CAMBRIAN PALAEONTOLOGY OF FENNOSCANDIAN BASEMENT FISSURES

Fossils from two ecologically and palaeogeographically interesting Cambrian outliers in the Precambrian basement of Fennoscandia are described or discussed. One of the localities is a fissure system, filled with sandstone, at Långbergsöda-Öjen in Saltvik, Province of Aland, Finland. The occurrence of the brachiopod Ceratreta tanneri (Metzger 1922) indicates a Late Cambrian age rather than Early Cambrian as assumed hitherto. The other locality, about 1.5 km W of the Fortress of Bohus, at Kungälv, Province of Bohuslän, Sweden, consists of a fissure and a trench-like crevice bounded by joints, containing a sequence of calcareous sedimentary breccia, grey limestone with coarse sand, and a late Medial Cambrian fauna. The sequence continues with black shale and bituminous dark limestone, containing large fragments of older igneous and sedimentary rocks as well as a Late Cambrian fauna.     

First evidence for Cambrian glaciation provided by sections in Avalonian New Brunswick and Ireland: Additional data for Avalon–Gondwana separation by the earliest Palaeozoic

The first evidence for Cambrian glaciation is provided by two successions on the Avalon microcontinent. The middle lowest Cambrian (middle Terreneuvian Series and Fortunian Stage–Stage 2 boundary interval) has an incised sequence boundary overlain by a fluvial lowstand facies and higher, olive green, marine mudstone on Hanford Brook, southern New Brunswick. This succession in the lower Mystery Lake Member of the Chapel Island Formation may be related to melting of an ice sheet in Avalon. The evidence for this interpretation is a muddy diamictite with outsized (up to 10 cm in diameter), Proterozoic marble and basalt clasts that penetrated overlying laminae in the marine mudstone. That eustatic rise was associated with the mudstone deposition is suggested by an approximately coeval rise that deposited sediments with Watsonella crosbyi Zone fossils 650 km away in Avalonian eastern Newfoundland. A sea-level rise within the Watsonella crosbyi Chron, at ca. 535 Ma, may correspond to a unnamed negative ¹³C excursion younger than the basal Cambrian excursion (BACE) and the ZHUCE excursion in Stage 2 of the upper Terreneuvian Series. Cambrian dropstones are now also recognized on the northern (Gander) margin of Avalon in continental slope–rise sedimentary rocks in southeast Ireland. Although their age (Early–Middle Cambrian) is poorly constrained, dropstones in the Booley Bay Formation provide additional evidence for Cambrian glaciation on the Avalon microcontinent. Besides providing the first evidence of Cambrian glaciation, these dropstone deposits emphasize that Avalon was not part of or even latitudinally close to the terminal Ediacaran–Cambrian, tropical carbonate platform successions of West Gondwana.     

Lapworthellids and other skeletonised microfossils from the Cambrian Stage 3 of the northern Montagne Noire,southern France

