6 protein is present in all placental animals. ...
It is well known that not all biological findings derived from animals can be directly applied to humans. The TRPV6 protein may serve as an example which highlights these inter-species differences as an example of parallel evolutionary pathways. TRPV6 (and TRPV5) belong to a family of ion channels from the transient receptor potential group but are selectively permeable for Ca2+, in contrast to other members of the family. Sequences with recognizable similarity to TRPV6 can already be found in archaebacteria. These ancient sequences show clear similarity to the ion-conducting pore of TRPV6. Over the course of evolution, the duplication of the TRPV6 gene gave rise to TRPV5. Duplications of the complete genome as well as subsequent loss of genetic material have led to a variety of different TRPV5/6 combinations. In addition, there is an N-terminal extension of the protein in placental animals. This extension causes translation of TRPV6 to be initiated from an ACG codon. Inactivation of one TRPV6 allele can be correlated with alcohol-independent pancreatitis in humans while inactivation of both alleles leads to skeletal dysplasia of newborn babies. The latter effect is not observed in mice, implying that the effects due to perturbations in TRPV6 levels are much more pronounced in humans.
... Plant phylogeny showed that the higher plants possessed more MYB genes than the lower plants, such as green algae (e.g., Ostreococcus lucimarinus, Volvox carteri, and Chlamydomonas reinhardtii). A signi cant expansion of MYB genes was observed after the Cambrian (about 540~480MYA), demonstrating an explosive biological diversi cation episode near the early period [26]. Most of the phylogenetic nodes of plant species were observed in the Cretaceous, a geological period when a typical global warming climate contributed to the diversity of the terrestrial species [27]. ...
Background: Sugarcane (Saccharum) is the most important sugar crop in the world. As one of the most enriched transcription factor families in plants, MYB genes display a great potential to contribute to sugarcane improvement by trait modification. We have identified the sugarcane MYB gene family at a whole-genome level through systematic evolution analyses and expression profiling. R2R3-MYB is a large subfamily involved in many plant-specific processes.
Results: A total of 202 R2R3-MYB genes (356 alleles) were identified in the polyploid Saccharum spontaneum genome and classified into 15 subgroups by phylogenetic analysis. The sugarcane MYB family had more members by a comparative analysis in sorghum and significant advantages among most plants, especially grasses. Collinearity analysis revealed that 70% of the SsR2R3-MYB genes had experienced duplication events, logically suggesting the contributors to the MYB gene family expansion. Functional characterization was performed to identify 56 SsR2R3-MYB genes involved in various plant bioprocesses with expression profiling analysis on 60 RNA-seq databases. We identified 22 MYB genes specifically expressed in the stem, of which MYB43, MYB53, MYB65, MYB78, and MYB99 were validated by qPCR. Allelic expression dominance in the stem was more significant than that in the leaf, implying the differential expression of alleles may be responsible for the high expression of MYB in the stem. MYB169, MYB181, MYB192 were identified as candidate C4 photosynthetic regulators by C4 expression pattern and robust circadian oscillations. Furthermore, stress expression analysis showed that MYB36, MYB48, MYB54, MYB61 actively responded to drought treatment; 19 and 10 MYB genes were involved in response to the sugarcane pokkah boeng and mosaic disease, respectively.
Conclusions: A Genome-wide expression analysis demonstrated that SsMYB genes were involved in stem development and stress response. This study largely contributed to understanding the extent to which MYB transcription factors investigate regulatory mechanisms and functional divergence in sugarcane.
... Previously reported data from Namibia suggest that the adjusted age of the base of the Cambrian is between 538 and 540 Ma Grotzinger et al., 1995). These data, plus the new estimates (Linnemann et al., 2019), approximate other zircon dates, stratigraphically less constrained, from Siberia (Bowring et al., 1993) and from the upper part of the Ediacaran (Grotzinger et al., 1995;Tucker and McKerrow, 1995). ...
Appearance of metazoans with mineralized skeletons, “explosion” in biotic diversity and disparity, infaunalization of the substrate, occurrence of metazoan Konservat Fossil-lagerstätten, establishment of most invertebrate phyla, strong faunal provincialism, dominance of trilobites, generally warm climate but with possible glacial–interglacial cycles in the later part, opening of the Iapetus Ocean, progressive equatorial drift and separation of Laurentia, Baltica, Siberia, and Avalonia from Gondwana characterize the Cambrian Period.
... The succession hosts laterally discontinuous stratiform tuff breccia and tuffites, occasionally basaltic flows, and is penetrated by numerous diatremes composed of tuff breccia and basalts, which are considered to be feeders for the stratified bodies. A U-Pb zircon date of 543.9 ± 0.24 Ma for a diatrem tuff breccia (Bowring et al., 1993) has long been used to constrain the age of the Ediacaran/Cambrian boundary in Siberia; however, the origin of the zircons remains ambiguous (Kiselev et al., 2016;Rogov et al., 2015;Vishnevskaya et al., 2017) and is in need of revision. A comprehensive field study conducted in 2018 allowed us to reexamine and re-sample a wide range of magmatic and volcanic formations cropping out in the middle reaches of the Khorbusuonka River, Olenek Uplift. ...
The genetic code is profoundly shaped by an origin in ancient RNA-mediated interactions, needing an extended development to reach the Standard Genetic Code (SGC). That development can serially use RNA specificities, a ribonucleopeptide transition (RNPT), finally code escape and diaspora. An index of evolutionary plausibility based on least selection takes simultaneous account of speed and accuracy of evolution, identifying favored evolutions. Combining RNA world specificities allowed convergence of early coding to SGC assignments. Secondly, this was sufficient to launch a post-RNA-world RNPT. The RNPT allowed biosynthesis of complex amino acids, depending heavily on late code fusions between coexisting independent codes. Thirdly, escape from fluctuating, but highly-evolved codes of the RNPT applied a near-ideal selection for fastest-evolving and most accurate/useful genetic codes. Concurrently, a code and its microbial carrier suited to a free-living existence necessarily evolved. The established unity of life on Earth likely traces to SGC ascendancy during escape from the RNPT, and code diaspora.
