Andrey Yu. Zhuravlev

Anoxia as the cause of the mid-Early Cambrian (Botomian) extinction event

New and revised Early Cambrian biostratigraphic data allow a quantitative analysis of changes in biotic diversity and extinction rate. The mid-Early Cambrian extinction can now be resolved into two distinct events: the well-known early Toyonian Hawke Bay regression event, and a newly observed but more severe disruption during the early Botomian, here named the Sinsk event. During the Sinsk event, the shallow-water benthos of the so-called Tommotian fauna, together with archaeocyaths and some trilobites, underwent a rapid decline. The Sinsk event is characterized by the significant accumulation of nonbioturbated laminated black shales in tropical shallow waters. Lamination is due to the fine alternation of clay- and organic-rich laminae with calcite-rich laminae containing abundant monospecific acritarchs. These shales are enriched by pyrite and elements typical of anoxic conditions and support a benthic biota of dysaerobic character. Our observations suggest that the extinction during the early Botomian was caused by extensive encroachment of anoxic waters onto epicontinental seas, associated with eutrophication and resultant phytoplankton blooms.

The Lower Cambrian Fossil Record of the Soviet Union

Lower Cambrian deposits are found throughout the Soviet Union and Mongolia (Fig. 1). They occur over the entire Siberian Platform, much of the East European Platform, and widely in the fold belts bordering the southern Siberian Platform (the Altay-Sayan Fold Belt, Mongolia—Okhotsk Fold Belt), Kazakhstan, Central Asia, and the Urals. In most cases, they are conformably underlain by Vendian rocks. The most important sections containing Early Cambrian fossils are in Siberia, Open image in new window Figure 1 Tectonic scheme of the USSR and nearby territories [modified from Yanshin et al. (1984)]. (1, 2) Ancient cratons including framing mobile belts: (1) Laurasian affinity; (2) “Pacific” affinity. (3) The main regions of Cambrian sedimentation: (a) Baltic Syneclise of the East European Platform, (b) Lena-Aldan area of the Siberian Platform, (c) Altay-Sayan Fold Belt, (d) western Mongolia, (e) Maly Karatau (south Kazakhstan), (f) South Urals, (g) central Kazakhstan, (h) Transbaikalian, (i) Far East, (j) Kolyma Basin. the East European Platform and Kazakhstan in the USSR, and in western Mongolia. The key region for the Cambrian stages adopted in the USSR is the Siberian Platform.

Eve of biomineralization: Controls on skeletal mineralogy

Carbonate mineralogies have oscillated between aragonite and calcite seas through geological time, proposed to be due mainly to secular variation in the magnesium/calcium ratio driven by changing rates of ocean crust production. A quantitative compilation of inorganic and biominerals from the onset of biomineralization (late Ediacaran–Middle Ordovician) reveals a correspondence between seawater chemistry and the first adopted mineralogy of skeletal clades. Ediacaran–Tommotian skeletons and inorganic precipitates were composed exclusively of aragonite or high-Mg calcite, but these were replaced by low-Mg calcite mineralogies during the early Atdabanian, implying the onset of a calcite sea. This transition is empirically constrained by fluid inclusion data. Late Atbadanian–Botoman inorganic precipitates returned to aragonite, with high-Mg calcite echinoderms and solitary tabulaconids and massive aragonitic tabulaconids originating during this interval. Middle Cambrian–Ordovician inorganic precipitates were low-Mg calcite, and the Ordovician radiation in skeletal expression was due mostly to groups with low-Mg calcite mineralogies. These short-lived transitions can be most parsimoniously explained by minor oscillations of mMg:Ca around ~2 during this period, possibly combined with the progressive onset of greenhouse conditions during the mid-Late Cambrian.

Structure and diversity of oldest sponge-microbe reefs: Lower Cambrian, Aldan River, Siberia

The oldest sponge reef is a small Early Cambrian bioherm at the base of the Tommotian Stage (∼535–540 Ma) in southeast Siberia. The mainly archaeocyath construction may be a response to turbid conditions. Cambrocyathellus bowls fused to create a rigid cavernous frame colonized by cryptic Archaeolynthus and calcified microbes ( Renalcis ). In addition to these constructors and binders, other reef guilds are present: bafflers (other archaeocyaths, spiculate sponges, and hyoliths) and dwellers (hyoliths, mollusks, and many others). This is the oldest known reef possessing an open skeletal frame structure built by animals and a mixed animal-autotroph composition. It provides a blueprint for younger Phanerozoic reefs.