The Montagne Noire (southern France) possesses one of the most complete Cambrian successions in the western peri-Gondwana margin and might provide a good stratigraphic reference for both regional charts and international correlations. However, to date, the lower Cambrian succession of the northern Montagne Noire has been supposed to be devoid of biostratigraphically significant fossils. The complex tectonostratigraphic framework of the area (a range divided into Axial Zone, northern and southern Montagne Noire) exacerbated problems related to regional correlations and palaeogeographic reconstructions. As a result, the chronostratigraphic context of the lower Cambrian of northern Montagne Noire is still uncertain and stratigraphic reports have broadly relied on putative lithostratigraphic correlations with the southern Montagne Noire. The purpose of this study is to characterise, for the first time, the fossil record of carbonate beds and lenses of the northern Montagne Noire occurring at the top of the siliciclastic-dominated Marcory Formation, in order to provide regional bio- and chronostratigraphic constraints on lower Cambrian strata. Moreover, this study is aimed at improving international chronostratigraphic correlation. Carbonate beds and lenses cropping out along the Orque Cliff, in the Mélagues thrust slice, were investigated. They yielded a faunal assemblage constituted of molluscs (Igorella cf. ungulata and Igorella moncereti n. sp.), hyoliths (Conotheca brevica), chancelloriids (Archiasterella cf. pentactina and Allonnia sp.) and tommotiids (Lapworthella rete). L. rete is recorded in upper Meishucunian (Cambrian Stage 3) strata of the Yangtze Platform (South China) where it is used as index fossil of the Cambrian Stage 3 Sinosachites flabelliformis–Tannuolina zhangwentangi Assemblage Zone. Therefore, the presence of this tommotiid provides evidence that the studied carbonate beds and lenses are Cambrian Age 3 (Atdabanian according to the Siberian chart). The upper part of the Marcory Formation in the Mélagues slice, dated as Cambrian Stage 3, might represent a lateral equivalent of the mixed (carbonate-siliciclastic) Pardailhan Formation defined in the southern Montagne Noire.     

Trace fossils from arenig flysch sediments of eire and their bearing on the early colonisation of the deep seas

A sequence of Lower Ordovician (Arenig) turbidites in Co. Wexford, Eire, has yielded one of the earliest diverse ichnofaunas yet recorded from deep water sediments comprising: Chondrites, Glockerichnus, Gordia, Helminthopsis, Lorenzinia, Neonereites, Palaeophycus, Paleodictyon, Planolites, Sublorenzinia, Taenidium, Taphrhelminthopsis, Teichichnus and Tomaculum. This ichnofauna is critical in any analysis of the colonisation of the deep seas by trace fossil‐producing animals.

A world‐wide review shows that the earliest trace fossils are mainly from Late Precambrian shelf sea environments, but many more evolved during very rapid diversification in the pre‐trilobite Lower Cambrian.

There was little increase in diversity in shallow water after the Lower Cambrian but a progressive colonisation of the deep ocean took place and this accelerated during the Ordovician, when the main lineages of deep sea trace fossils were established there. Rosetted, patterned, meandering and simple spiral forms evolved in shallow water in the Upper Precambrian and pre‐trilobite Lower Cambrian and only later migrated into the deep sea, whereas complex, closely programmed, spiral traces may have evolved there.     

Variations in the Properties of Extractable “Humic Matter” and Associated Kerogen in Sediments through Geologic Time: Their Significance for Precambrian Biological Evolution and Paleoecology

Chemical properties and δ¹³C values of benzene/methanol-extractable “humic matter” and associated kerogen in a large, diverse collection of Precambrian and Phanerozoic sediments from different parts of the world showed complex systematic variations through geologic time, reflecting major developments in the history of Precambrian life, and different kinds of sediment yielded similar patterns of variation. Moreover, certain data differentiate clearly between glacial and nonglacial detrital sediments, or between lacustrine and marine sediments, and some data suggest the occurrence of Precambrian land life. The abundance of aromatic groups and the proportion of aliphatic to aromatic groups in the extracts showed little variation from the early Archean (ca. 3.3 Ga) to the mid-Proterozoic (ca. 1.6-1.3 or 1.3-1.1 Ga), whereupon they increased sharply, peaked ca. 1.1-0.900 Ga, and then plunged to a minimum in the late Proterozoic (ca. 0.800 Ga) or early Phanerozoic. This is interpreted as indicating that cyanobacteria were the dominant photoautotrophs until the mid-Proterozoic, when algae evolved, proliferating until the late Proterozoic, whereupon their populations were depleted by herbivorous metazoans. Nitrogenous aromatic material increased to a maximum ca. 3.4–3.3 Ga and then decreased steadily to ca. 1.3 Ga, suggesting that early Precambrian cyanobacteria were enriched in photoprotective as well as photosynthetic tetrapyrrole pigments owing to the lack of ultraviolet radiation-shielding atmospheric O₂ and O₃. The concentration increased again starting ca. 1.3–0.800 Ga, reflecting the rise of algae, peaked ca. 0.680 Ga, and dropped catastrophically to a much lower value in the Cambrian (ca. 0.510 Ga), suggesting mass mortality at the Precambrian-Cambrian boundary.     