High-precision U-Pb zircon ages on SE Newfoundland tuffs now bracket the Avalonian Lower–Middle Cambrian boundary. Upper Lower Cambrian Brigus Formation tuffs yield depositional ages of 507.91 ± 0.07 Ma ( Callavia broeggeri Zone) and 507.67 ± 0.08 Ma and 507.21 ± 0.13 Ma ( Morocconus-Condylopyge eli Assemblage interval). Lower Middle Cambrian Chamberlain’s Brook Formation tuffs have depositional ages of 506.34 ± 0.21 Ma ( Kiskinella cristata Zone) and 506.25 ± 0.07 Ma ( Eccaparadoxides bennetti Zone). The composite unconformity separating the Brigus and Chamberlain’s Brook formations is constrained between these ages. An Avalonian Lower–Middle Cambrian boundary between 507.2 ± 0.1 and 506.3 ± 0.2 Ma is consistent with maximum depositional age constraints from southwest Laurentia, which indicate an age for the base of the Miaolingian Series, as locally interpreted, of ≤ 506.6 ± 0.3 Ma. The Miaolingian Series’ base is interpreted as correlative within ≤ 0.3 ± 0.3 Ma between Cambrian palaeocontinents, although its exact synchrony is questionable due to taxonomic problems with a possible Oryctocephalus indicus -plexus, invariable dysoxic lithofacies control of O. indicus and diachronous occurrence of O. indicus in temporally distinct δ ¹³ C chemozones in South China and SW Laurentia. The lowest occurrence of O. indicus assemblages is linked to onlap (epeirogenic or eustatic) of dysoxic facies. A united Avalonia is shown by late Early Cambrian volcanics in SW New Brunswick; Cape Breton Island; SE Newfoundland; and the Wrekin area, England. The new U-Pb ages revise Avalonian geological evolution as they show rapid epeirogenic changes through depositional sequences 4a–6.
Fossil material assigned to Nenoxites from the late Ediacaran Khatyspyt Formation of Arctic Siberia (550–544 Ma) has been presented as evidence for bioturbation prior to the basal Cambrian boundary. However, that ichnological interpretation has been challenged, and descriptions of similar material from other global localities support a body fossil origin. Here we combine x-ray computed tomography, scanning electron microscopy and petrographic methods to evaluate the body or trace fossil nature of Nenoxites from the Khatyspyt Formation. The fossilized structures consist of densely packed chains of three-dimensionally preserved silicic, bowl-shaped elements surrounded by distinct sedimentary halos, in a dolomitized matrix. Individual bowl-shaped elements can exhibit diffuse mineralogical boundaries and bridging connections between elements, both considered here to result from silicification and dolomitization during diagenesis. This new morphological and petrological evidence, in conjunction with recent studies of the late Ediacaran tubular taxa Ordinilunulatus and Shaanxilithes from China, suggest that the Khatyspyt specimens most probably reflect a coquina deposit of Shaanxilithes-like body fossils. Our data support the possibility of Shaanxilithes-like organisms representing total group eumetazoans.
The way towards the stage subdivision of the Vendian starts with the assemblage of a complete composite geological section of this interval of the Earth’s history, a succession of geological bodies each reflecting a certain ecosystem state. A Vendian composite section of the Siberian Platform, which is a succession of regional to sub-global scale geological bodies reflecting particular states or unidirectional transformations determined by episodic expansion of relatively oxygen-rich environments onto the shelf and alternating with large-scale oceanic anoxia and euxinia events is proposed as a reference for constructing the stage subdivision of the Vendian. The redox instability had to be accompanied by changes in nutrient availability and could not but affect the course of macroevolution and macroecology. The geological record of the Vendian of the Siberian Platform is also marked by episodic increase in alkalinity of the World Ocean. At least five such alkalinity events could be provisionally identified in the composite section of the Vendian of the Siberian Platform. A hypothesis is proposed suggesting that the alkalinity events could control the appearance and disappearance in the geological record of a distinctive suite of sedimentary structures in carbonates, the discrete nature of early diagenetic cementation of aluminosilicoclastic sediments, and fossilization of soft-bodied organisms in the Vendian.
The delay between the origin of animals in the Neoproterozoic and their Cambrian diversification remains perplexing. Animal diversification mirrors an expansion in marine shelf area, but the mechanisms linking this environmental change to early organismal ecology are unclear. In this study, we used a biogeochemical model to consider oxygen dynamics at the sunlit sediment–water interface over night-day (diel) cycles in the early Cambrian under a greenhouse climate. We found that temperatures dictated the diurnal benthic oxic-anoxic shifts that were physiologically stressful to early animals and this dynamic expanded over nutrient-rich shelf areas as continents were flooded in the Cambrian. We suggest that the combination of a greenhouse climate and continental flooding combined to amplify diurnal benthic redox variability, which promoted the adaptive radiation of animals tolerant to oxygen fluctuations and stress, the Metazoa.
Plain Language Summary
The earliest appearance of rooted plant fossils coincides with widespread evidence of meandering rivers in the geologic record. This correlation has led researchers to suggest that meandering rivers only existed on our planet since terrestrial plants colonized the continents, and that pre‐vegetation rivers were predominantly characterized by shallow and multi‐thread channels. While there is growing evidence of deep, single‐thread rivers predating the rise of land plants, it is currently unclear how these rivers maintained stable banks. Here, we combine observations of 1.2‐billion‐year‐old river sediments in NW Scotland with mechanistic theories of river dune formation to constrain the geometry of pre‐vegetation rivers. We show that our field observations are consistent with deposition by deep, low‐sloping, and single‐thread rivers, whose reconstructed geometry is similar to modern‐day meandering, rather than braided, rivers. We also demonstrate that the mud fraction of the floodplain sediment could have provided sufficient cohesion to resist erosional forces in deep, low‐sloping rivers. Together, our results indicate that single‐thread rivers could have been prevalent before the rise of land plants, and that mud can provide sufficient bank strength for the development of deep rivers.
The advent of greatly improved radiometric dating techniques with lower uncertainties, the development of new dating and correlation techniques, including vastly expanded quantitative biostratigraphic methods, and the possibility of reliable extension of orbital cyclostratigraphy into the Paleozoic all promise a great improvement in the ability of geologists to construct high-resolution temporal frameworks far deeper into the past. Such techniques have already allowed the generation of a high-resolution temporal framework for the Ediacaran-Cambrian radiation of metazoa, helped greatly narrow the duration of the great Permo-Triassic mass extinction and eliminated several hypothesized causes, and narrowed the duration of the oceanic anoxic event at the Cenomanian-Turonian (Late Cretaceous boundary). Temporal resolution of 100 kyr (0.02%) or even better into the early Paleozoic now seems likely, opening a host of new questions for reliable investigation. Further exploiting the possibilities of these techniques will require paleontologists to improve methods to integrate these disparate techniques, improve our understanding of the analysis of evolutionary rates, and confront the challenges of settings where geochronologic resolution may be greater than paleontologic resolution.