Functional biology and ecology of Archaeocyatha

A modular organization confers many ecological advantages in reef-building settings, such as indeterminate growth leading to larger size, greater powers of regeneraion, and an ability to encrust and gain secure attachment to substrates. The Archaeocyatha, an early Cambrain group of calcified sponges, were the first skeletal metazoans to develop a modular habit and to be associated with reefs. Archaeocyaths show the predicted ecological changes with the appearance of modularity. Supposed species may be facultatively modular, with modular ecophenotypes always dominating biohermal settings. However, only genera possessing porous septa show any development of modularity, seggesting that an initially well-integrated soft-tissue is a prerequisite for acquisition of the habit

Middle–Late Cambrian Rankenella–Girvanella reefs of the Mila Formation, northern Iran

Following the collapse of the Early Cambrian archaeocyathan–calcimicrobial reef consortium, the Middle–Late Cambrian – Furongian was an interval dominated by purely microbial dendrolite and stromatolite reefs. However, among these latter, a few exceptional occurrences of metazoan reefs are known. One such reef complex occurs in the late Middle – early Late Cambrian – Furongian portion of the Mila Formation of northern Iran. In the otherwise low-energy interval of this formation, the anthaspidellid demosponge Rankenella hamdii sp. nov. is associated with encrusting Girvanella, eocrinoid plates, rhynchonelliformean brachiopod valves and subordinate hyoliths and trilobites in tempestite shell beds; these beds underwent synsedimentary cementation on the seafloor to form hardgrounds. In the succeeding, higher energy interval, a complex of metre-scale bioherms and (or) taphoherms incorporates toppled or transported Rankenella hamdii in association with brachiopods, echinoderm plates, trilobites and some red arg...

Latest Middle Cambrian metazoan reef from northern Iran

Middle and Late Cambrian reefs were built mainly by cyanobacterial communities. A few reefs with a metazoan as well as an algal component, however, are known from this interval. A Middle Cambrian reef formed primarily by spicular demosponges is described here from the Mila Formation in the Elburz Mountains, northern Iran. The reef is enclosed within calcareous grainstones which contain terminal Middle Cambrian (late Mayan) trilobites. The Mila Formation reef was constructed by sponges of the family Anthaspidellidae and bacterial (algal?) sheaths, and is the earliest metazoan reef to be documented from the interval after the demise of archaeocyath sponges. The reefal community is typical of subsequent reefal communities of Early–Middle Ordovician age. The Ordovician examples differ only by the incorporation of additional metazoan elements.

First finds of problematic Ediacaran fossil Gaojiashania in Siberia and its origin

We describe the first occurrence of the problematic fossil Gaojiashania outside China, in the Ediacaran Yudoma Group of the Siberian Platform. In both areas, Gaojiashania characterizes the lower upper Ediacaran strata and precedes the appearance of Cloudina and other skeletal fossils, which highlights its significance for the Ediacaran subdivision and correlation. Features of this fossil such as indeterminate length, the absence of a distinct growth pattern, and self-avoiding behaviour indicate its trace fossil origin but do not necessarily imply metazoan affinities for its producers. Several organisms including stem-group social amoebozoans and unicellular protists may have been Proterozoic trace fossil producers.

Evolution of archaeocyaths and palaeobiogeography of the Early Cambrian

In the Early Cambrian, there were two peaks of the increase in number of new archaeocyathan genera. These diversification bursts are, perhaps, related to significant changes in Early Cambrian palaeogeography. The first burst at the beginning of the Atdabanian was, perhaps, connected with the initiation of archaeocyathan expansion beyond the Siberian Platform. During the second half of the Atdabanian, closely related archaeocyathan assemblages were established from North Africa to Australia because neither climate, nor geographic isolation could have affected their differentiation in that time. At the end of the Atdabanian and the beginning of the Botomian the number of archaeocyathan genera again increased sharply, and isolated provinces were established. The American–Koryakiyan province was formed in western North America (from Alaska to Sonora) and included the Koryakiya, while the Afro–Siberian–Antarctic province probably extended from North Africa and Western Europe to Siberia, Australia, and Antarctica. The main difference between the regular archaeocyathan assemblages lies in what skeletal elements filled the intervallum. There was a certain stability in the distribution of skeletal elements of high taxonomic rank and an interchangeability of elements of low taxonomic rank.

Controls on carbonate skeletal mineralogy: Global CO2 evolution and mass extinctions

A quantitative compilation of carbonate skeletal mineralogy through the Phanerozoic shows a progressive replacement of low-Mg calcite by aragonite. This general trend overrides the subsidiary trend of greenhouse intervals favoring biogenic low-Mg calcite mineralogies (calcite seas), and icehouse intervals facilitating aragonite + high-Mg calcite mineralogies (aragonite seas). The replacement of low-Mg calcite by aragonite was, however, achieved episodically at mass extinction intervals. In particular, the end-Permian extinction both preferentially removed species bearing “unfavorable” low-Mg calcite, and allowed the selective radiation of biota with “favorable” aragonite. This demonstrates the importance of incumbency in the evolution of skeletal mineralogy. We suggest that the broad increase of aragonitic biota has been controlled by changes in atmospheric carbon dioxide partial pressure ( p CO2) via carbonate mineral kinetics. Through the Phanerozoic, broadly decreasing p CO2 levels led to decreasing total alkalinity and dissolved inorganic carbon, and increasing oceanic pH. Superimposed upon this general trend, there are cyclic episodes of relatively high p CO2 and saturation state combined with a lower ratio of magnesium to calcium ions in seawater driven by the relatively slow changes in mid-ocean ridge expansion rates. Mass extinction events, many of which may have been caused by rapid global changes in temperature and/or p CO2, represent major intervals of turnover.