Buschmannia roeringi n. gen., n. sp. (Archaeocyatha) aus der Nama-Gruppe Südwestafrikas

From the Buschmann-Klippe-Formation, mainly considered as Late Precambrian in age until now, the ArchaeocyathaBuschmannia roeringi, n. gen., n. sp. is described. Based on this discovery, an Early Cambrian age for the Nama-group is assumed.     

The importance of Planolites in the Cambrian substrate revolution

Early Cambrian subtidal shelf substrates were characterized by low water content and steep chemical gradients, conditions likely facilitated by the presence of microbial mats as reflected by an abundance of microbially-mediated sedimentary structures in Lower Cambrian strata. Such substrate conditions would have been unfavourable for burrowing by benthic metazoans. A combination of environmental restrictions and a lack of adaptations to vertical burrowing likely prevented most benthic metazoans from burrowing infaunally in Early Cambrian subtidal shelf substrates. The eventual acquisition of burrowing adaptations by benthic metazoans later in the Cambrian promoted an increase in the depth and intensity of bioturbation and initiated a transition toward well-hydrated substrates in which extensive infaunal activity was possible.Siliciclastic units of the Lower Cambrian succession in the White–Inyo Mountains, eastern California, contain abundant horizontal bioturbation on bedding planes, as documented by bedding plane bioturbation indices, but little vertical bioturbation, as shown by ichnofabric indices and x-radiography. Planolites, a simple horizontal trace fossil, represents the dominant type of bioturbation in these units. Planolites is found in a range of diameters, indicating that more than one species of tracemaker likely produced this type of trace. Although these Planolites do not have a vertical component, their abundance on bedding planes indicates that the activities of Planolites tracemakers had a significant impact on subtidal shelf substrates, represented by Lower Cambrian units in the White–Inyo Mountains, early in the Cambrian substrate revolution.     

Origin and the Early Evolution of the Phylum Mollusca

The paper gives an overview of the modern hypotheses on the origin of the phylum Mollusca and the formation of its main classes. The Cambrian stage of molluscan evolution is characterized based on the paleontological material. The doubtfulness of assignment of the Precambrian (Vendian) soft-bodied fossil Kimberella to mollusks is argued. Judging from the interpretation of the morphologically diverse Cambrian fossils, it is suggested that the classes Polyplacophora, Monoplacophora, Gastropoda and Bivalvia formed already near the Precambrian–Cambrian boundary, i.e., from the beginning of the paleontologically documented evolutionary history of the phylum, whereas the assumption of the later origin of these taxa is unconvincing. The remaining classes of mollusks arose later, i.e., Cephalopoda, in the Late Cambrian; Scaphopoda, in the Ordovician; and Aplacophora, in the Silurian.     