One of my favorite places from which to collect fossils is a steep, high railway cut in a rural part of southern Kentucky, where a ridge made of limestone sat right across the most efficient route the railway could take. When that cut was first made, professional paleontologists and amateur fossil collectors came by to have a look at the newly exposed rock; but although the fossils could be seen, we could hardly get anything out because they were almost all too firmly locked within the rock. Within just a couple of years, weathering had caused some of the weaker, clay-rich layers to break down a little, yielding loads of fossils. The rock around the fossils broke apart into small grains, and the fossils themselves, skeletal remains of marine animals, could be picked up by the hundreds of thousands: crinoids, blastoids, brachiopods, screw-shaped parts of bryozoans, trilobites, individual cone-shaped corals, strange conical snails, even rare starfish.
One of my favorite places to collect fossils is a steep, high railway cut in a rural part of southern Kentucky, where a ridge made of limestone sat right across the most efficient route the railway could take. When that cut was first made, professional paleontologists and amateur fossil collectors came by to have a look at the newly exposed rock; but although the fossils could be seen, we could get hardly anything out because they were almost all too firmly locked within the rock. Within just a couple of years, weathering had caused some of the weaker, clay-rich layers to break down a little, yielding loads of fossils. The rock around the fossils broke apart into small grains, and the fossils themselves, skeletal remains of marine animals, could be picked up by the hundreds of thousands: crinoids, blastoids, brachiopods, screw-shaped parts of bryozoans, trilobites, individual cone-shaped corals, strange conical snails, even rare starfish.
The Olenek uplift is a highly promising region for the search of oil fields. The current work is devoted to paleogeographic position of Siberia in the Late Vendian-Early Cam-brian. The results of studying the composition of the magnetization of the Turkut, Kesyushin and Yerkeket formations in the middle course of the Khorbusuonki River are presented. The position of the sedimentation basin at the time of formation of the studied rocks corresponds to 43,6° south latitude.
We report results of paleomagnetic studies of mafic dikes and sills from the Tas-Yuryakh magmatic
complex on the Olenek uplift in the northeast of the Siberian platform. The paleomagnetic record in the rocks
corresponds to an episode of anomalous state of the geomagnetic field that persisted from the Ediacaran
period (~580 Ma and younger) to the end of the Fortunian age. Paleointensity measurements indicate an
extremely low value of the virtual dipole moment during this time. This presumably caused a disruption of
the normal Geocentric Axial Dipole model, so much so that the world magnetic anomalies made a substantial
contribution. We propose that the Antarctic anomaly influenced the magnetization of the Siberian craton
rocks during this period of very low dipole moment. The high latitudes corresponding to the observed paleopole
do not correspond to the actual paleogeography of Siberia and can be used for paleoreconstructions only
after adjusting for this anomaly. The true position of the Olenek uplift at the Precambrian–Paleozoic boundary
was close to 30° S above the southeastern periphery of the African (Tuzo) mantle hot field.
Tephrochronology is one of the most effective ways to correlate and date Quaternary deposits across large distances. However, it can be challenging to obtain direct ages on tephra beds when they are beyond the limit of radiocarbon dating, do not contain mineral phases suitable for ⁴⁰K-⁴⁰Ar (or ⁴⁰Ar/³⁹Ar) dating, or suitable glass shards for fission-track dating are not available. Zircon U-Pb dating by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) is an emerging technique for dating young (<1 Ma) tephra. Here, we demonstrate that LA-ICP-MS zircon U-Pb dating can produce reliable ages for key tephra beds found in Yukon and Alaska. We assessed five different techniques for calculating tephra maximum depositional ages from zircon U-Pb ages for eight tephra beds. Our preferred zircon U-Pb ages (reported with 2σ uncertainties), based on a Bayesian model for calculating maximum depositional ages, are broadly consistent with previously established chronology constructed from stratigraphy, paleomagnetism, and/or glass fission track and ⁴⁰Ar/³⁹Ar ages: Biederman tephra (178 ± 17 ka), HP tephra (680 ± 47 ka), Gold Run tephra (688 ± 44 ka), Flat Creek tephra (708 ± 43 ka), PA tephra (1.92 ± 0.06 Ma), Quartz Creek tephra (2.62 ± 0.08 Ma), Lost Chicken tephra (3.14 ± 0.07 Ma), and GI tephra (542 ± 64 ka). We also present newly revised glass fission-track and ⁴⁰Ar/³⁹Ar ages recalculated from previous determinations using updated ages for the Moldavite tektite and Fish Canyon Tuff standards, and updated K decay constants. For Pleistocene age zircon crystals, corrections for ²³⁰Th disequilibrium and common-Pb are significant and must be treated with caution. Similarly, apparent tephra ages are sensitive to the choice of method used to calculate a maximum depositional age from the assemblage of individual crystallization ages. This study demonstrates that LA-ICP-MS zircon U-Pb dating can be successfully applied to numerous Pliocene-Pleistocene Alaskan-Yukon tephra, providing confidence in applying this method to other stratigraphically important tephra in the region.
