A. Yu. Zhuravlev

Sr and C isotopes in Lower Cambrian carbonates from the Siberian craton: A paleoenvironmental record during the ‘Cambrian explosion’

We report 87Sr86Sr measurements on a suite of well preserved sedimentary carbonates from Lower Cambrian strata of the Lena River region of Siberia. Stable isotopes and major and trace element chemistry have been used to identify potentially unaltered samples for Sr isotopic measurements. The Sr data define a smooth curve of paleoseawater 87Sr86Sr values from the Tommotian through to the early Middle Cambrian. During the Tommotian-Atdabanian interval, 87Sr86Sr rose rapidly from 0.7081 to 0.7085. The rate of change in Sr ratios decreased during the Botomian but rose to 0.7088 in the late Toyonian to early Middle Cambrian. The rate of 87Sr86Sr increase during the Tommotian-Atdabanian was ca. 0.0001/m.y., comparable to the late Miocene change in seawater Sr. We infer that an interval of enhanced erosion during the ‘Cambrian explosion’ was responsible for this increase. An important source for radiogenic Sr to the oceans may have been erosion of the Pan-African orogenic belt of southern Africa. The rapid change in paleoseawater Sr corresponds with an interval of highly variable marine δ13C values. Model results for the Sr and C isotopic records suggest that the quasi-periodicity in the δ13C record is not a consequence of direct erosional forcing. However, our inference of high erosion rates during the Tommotian-Atdabanian implies enhanced fluxes of nutrient elements such as P to the oceans. Phosphorite deposits and black shale deposition in coeval strata suggest that periods of high marine productivity and anoxia may be in part related to enhanced river dissolved fluxes. Our results thus provide some insight into environmental conditions during the ‘Cambrian explosion.’

Precambrian-Cambrian boundary problem: carbon isotope correlations for Vendian and Tommotian time between Siberia and Morocco

At least three distinctive cycles are preserved in the carbon-isotope patterns for the Vendian/Tommotian interval of the Siberian platform and the Anti-Atlas Mountains of Morocco. In Siberia, an early Tommotian carbon cycle provides the first detailed test of correlations based on archaeocyathid biostratigraphy between classic sequences on the Aldan and Lena rivers, and demonstrates that the Early Cambrian zone boundaries are reasonably good time lines. The carbon signal is preserved in both limestone and dolomite and across regional faties variations. The δ^(13)C results from the late Vendian section in Morocco display a pattern similar to that found in Siberia, suggesting that carbon isotopes can be used to test stratigraphic correlations on an intercontinental scale.

A carbon isotope reference scale for the Lower Cambrian succession in Siberia: report of IGCP Project 303

Four sections from the mid-Atdabanian to lowest Toyonian (middle Cambrian) along the Lena River of Siberia were sampled for carbon isotope stratigraphy. These show a mainly heavy but highly oscillatory SC signature for the Atdabanian to mid-Botomian interval, coincident with the major phase of invertebrate innovation. A prolonged interval of negative <SC followed until late Toyonian times, coincident with Botomian-Toyonian mass extinctions. Eleven carbon isotope cycles are identified through the lower Cambrian, which should now be tested for their utility in global correlation and relationship to bioevents in the Cambrian explosion.

Archaeocyaths from Yorke Peninsula, South Australia and archaeocyathan Early Cambrian zonation

Zhuravlev, A. Yu., & Gravestock, D. I., 1994:03:28. Archaeayaths from Yorke Peninsula, South Australia and archaeocyathan Early Cambrian zonation. Alcheringa 18, 1–54. ISSN 0311-5518. Two assemblages of archaeocyaths are documented from Lower Cambrian outcrops and drillholes on Yorke Peninsula. South Australia. The older assemblage (11 species) occurs in the uppermost Kulpara Formation and conformably overlying basal Parara Limestone, and is equivalent to Lower Faunal Assemblage II in the Flinders Ranges. The younger assemblage (28 species plus Acanthhcyathus and Rodiocyathus) occurs in the Koolywurtie Member near the top of the Parara Limestone. Equivalent taxa are widespread in the Flinders Ranges, western New South Wales and Antarctica. Archaeocyathan distribution is now sufficiently well known to propose three assemblage zones and two informal assemblages for regional correlation. No new taxa have been added, but Erugatocyathus scutatus (Hill) and Pycnoidocyathus latiloculatus (Hill), hitherto known o...