云南澄江早寒武世地层并论中国的Parabadiella,摩洛哥的Abadiella及澳大利亚的Abadiella huoi

根据近来的发现,区域地层的阐明及生物在寒武系界线上下的变化,澄江早寒武世地层划分作了修订。早寒武世岩石地层名称的更换,年代地层及生物地层单位的分带这里作了简述.目前的岩石地层单位从上到上是渔户村组（梅树阶）,黑林铺组（筇竹寺阶）,红井哨组、乌龙箐组（沧浪铺阶）及山邑村组（龙王庙阶）。此层序可分为2个亚带。所有的组合带在中国西南地台区都有存在,但两个亚带在澄江是区域性的。云南东部震旦系与寒武系的界线暂时置于渔户村且小歪头山段的底界,位于小壳化石最早出现的下部。中国的Parabadiella,摩洛哥的Abadiella及澳大利亚的Abadiella huoi这里重新作了研究。Parabadiella及Abadiella是两个不同的属。模式标本的图片在这里再次刊出。澳大利亚的Abadiella huoi一种应是Wutingaspis的一个新种（Wutingaspis jelli Chang sp.nov)。Opik(1975）的Dolerolenus(?)sp.nov.一种应归属Parabadiella huoi一种。Parabadiella是中国也是澳大利亚寒武系最古老的三叶虫。澳大利亚的Wutingaspis jelli的层位与摩洛哥Abadiella的层位相当,可与中国的Wutingaspis-Eoredichia带的上部对比。     

通过C27/C29甾烷和有机碳同位素来判断早古生代和前寒武纪的烃源岩的生物来源

在早古生代与前寒武纪的石油与源岩中,经常出现令人困惑的C29甾烷优势,而在有机地球化学中C29甾烷优势被认为是陆相有机质输入的标志。作者通过对比文献中各种海相藻类的热模拟产物分析,提出了早古生代与前寒武纪的石油与源岩中C29甾烷优势主要来源于浮游绿藻的观点,并通过C27甾烷/C29甾烷的比值提出了区分早古生代及前寒武纪石油源岩沉积环境的方法,对中国南方前寒武纪和早寒武世黑色岩系的有机地球化学分析也支持上述推断。此外,本文还提出了用有机碳同位素来进一步区分C29甾烷优势生物来源的方法。     

Structures of biogenic origin from Early Precambrian rocks of Euro-Asia

Spheroidal microfossils mainly 20 to 100 μm in diameter and exhibiting granular surface textures have been recovered from Early Precambrian rocks by applying a new method of water separation in combination with thin chemical preparation. In contrast to the Acritarcha, these microfossils are characterized by a relatively low specific weight (close to one) and considerable fragility due to impregnation by mineral matter. They occur in Archean sediments of Hindustan, in rocks of the Baltic and Aldan Shields with ages of 3.0 to 3.5 billion (10⁹) years, and in Proterozoic deposits in many regions of Euro-Asia. They commonly occur in great number in Precambrian sediments of West Africa. Australia and North America. These forms are here regarded asMenneria Lopuchin and are considered to be bluegreen algae.Menneria resembles alga-like forms reported by Engel, Nagy and their co-workers from the Onverwacht Series and microfossils reported by Schopf and Barghoorn from the Fig Tree Series, both of the Swaziland System of southern Africa. In addition to spheroidalmicrofossils, ribbon-like and filiform microstructures are here reported from Archean deposits. The biogenic structures here described from the Early Precambrian of Euro-Asia are considered to have been photosynthetic and planktonic. Their progressive evolution, intensive production of organic matter, and biogeochemical role in concentration of rare elements is discussed.     

Biogenicity of an Early Quaternary iron formation,Milos Island,Greece

A ~2.0‐million‐year‐old shallow‐submarine sedimentary deposit on Milos Island, Greece, harbours an unmetamorphosed fossiliferous iron formation (IF) comparable to Precambrian banded iron formations (BIFs). This Milos IF holds the potential to provide clues to the origin of Precambrian BIFs, relative to biotic and abiotic processes. Here, we combine field stratigraphic observations, stable isotopes of C, S and Si, rock petrography and microfossil evidence from a ~5‐m‐thick outcrop to track potential biogeochemical processes that may have contributed to the formation of the BIF‐type rocks and the abrupt transition to an overlying conglomerate‐hosted IF (CIF). Bulk δ¹³C isotopic compositions lower than ‐25‰ provide evidence for biological contribution by the Calvin and reductive acetyl–CoA carbon fixation cycles to the origin of both the BIF‐type and CIF strata. Low S levels of ~0.04 wt.% combined with δ³⁴S estimates of up to ~18‰ point to a non‐sulphidic depository. Positive δ³⁰Si records of up to +0.53‰ in the finely laminated BIF‐type rocks indicate chemical deposition on the seafloor during weak periods of arc magmatism. Negative δ³⁰Si data are consistent with geological observations suggesting a sudden change to intense arc volcanism potentially terminated the deposition of the BIF‐type layer. The typical Precambrian rhythmic rocks of alternating Fe‐ and Si‐rich bands are associated with abundant and spatially distinct microbial fossil assemblages. Together with previously proposed anoxygenic photoferrotrophic iron cycling and low sedimentary N and C potentially connected to diagenetic denitrification, the Milos IF is a biogenic submarine volcano‐sedimentary IF showing depositional conditions analogous to Archaean Algoma‐type BIFs.     