The diversification of animals during the Cambrian Period is one of the most significant evolutionary events in Earth’s history. However, the sequence of events leading to the origin of ‘modern’ ecosystems and the exact temporal relationship between Ediacaran and Cambrian faunas are uncertain, as identification of the Ediacaran–Cambrian boundary and global correlation through this interval remains problematic. Here we review the controversies surrounding global correlation of the base of the Cambrian and present new high-resolution biostratigraphic, lithostratigraphic and δ¹³C chemostratigraphic data for terminal Ediacaran to basal Cambrian strata in the Zavkhan Basin of Mongolia. This predominantly carbonate sequence, through the Zuun-Arts and Bayangol formations in southwestern Mongolia, captures a distinct, negative δ¹³C excursion close to the top of the Zuun-Arts Formation recognized as the BAsal Cambrian carbon isotope Excursion (BACE). In this location, the nadir of the BACE closely coincides with first occurrence of the characteristic early Cambrian protoconodont Protohertzina anabarica. Despite recent suggestions that there is an evolutionary continuum of biomineralizing animals across the Ediacaran–Cambrian transition, we suggest that this continuum is restricted to tubular forms and that skeletal taxa such as Protohertzina depict ‘true’ Cambrian representatives and are restricted entirely to the Cambrian. Employing the first appearance of the trace fossil Treptichnus pedum to define the base of the Cambrian suffers significant drawbacks, particularly in carbonate settings where it is not commonly preserved. As T. pedum is the only proxy available to correlate the Cambrian Global boundary Stratotype Section and Point (GSSP) defined at Fortune Head, Newfoundland, we suggest that the GSSP be redefined elsewhere, in a new stratigraphic section that contains secondary markers that permit global correlation. We propose the nadir of the BACE as the favored candidate to define the base of the Cambrian. However, it is essential that the BACE be complemented with secondary markers. In many global sections the nadir of the BACE and the first occurrence of the genus Protohertzina are closely juxtaposed, as are the BACE and T. pedum. Hence these taxa provide essential biostratigraphic control on the BACE and increase potential for effective global correlation. We also recommend that an Auxiliary boundary Stratotype Section and Point (ASSP) be simultaneously established in order to incorporate additional markers that will aid global correlation of the Ediacaran–Cambrian boundary. The BAY4/5 section through the upper Zuun-Arts and Bayangol formations yields key shelly fossils and δ¹³C values and is therefore an ideal candidate for consideration as the GSSP for the Ediacaran–Cambrian boundary.
THE Chengjiang fossil Lagerstätte is one of the earliest and most important palaeontological sites from the Phanerozoic era1,2, about 530 million years ago3. It yields extremely abundant and remarkably preserved soft-bodied fossils and shells with soft parts of various kinds, including bradoriids4–6, trilobites7,8, crustaceans9, brachiopods, worms, sponges, algae and many unknown forms10–13. One of these fossils is Yunnanozoon
14, which we reinterpret
here as the earliest known hemichordate. Possessing half of the characteristic chordate features and providing an anatomical link between invertebrates and chordates15, Hemichor-data is a minor but important phylum in evolutionary biology. Hemichordates comprise two main groups: the enteropneusts, or 'acorn worms', and the pterobranchs. Apart from the presumable inclusion of graptolites in pterobranchs16–19, there are very few hemichordate fossils2,17,20. Although Yunnanozoon is superficially similar to the chordates21,
its typical tripartite body plan is broadly consistent with that of living balanoglossid hemichor-dates (enteropneusts).
The rise of complex macroscopic life occurred during the Ediacaran Period, an interval that witnessed large-scale disturbances to biogeochemical systems. The current Ediacaran chronostratigraphic framework is of insufficient resolution to provide robust global correlation schemes or test hypotheses for the role of biogeochemical cycling in the evolution of complex life. Here, we present new radio-isotopic dates from Ediacaran strata that directly constrain key fossil assemblages and large-magnitude carbon cycle perturbations. These new dates and integrated global correlations demonstrate that late Ediacaran strata of South China are time transgressive and that the 575-to 550-Ma interval is marked by two large negative carbon isotope excursions: the Shuram and a younger one that ended ca. 550 Ma ago. These data calibrate the tempo of Ediacaran evolution characterized by intervals of tens of millions of years of increasing ecosystem complexity, interrupted by biological turnovers that coincide with large perturbations to the carbon cycle.
We combine U-Pb in-situ carbonate dating, elemental and isotope constraints to calibrate the synergy of integrated mountain-basin evolution in western Gondwana. We show that deposition of the Bambuí Group coincides with closure of the Goiás-Pharusian (630-600 Ma) and Adamastor (585-530 Ma) oceans. Metazoans thrived for a brief moment of balanced redox and nutrient conditions. This was followed, however, by closure of the Clymene ocean (540-500 Ma), eventually landlocking the basin. This hindered seawater renewal and led to uncontrolled nutrient input, shallowing of the redoxcline and anoxic incursions, fueling positive productivity feedbacks and preventing the development of typical Ediacaran-Cambrian ecosystems. Thus, mountains provide the conditions, such as oxygen and nutrients, but may also preclude life development if basins become too restricted, characterizing a Goldilocks or optimal level effect. During the late Neoproterozoic-Cambrian fan-like transition from Rodinia to Gondwana, the newborn marginal basins of Laurentia, Baltica and Siberia remained open to the global sea, while intracontinental basins of Gondwana became progressively landlocked. The extent to which basin restriction might have affected the global carbon cycle and climate, e.g. through the input of gases such as methane that could eventually have collaborated to an early Cambrian greenhouse world, needs to be further considered.
This chapter discusses evolution of the crust and mantle through 4.6 Gyr of Earth's history. It begins with a discussion of the Hadean era focusing on new and exciting research on the ages of detrital zircons and the origin and composition of the earliest crust. It includes a brief summary of Earth's oldest rocks. Models of crustal origin and growth are presented with a detailed discussion of the growth rate of continental crust including problems of recycling into the mantle and juvenile crust production. The crustal age gap at 2.4–2.2 Ga is discussed, as are secular changes in the composition, rock associations, and tectonic settings of the crust, the controversial topic of when plate tectonics began. The chapter ends with discussion of secular changes in the mantle, including the lithosphere, and models for Earth's thermal history.
Research subject . Main problems of the General Stratigraphic Scale (GSS) of the Upper Precambrian including uncertainties in the hierarchy of subdivisions are analyzed.
Results. Prospects for detailing the Upper Precambrian GSS are discussed, along with questions of its correlation with International Chronostratigraphic Chart (ICSC) and establishing the lower boundaries of chronostratigraphic subdivisions. The importance of unifying the existing views is emphasized.