Palaeomagnetism of the Bayan Gol formation, western Mongolia

Oriented samples of the Lower Cambrian Bayan Gol Formation from Salaany Gol, Mongolia, were collected at roughly 5 m stratigraphic intervals for palaeomagnetic analysis. Progressive alternating-field and thermal demagnetization isolated two magnetic components: a present-field overprint, typically removed by 10 mT fields and ~200°C heating; and a high-coercivity, high-unblocking-temperature (550–600 °C), predominantly single-polarity component that was imparted to the rocks prior to early or middle Palaeozoic deformation. Single-polarity magnetization at Salaany Gol contrasts with results from Lower Cambrian rocks on the Siberian platform, previously considered correlative with the Bayan Gol Formation, which show a prominent change in polarity bias near the top of the Tommotian Stage. Two hypotheses can explain this discrepancy. First, the entire Bayan Gol Formation may correlate with the predominantly reversely polarized, lower half of the Tommotian Stage in Siberia. This model is consistent with plausible interpretations of δ^(13)C profiles for the Zavkhan basin and the Siberian platform. Alternatively, the characteristic magnetic direction from our samples may be a pre-fold overprint. If post-accretionary, then comparison with Siberian palaeomagnetic results suggest a Silurian–Devonian remagnetization age, and existing bioand chemostratigraphic correlations provide the most reliable spatial and temporal links between the Zavkhan basin and the Siberian platform. If the observed magnetic directions are primary or an immediate overprint then they may be used to constrain the early Cambrian palaeogeography of the Zavkhan basin and the Palaeo-Pacific Ocean. Mean inclination of 62 ± 4° corresponds to a palaeolatitude of 44 ± 5°, several thousand kilometres from the equatorial Siberian craton.

Cambrian food web: A brief review

Abstract Cambrian trophic web guilds consist of 1/Primary producers: calcified and non calcified bacteria, phytoplankton (acritarchs) and dissolved organic matter and 2/Primary consumers: mainly filter/suspension-feeders, some carnivorous, probable deposit feeders and rare grazers; parasites can only be surmised. Trophic modes are illustrated by examples taken in the different Cambrian communities.

The oldest evidence of bioturbation on Earth: COMMENT

Recently, [Rogov et al. (2012)][1] discovered dense bioturbation fabrics, the preserved depth of which reached 5 cm, from the Ediacaran Khatyspyt Formation of the northern Siberian Platform. The age of the bioturbated beds is estimated at ca. 555 Ma. The ichnotaxon itself is attributed to the

A synonymized nomenclature for calcified sponges

A synonymized nomenclature for calcified sponges (archaeocyaths, stromatoporoids, chaetetids, sphinctozoans and pharetronids) is here presented to enable comparison of suggested homologous structures. The rediscovery of living calcified sponges has renewed interest in the biology, phylogeny and ecology of related fossils, and many of these forms have been rescued from groups of previously problematic status. If we can prove poriferan affinity for these groups the archaeocyaths, stromatoporoids, chaetetids and sphinctozoans then this will have considerable implications for our understanding of the evolution of the Porifera as well as the evolution of fossil communities, especially within reefs. However, these groups have been worked on by specialists in isolation, and complex Table 1. Homologous structures and supposed synonyms for calcified sponges Stromatoporoid and chaetetid (Wood, 1987) Sphinctozoan (Finks, 1983) Archaeocyath (Debrenne, Zhuravlev & Rozanov, 1989) aquiferous unit space aquiferous unit aquiferous system (3) astrorhizae (3) astrorhizal canal (4) (calcareous) skeleton (5) [calicle]

Etheridge collection: systematic revision of some of the first archaeocyaths discovered in Australia

The twelve specimens described by Etheridge (1890) and Tate (1892) originate from the first Australian collections of Archaeocyatha. Few of these are suitable for modern studies and most must be considered as nomina dubia. Only two taxa may be determined at species level and one at genus level. The species Coscinocyathus tatei Etheridge 1890 as restricted by Tate (1892) is a senior synonym of Veronicacyathus frondeus Debrenne 1973, and consequently becomes the type species of Veronicacyathus Debrenne 1973.

The early history of the metazoa—a paleontologist’s viewpoint

Successful molecular biology, which led to the revision of fundamental views on the relationships and evolutionary pathways of major groups (“phyla”) of multicellular animals, has been much more appreciated by paleontologists than by zoologists. This is not surprising, because it is the fossil record that provides evidence for the hypotheses of molecular biology. The fossil record suggests that the different “phyla” now united in the Ecdysozoa, which comprises arthropods, onychophorans, tardigrades, priapulids, and nematomorphs, include a number of transitional forms that became extinct in the early Palaeozoic. The morphology of these organisms agrees entirely with that of the hypothetical ancestral forms reconstructed based on ontogenetic studies. No intermediates, even tentative ones, between arthropods and annelids are found in the fossil record. The study of the earliest Deuterostomia, the only branch of the Bilateria agreed on by all biological disciplines, gives insight into their early evolutionary history, suggesting the existence of motile bilaterally symmetrical forms at the dawn of chordates, hemichordates, and echinoderms. Interpretation of the early history of the Lophotrochozoa is even more difficult because, in contrast to other bilaterians, their oldest fossils are preserved only as mineralized skeletons. However, the unity of the microstructures of mollusks, brachiopods, and bryozoans, which is absent in other metazoans, is indicative of the presence of close relatives among the various earliest lophotrochozoans, some of which were sedentary suspension-feeders while others were mobile epibenthic detritophages. In the aggregate, modern data from molecular biology, palaeontology, and comparative embryology/morphology, having been revitalized by the introduction of new microscopy techniques, imply that the hypothesized planktotrophic gastrae-like common ancestor is the least likely of the diverse suggestions on the origins of the Metazoa. The common ancestor of the Bilateria had to be a motile epibenthic animal, and the explosive metazoan diversification embracing the Late Ediacaran–Early Cambrian interval (c. 40 Ma) was probably a real event, which was predated by a long (ca. a billion years) period of the assembly of the metazoan genome within the unicellular and colonial common ancestors of the Opisthokonta, and possibly even the entire Unikonta.