Devonian and modern relatives of the Precambrian Eosphaera: possible significance for the early eukaryotes

The Middle Precambrian problematical microorganism Eosphaera Barghoorn and Eosphaera- like structures known from Early and Middle Precambrian banded iron formations have been compared with the recently discovered Devonian volvocacean alga Eovolvox Kaźmierczak and some modern colonial Volvocales. The volvocacean interpretation of Eosphaera implies that algal eukaryotes (green phytoflagellates) werc already prewmt in the earth biosphere before at least 1.9 b. y. ago (Gunflint Iron Formaticn), and probably before 2.7 b. y. ago (Soudan Iron Formation). The type of metabolism and thc oxygen requirements of modern colonial Volvocales indicate that Eosphaera was most probably a photoorgano-trophic (mixotrophic) organism able to live in the extremely oxygen-deficicnt or anoxy-genous Early Precambrian environment. As an oxygen-releasing photosynthesizer, Eosphaera could have played a considerable role in the production of free oxygen during the Precambrian. The abundance of Eospkaera- like ferriferous structures in the iron microbands of many banded iron formations implies active participation of these organisns in the formation of Precambrian sedimentary iron ores. The exclusively fresh-water habitat of extant volvocacean algae suggests that the Procambrian environments inhabited by Eosphaera were non-marine.     

The effect of geological biases on our perception of early land plant radiation

The Silurian–Devonian plant radiation was a critical development in the evolution of early terrestrial ecosystems. Characterizing the diversity dynamics of this radiation has been a focus of numerous studies. However, little is known about the impact of geological bias on our perception of this biodiversification. Here, we use a new, comprehensive compilation of plant occurrences from North America, together with a Macrostrat lithological dataset, to elucidate the relationships between the palaeobotanical and geological records of early land plants. Results show that observed raw diversity patterns at both species and genus rank are significantly correlated with fluctuations of sedimentary rock volume, especially of non-marine fossiliferous deposits. The lack of terrestrial sedimentary deposits before the Emsian (Early Devonian) makes it difficult to obtain an accurate depiction of the pre-Emsian plant diversification in North America. However, complementary analyses reveal that sampling-standardized diversity patterns partially correct the raw trajectories, especially at the genus-level if enough preserved non-marine sediments are available for sampling. Our findings highlight that geological incompleteness remains a fundamental bias for describing early plant diversification. This indicates that, even when sampling is extensive, observed diversity patterns potentially reflect the heterogeneity of the rock record, which blurs our understanding of the early history of land vegetation.     

Palaeoecological significance of calcimicrobial communities during ramp evolution: An example from the Lower Cambrian of Germany