Conclusions. It is proposed to carry out the following reforms of GSS: to abolish Acrothemes / Acrons; to approve the Proterozoic (as well as the Archean) as an Eonotheme / Eon; to minimize the use of terms “Upper Proterozoic” and “Lower Proterozoic”; to assign the Riphean and Vendian to the rank of Erathem / Era (while preserving the status of the Vendian as a System / Period); to consider Burzyanian, Yurmatinian, Karatavian and Arshinian as Systems / Periods of the Riphean. Attention is focused on the Upper Riphean-Vendian interval. The lower boundary of the Upper Riphean (Karatavian) was proposed to establish according to the first appearance of the Trachyhystrichosphaera sp. microfossils. Then, the Terminal Riphean (Arshinian) lower boundary should be traced to the base of the tillites formed during the global Sturtian glaciation (which approximately corresponds to the base of the Cryogenian in ICSC). Apparently, the Vendian lower boundary may be raised to the level of the top of the Gaskiers tillites, as the deposits of the last major glaciation in the Precambrian. The indicated proposals are substantiated. It is necessary to form work groups to develop solutions.
Macrofossils of the late Ediacaran Period ( c. 579–539 Ma) document diverse, complex multicellular eukaryotes, including early animals, prior to the Cambrian radiation of metazoan phyla. To investigate the relationships between environmental perturbations, biotic responses and early metazoan evolutionary trajectories, it is vital to distinguish between evolutionary and ecological controls on the global distribution of Ediacaran macrofossils. The contributions of temporal, palaeoenvironmental and lithological factors in shaping the observed variations in assemblage taxonomic composition between Ediacaran macrofossil sites are widely discussed, but the role of palaeogeography remains ambiguous. Here we investigate the influence of palaeolatitude on the spatial distribution of Ediacaran macrobiota through the late Ediacaran Period using two leading palaeogeographical reconstructions. We find that overall generic diversity was distributed across all palaeolatitudes. Among specific groups, the distributions of candidate ‘Bilateral’ and Frondomorph taxa exhibit weakly statistically significant and statistically significant differences between low and high palaeolatitudes within our favoured palaeogeographical reconstruction, respectively, whereas Algal, Tubular, Soft-bodied and Biomineralizing taxa show no significant difference. The recognition of statistically significant palaeolatitudinal differences in the distribution of certain morphogroups highlights the importance of considering palaeolatitudinal influences when interrogating trends in Ediacaran taxon distributions.
Supplementary material: Supplementary information, data and code are available at https://doi.org/10.6084/m9.figshare.c.5488945
Thematic collection: This article is part of the Advances in the Cambrian Explosion collection available at: https://www.lyellcollection.org/cc/advances-cambrian-explosion
Обсуждаются новые данные о петрографических и геохимических особенностях карбонатных отложений юго-восточной части Анабаро-Синской структурно-формационной области. Впервые в пределах разреза тойонских отложений фации предрифового склона выявлены прямые и косвенные признаки синхронных седиментогенезу вулканогенной и эксгаляционной деятельности. Результаты проведенных исследований позволяют использовать положительные геохимические аномалии Co, Pb, Zn, Ba и Mnв коренных породах, а также величины отношений HREE/LREE, Ce/Ce* и Eu/Eu* в качестве одного из поисковых критериев проявления стратиформного Pb-Zn оруденения в пределах Синско-Ботомской структурно-формационной зоны.
The paper presents new data of petrographic and geochemical features of carbonate deposits in the southeastern part of the Anabar-Sinsk basin. Direct and indirect signs of volcanogenic and exhalation activity synchronous to sedimentogenesis have been identified in the section of Toyonian deposits of the pre-reef slope facies for the first time. Results of the studies allow using positive geochemical Co, Pb, Zn, Ba and Mn anomalies, the values of the HREE/LREE, Ce/Ce* and Eu/Eu* ratios in bedrocks as a search criterion for the lead-zinc Mississippi-Valley-type mineralization within the Sinsk-Botomian Group.
Carbonate units deposited during the Precambrian-Cambrian transition records a unique δ ¹³ C profile that is useful for chemostratigraphic correlation. However, the Precambrian-Cambrian boundary is currently defined within siliciclastic units where δ ¹³ C data are not available. The mixed siliciclastic-carbonate succession from the southern Great Basin, USA, records the appropriate fossils in the siliciclastic strata interbedded with carbonate strata that record the appropriate shifts in δ ¹³ C to facilitate correlation between the lithologic end-members. Ultimately, the integrated dataset demonstrates that vertical burrowing and the onset of widespread biomineralization was essentially synchronous.
High initial parent/daughter element ratios and a unique dual decay scheme make UPb zicron ages more precise and reliable than most isotopic ages, and thus inherently superior for time-scale calibration. Employing improved techniques to the conventional method of UPb dating, we have analyzed microgram-size (2–12 × 10−8 g) zircon fractions from biostratigraphically controlled volcanic ashes and dated key Paleozoic time-markers with a precision better than 1% (±2Ma). Four of the stratotype samples from Britain for which fission-track ages [1] were previously reported have yielded improved ages of:438.7 ± 2.0Ma for the lower Silurian zone of Coronograptus cyphus from Llandovery strata at Dob's Linn, southern Scotland;457.5 ± 2.2 Ma for a Middle Ordovician Caradoc (Longvillian) ash near Bala, North Wales, and;465.7 ± 2.1and464.6 ± 1.8 Ma for the Didymograptus artus Zone and the type Didymograptus Murchisoni Zone, respectively, of the Llanvirn Series at Arenig Fawr and Abereiddi Bay, Wales. Another sample from the zone of Dicellograptus anceps (P. pacificus Subzone) of the Ashgill Series at Dob's Linn has been dated at445.7 ± 2.4Ma, suggesting placement of the Ordovician-Silurian time boundary at approximately 441 Ma. A sixth bentonite from Caradocian age strata of North America (Spechts Ferry Shale, Decorah Formation, Missouri) is453.7 ± 1.8Ma old, indicating that the Rocklandian Stage of the Mohawkian Series is only slightly younger than the Longvillian Stage of the Caradoc Series in Britain.
A simple equilibrial model for the growth and maintenance of Phanerozoic global marine taxonomie diversity can be constructed from considerations of the behavior of origination and extinction rates with respect to diversity. An initial postulate that total rate of diversification is proportional to number of taxa extant leads to an exponential model for early phases of diversification. This model appears to describe adequately the “explosive” diversification of known metazoan orders across the Precambrian-Cambrian Boundary, suggesting that no special event, other than the initial appearance of Metazoa, is necessary to explain this phenomenon. As numbers of taxa increase, the rate of diversification should become “diversity dependent.” Ecological factors should cause the per taxon rate of origination to decline and the per taxon rate of extinction to increase. If these relationships are modeled as simple linear functions, a logistic description of the behavior of taxonomie diversity through time results. This model appears remarkably consistent with the known pattern of Phanerozoic marine ordinal diversity as a whole. Analysis of observed rates of ordinal origination also indicates these are to a large extent diversity dependent; however, diversity dependence is not immediately evident in rates of ordinal extinction. Possible explanations for this pattern are derived from considerations of the size of higher taxa and from simulations of their diversification. These suggest that both the standing diversity and the pattern of origination of orders may adequately reflect the behavior of species diversity through time; however, correspondence between rates of ordinal and species extinction may deteriorate with progressive loss of information resulting from incomplete sampling of the fossil record.
The kinetic model of taxonomic diversity predicts that the long-term diversification of taxa within any large and essentially closed ecological system should approximate a logistic process controlled by changes in origination and extinction rates with changing numbers of taxa. This model is tested with a new compilation of numbers of metazoan families known from Paleozoic stages (including stage-level subdivisions of the Cambrian). These data indicate the occurrence of two intervals of logistic diversification within the Paleozoic. The first interval, spanning the Vendian and Cambrian, includes an approximately exponential increase in families across the Precambrian-Cambrian Boundary and a “pseudo-equilibrium” through the Middle and Late Cambrian, caused by diversity-dependent decrease in origination rate and increase in extinction rate. The second interval begins with a rapid re-diversification in the Ordovician, which leads to a tripling of familial diversity during a span of 50 Myr; by the end of the Ordovician diversity attains a new dynamic equilibrium that is maintained, except for several extinction events, for nearly 200 Myr until near the end of the Paleozoic. A “two-phase” kinetic model is constructed to describe this heterogeneous pattern of early Phanerozoic diversification. The model adequately describes the “multiple equilibria,” the asymmetrical history of the “Cambrian fauna,” the extremely slow initial diversification of the later “Paleozoic fauna,” and the combined patterns of origination and extinction in both faunas. It is suggested that this entire pattern of diversification reflects the early success of ecologically generalized taxa and their later replacement by more specialized taxa.
Cohort analysis provides an effective method of analysing taxonomic survivorship in the fossil record where large data sets are available. An analysis of the Stratigraphic ranges of about 8,500 fossil genera and subgenera shows that survivorship patterns are substantially the same throughout the Phanerozoic. These patterns are used to calculate an average value for mean species duration among fossil invertebrates (11.1 Myr.). Also, the extra extinctions near the Permo-Triassic boundary are shown to be equivalent to about 85 Myr of normal, background extinction.
Cohort analysis is used to investigate survivorship of trilobites originating during the Cambrian and Ordovician. Using a time-homogeneous branching model, it is estimated that trilobite genera and species originating during the Ordovician survived three times longer than Cambrian genera and species. Monte Carlo simulation of survivorship is used to show that (1) Cambrian and Ordovician survivorship are significantly different, (2) Ordovician cohorts conform more closely to the time-homogeneous model than do Cambrian cohorts, and (3) deviations from temporal homogeneity are more often produced by extraordinary extinction than by unusually slow turnover.
When Early Ordovician cohorts are decomposed into genera within families that originated in the Cambrian versus the Ordovician, no evidence that Cambrian and Ordovician survivorship differences are clade-specific can be found. Ordovician genera of Cambrian affinity and of Ordovician affinity become extinct at similar rates.
Some of the ultimate causes of these differences in survivorship include (1) taxonomic inconsistency, (2) greater environmental stability in the Ordovician, and (3) more highly structured ecosystems in the Ordovician that may have led to the weeding out of extinction-prone taxa.
New stratigraphic correlations and U-Pb zircon ages on intercalated volcanic ash beds calibrate the temporal evolution of the passive margin sequence exposed in Wopmay orogen and require revision of tectonic models to explain it. The age of initial passive margin sedimentatin in Wopmay orogen is 1969 Ma, and its destruction during collision and initiation of foreland basin subsidence occurred by 1882 Ma. Therefore, the duration of the passive margin was on the order of 80 to 90 my. The new age data required reconsideration of the stratigraphic position and tectonic significance of the Akaitcho Group. Accordingly, a new tectonic model for Wopmay orogen is proposed that involves collison of an extending, arc-bearing micro-continent with a west-facing passive margin developed on the edge of Slave craton. -from Authors
Cambrian carbonate strata of the Great Basin deposited below fairweather wave base but above mean storm wave base (inner shelf), and below mean storm wave base but above maximum storm wave base (middle shelf) were examined for extent and depth of bioturbation. Data indicate that initial extensive colonization of marine infaunal habitats occurred between Tommotian and Atdabanian time. Throughout the remaining 65 m.y. of Cambrian time bioturbation increased, although depth of bioturbation remained relatively constant at less than 6 cm. During this time, the amount of bioturbation in the inner shelf was greater than that in the middle shelf.
Contributions to a symposium of the IUGS Subcommission on Cambrian Stratigraphy at the International Geological Congress in Montreal in 1972 are the basis of this paper which is intended to give guidance to further work of what is now a Working Group on the Precambrian---Cambrian boundary of the International Geological Correlation Programme (UNESCO/IUGS). The proposed principles for the definition of the boundary are based on those accepted by the Commission on Stratigraphy, particularly on stratigraphic classification and on defining the Silurian---Devonian boundary. These matters are discussed by the authors in general terms and by W.B. Harland in terms of procedure. They are illustrated by examples of stratigraphic sequences across the boundary in Siberia given by A.Yu. Rozanov and discussed critically by V.E. Savitsky. New relevant discoveries of Early Cambrian faunas and their stratigraphic setting are reviewed by A. Boudda et al. P.E. Cloud's contribution, published elsewhere, and work in Australia are reviewed by the authors. The conclusions, based not only on the symposium material but also on current opinion, are that no single sequence which could serve as a reference standard for the Precambrian---Cambrian boundary has yet been identified but that many significant ones have been described which will be further examined. While the general limits for the choice of a boundary definition should lie between the horizons with fossils of Ediacaran type and those with diversified trilobite assemblages, a number of ``Working Reference Points'' exist in that interval which are correlatable horizons of faunal change. These cannot be defined simply as horizons of first appearance.
Ion microprobe U-Pb dating of zircons extracted from a narrow tuff band in the Lower Cambrian Heatherdale Shale of South Australia produced an age of 526 +/- 4 Ma (95% confidence limits). Local and regional trilobite and archaeocyathan evidence indicates that the tuff is latest Atdabanian or Botomian. The tuff shortly predates the onset of the Delamerian Orogeny.
Acritarch assemblages at the Cambrian-Precambrian boundary correlate the Siberian Tommotian deposits with the East European Lontova horizon, and the Nemakit-Daldyn horizon with Vendian deposits in Malyy Karatau.
At the International Geological Congress held in Sydney, Australia, in August 1976 a Symposium of papers and discussion on ‘The Precambrian–Cambrian Boundary Problem– was organized by the author in his capacity as Project-Leader/Chairman of the international Project 29 Working Group on the Precambrian–Cambrian Boundary. The following seven papers arose from this symposium. Project 29 is part of the current research under the auspices of the International Geological Correlation Programme (I.G.C.P.). which is sponsored jointly by the United Nations Educational. Scientific and Cultural Organization (U.N.E.S.C.O.) and the International Union of Geological Sciences (I.U.G.S.).
Single zircons from two Early Cambrian volcanic horizons have been analysed using the SHRIMP ion microprobe. Full details of the analytical procedures and data reduction are given. Zircons from tuff within the Lie de Vin Formation, near Tiout, Morocco, show little spread in U-Pb age and have a mean value of 521 +/- 7 Ma (2-sigma). Those from a bentonite within unit 5 of the Meishucun section near Kunming, southern China, show relatively dispersed U-Pb ages, revealing the presence of both detrital or xenocrystic grains as well as areas within grains that have lost radiogenic Pb. The main population has as mean age of 525 +/- 7 Ma, but a mean Pb-207/Pb-206 age of 539 +/- 34 Ma which is a maximum estimate for the bentonite age. These results conflict with previous Rb-Sr whole rock ages of c. 580 Ma for overlying Cambrian shales at Meishucun, and c. 570 Ma for Atdabanian shales from the E. Yangtse Gorges area.
Seven new U-Pb zircon ages for volcanic and intrusive igneous rocks from southern Britain are as follows: 613 +/- 4 Ma for the Coedana Granite, Anglesey, and 614 +/- 2 Ma for the Padarn Tuff Formation of the Arfon Group, North Wales; 677 +/- 2 Ma for a deformed granite of the Malvernian Complex and 566 +/- 2 Ma for a rhyolitic tuff in the Warren House Formation, Malvern Hills; 603 +/- 2 Ma for a granophyric diorite ('markfieldite') intrusive into the Caldecote Volcanic Formation at Nuneaton; 566 +/- 2 Ma for the Uriconian volcanic group, Shropshire, and 560 +/- 1 Ma for the Ercall Granophyre, which is intrusive into Uriconian volcanic rocks. Three of the four dated intrusive rocks contain a component of inherited zircon, but the dated volcanic rocks are composed of simple, single-stage zircon populations with no detectable amount of secondary Pb-loss. The ages clarify the timing of Precambrian magmatic events in southern Britain, and conform well with those in neighbouring Avalonian terranes. The new age of the Ercall Granophyre provides a reliable maximum age for the Early Cambrian (Atdabanian-late Tommotian) strata of England, and reconfirms a Precambrian age for the Padarn Tuff Formation and granophyric diorite ('markfieldite') which demonstrably underlie Early Cambrian strata.
Rb-Sr age data are reported for late Precambrian to Cambrian igneous and metamorphic rocks from the border region of England and Wales. A felsic intrusion from Stanner Hill defines a good whole-rock isochron of 702 ±8 Ma. Uriconian pyroclastic rocks from the Wrekin area give a whole-rock age of 558± 16 Ma, while the late- or post-Uriconian Ercall granophyre from the same area gives a good whole-rock isochron of 533 ±13 Ma. A biotite cooling age from the nearby pre-volcanic Rushton Schist is consistent with these dates and suggests that the region cooled after the close of Uriconian magmatism at 536 ±8 Ma. All these results relate to widespread igneous/metamorphic activity in latest Precambrian to Lower Cambrian time.
The Ercall granophyre and Rushton schist are directly overlain by Lower Cambrian sediments and 533±13Ma is thus a maximum age for the deposition of these rocks.
In Siberia Vendian is equated with a Yudoma Complex or Yudomian. Yudomian deposits of the Siberian Platform and adjacent geosynclines differ greatly in facies and thickness. According to the composition and structure of Yudomian deposits, four facies provinces may be recognized on the platform. Local stratigraphic charts for each province are presented and their correlation and possibility to subdivide the Vendian System in Siberia into three horizons or stages are substantiated. The upper stage (Nemakit–Daldyn), on the basis of palaeontological evidence, is in its turn subdivided into two zones: Anabarites trisulcatus and Purella antiqua .
The most important Early Baikalian rearrangement is proved to take place around 800±50 Ma. The pre-Vendian (Late Baikalian) movements, though less intensive, make the determination of the Yudomian lower boundary easier.
The specific character of Vendian biostratigraphy is discussed involving all groups of the organic remains, that causes the necessity of recognition, subdivision and correlation on the basis of the whole complex of data.
The boundary between the Vendian System and the Lower Cambrian Tommotian Stage (in type section) is proved to conform to the base of the Pestrozvet Formation, which divides the A. sunnaginicus and P. antiqua zones.
Acritarchs of Lower Cambrian age have been recovered from a clay horizon within conglomerates 6 m above the unconformity described by Cope & Gibbons (1987). The Wrekin Quartzite (34 m thick) represents beach deposits grading upwards into shelf sandstones. The stratigraphie evidence and recent age determinations suggest that although the granophyre was intruded at 560 ± 1 Ma the Cambrian Unconformity is younger than 533 ± 13 Ma.
This chapter discusses the origin and early evolution of the earth. The Earth is the end-product of a series of evolutionary processes that began at the time the first elements were produced—that is, 10–20 billion years (Ga) ago. The Earth's composition, its evolution, and many of its chemical and physical cycles are influenced to various degrees by processes that occurred during or before the formation of the solar system. These processes include nuclear reactions to produce the elements, gravitational collapse to produce stars and protoplanetary systems, condensation to produce solid grains, and accretion to accumulate grains into planets. Probably, the most remarkable and even haunting aspect of the Earth's genesis is that in the planetary system, the only one known, the Earth has unique properties. These unique features include a hydrosphere, vigorous plate tectonics, a large moon, and an oxygen-rich atmosphere. The composition of the Earth was determined both by the chemical composition of the solar nebula, from which the Sun and planets formed, and by the nature of the physical processes that concentrated materials to form planets. The bulk elemental and isotopic composition of the nebula is believed or usually assumed to be identical to that of the Sun.
A simple procedure for the decomposition of zircon and the extraction of U and Pb for isotopic age determinations has been developed and tested (Krogh, 1971a,b). Samples are decomposed at 220°C with 48 per cent HF in a Teflon® capsule confined by a self-sealing stainless steel jacket. Uranium and lead are isolated on a Teflon® anion exchange column using Dowex 1 resin. Measured lead contamination levels range from 0.5 to 5.0 ng. In contrast, lead blanks for the borax fusion technique used in all previous zircon analyses are typically 0.2 to 1.0 μg.Eight small samples weighing 0.3 to 7 mg contained 30 to 260 ng of Pb206. The average value for the total amount of common lead present in the lead isotopic composition analysis, contributed from both the sample and the chemical procedures, was 1.4 ng. The highest ratio measured to date (126,000) was obtained on a 0.1-g sample that contained 50 ppm of Pb206. An exact determination of the amount of common lead in zircons is now possible. A maximum value of 0.3 ppm was found for fourteen nonmagnetic zircon fractions from granites and rhyolites. Higher values reported in the literature suggest that lead contamination levels are often underestimated in the analysis of zircons by the borax fusion technique.The silica-gel loading technique for lead provides stable emission for small samples as well as limited isotopic fractionation in the mass spectrometer. These features, combined with the low levels of lead contamination and the high precision of mass spectrometric analyses, make possible an average reproducibility (for duplicate decompositions of the same finely ground sample) of 0.3 m.y. for lead ages of 2750 m.y. The new method requires fewer reagents and is much easier than the borax fusion technique.
Abrasion combined with an improved paramagnetic separation technique eliminates 90 to 100 percent of discordance so that ages of unprecedented accuracy (±1 to 3 m.y.) can be achieved for virtually all 2700 m.y. old zircon populations from plutonic or volcanic rocks. The procedures work even better for younger zircons. Besides removing outer layers that may have been leached, high-U parts are preferentially removed by abrasion because they are softened by radiation damage. Altered and cracked zircons also tend to be eliminated.In most cases, the new concordant data move up the line established by previously analysed paramagnetic fractions but a number of anomalies have been found where old data give upper intersections that are in error by as much as 25 m.y. Reducing or eliminating paramagnetically correlated Pb loss greatly enhances our ability to define mixing lines for igneous or metamorphic rocks when two ages of zircon are present.The abrasion technique allows detection of an inherited component if it exists by enhancing the sample in core material. Abrasion in many cases removes about 80 percent of the common lead, thus allowing a direct evaluation of this component.When the outer parts of grains are removed, the correlation between magnetic susceptibility and uranium content is maintained but the usual correlation of uranium level with lead loss is reduced or eliminated. Therefore, only near surface uranium is involved in the classic discordance versus uranium level correlation of Silver (1963).
Parameters on which models for terrestrial lead isotope evolution are based have recently been revised. These parameters are the isotopic composition of troilite lead, the age of the meteorite system and the decay constants of uranium and thorium. As a result, the normal single-stage model in which the age of the earth is taken to be that of the meteorite system is now untenable.A two-stage model has been constructed which permits the age of the earth to be that of the meteorite system and which also yields good model ages for samples of all ages. The new model postulates that lead developed initially from a primordial composition assumed to be that of troilite lead beginning at 4.57 b.y. ago. The average values of 238U/204Pb and 232Th/204Pb for this first stage were 7.19 and 32.21 respectively. At approximately 3.7 b.y. ago differentiation processes brought about the conditions of a second stage, in which 238U/204Pb ≈ 9.74 and 232Th/204Pb ≈ 37.19 in those portions of the earth which took part in mixing events, giving rise to average lead.
Although the base of the Phanerozoic is an important date in the history of life and of the Earth's crust, few conclusive data have been available to estimate the numerical age of this boundary. Accurate and reliable modern data now exist, however, and those relevant modern radiometric dates are summarized in this review and are shown to be consistent with a considerably younger age for the Precambrian-Cambrian boundary than previously accepted.
U-Pb age determinations on four lunar zircons from existing thin-sections of one highland breccia, 73217, using the recently constructed ion microprobe SHRIMP, are reported. The analytical reproducibility of SHRIMP is demonstrated, and procedures for measuring Pb/U, Th/U, and corecting for initial Pb are explained. Electron microprobe analyses for the zircons are alsoar reported. The results show that the four zircons survived the lunar cataclysm without any identifiable effects on their U-Pb systematics. All four indicate a single age of 4356 +23 or -14 m.y. The zircons have experienced small variable amounts of Pb loss since crystallization, from almost zero up to about 10 percent. If this occurred during one later event, then age of the latter is between 1100 and 2300 m.y.
Global taxonomic richness is affected by variation in three components: within-community, or alpha, diversity, between-community, or beta, diversity; and between-region, or gamma, diversity. A data set consisting of 505 faunal lists distributed among 40 stratigraphic intervals and six environmental zones was used to investigate how variation of alpha and beta diversity influenced global diversity through the Paleozoic, and especially during the Ordovician radiations. As first shown by Bambach (1977), alpha diversity increased by 50 to 70 percent in offshore marine environments during the Ordovician and then remained essentially constant of the remainder of the Paleozoic. The increase is insufficient, however, to account for the 300 percent rise observed in global generic diversity. It is shown that beta diversity among level, soft-bottom communities also increased significantly during the early Paleozoic. This change is related to enhanced habitat selection, and presumably increased overall specialization, among diversifying taxa during the Ordovician radiations. Combined with alpha diversity, the measured change in beta diversity still accounts for only about half of the increase in global diversity. Other sources of increase are probably not related to variation in gamma diversity but rather to appearance and/or expansion of organic reefs, hardground communities, bryozoan thickets, and crinoid gardens during the Ordovician.
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