Summary The palaeoecology of calcimicrobial communities from the only Gondwana-related Lower Cambrian in Central Europe (eastern Germany, carbonates and siliciclastics) has been studied. Six morphological groups of calcimicrobes are described. Some of them show a significant ability for sediment stabilization and construction of biohermal reef mounds. Other types of calcimicrobes were more common in biostromal thickets. Some of them were capable of populating different environments, growing in different modes and on different substrates. AnEpiphyton-archaeocyathan reef mound is described, illustrating the importance of calcimicrobes for mound formation. The fossil communities together with a complex of sedimentary features allow a reconstruction of the depositional history of the environment. Based on comparison with similar Gondwanan Lower Cambrian successions (Sardinia/Italy, Spain) and facies development a model is proposed describing the sedimentary history. Three depositional stages are distinguished: (1) deep subtidal ramp, (2) shallow subtidal ramp, (3) shallow subtidal to intertidal mixed siliciclastic-carbonate ramp with migrating oolitic shoals. In spite of similarities with the deposits in southern Europe, some distinct differences exist with respect to the succession of facies, the completeness of the sections, the fossil spectrum, and the nature of the siliciclastic sediments. For the German Lower Cambrian, a facies development from a low energy deep environment to a high energy shallow environment (partly restricted and with some evaporites) can be reconstructed. As compared with Sardinia and Spain, the depositional environment of the eastern German Lower Cambrian successions was predominantly characterized by low-energy conditions.     

Possible ctenophoran affinities of the Precambrian "sea-pen" Rangea

The Namibian Kuibis Quartzite fossils of Rangea are preserved three-dimensionally owing to incomplete collapse of the soft tissues under the load of instantaneously deposited sand. The process of fossilization did not reproduce the original external morphology of the organism but rather the inner surface of collapsed organs, presumably a system of sacs connected by a medial canal. The body of Rangea had tetraradial symmetry, a body plan shared also by the White Sea Russian fossil Bomakellia and possibly some other Precambrian frond-like fossils. They all had a complex internal anatomy, smooth surface of the body, and radial membranes, making their alleged colonial nature unlikely. Despite a different style of preservation, the Middle Cambrian Burgess Shale frond-like Thaumaptilon shows several anatomical similarities to Rangea. The body plan of the Burgess Shale ctenophore Fasciculus, with its numerous, pinnately arranged comb organs, is in many respects transitional between Thaumaptilon and the Early Cambrian ctenophore Maotianoascus from the Chengjiang fauna of South China. It is proposed that the irregularly distributed dark spots on the fusiform units of the petaloid of Thaumaptilon represent a kind of macrocilia and that the units are homologous with the ctenophoran comb organs. These superficial structures were underlain by the complex serial organs, well represented in the fossils of Rangea. The Precambrian "sea-pens" were thus probably sedentary ancestors of the ctenophores.     

The upper Shemshak Formation (Toarcian–Aalenian) of the Eastern Alborz (Iran): Biota and palaeoenvironments during a transgressive–regressive cycle

The siliciclastic, up to 4,000 m thick Upper Triassic–Bajocian Shemshak Formation is widespread across the Iran Plate, especially in the Alborz Mountains of northern Iran. In contrast to its lower, generally non-marine part, the upper part is marine. Based on the Tazareh section of the eastern Alborz, an integrated analysis of this marine interval is presented. The 1,700 m thick marine sedimentary succession records a gradual deepening from inner to mid and outer shelf environments from the Middle Toarcian to early Late Aalenian. During the Late Aalenian–Early Bajocian, the trend was reversed and infilling of the basin by a large delta system occurred. This general facies development reflects a nearly symmetrical transgressive–regressive (T–R) megacycle, terminated by the inter-regional mid-Cimmerian tectonic event. A renewed transgression in the early Late Bajocian initiated a subsequent sedimentary megacycle. The bioturbated mid and outer shelf sediments contain a low to moderately diverse benthic fauna dominated by deep burrowing bivalves, often preserved in the growth position. A hierarchy of four orders of sedimentary cycles can be recognized (parasequences, parasequence sets, unconformity-bounded third-order depositional sequences, and the 13 ma long second-order T–R megacycle). A regional correlation with the Jajarm area (200 km to the east) shows a very similar temporal facies pattern of the upper Shemshak Formation. The eastern Alborz T–R cycle is completely out-of-phase with other (eustatic) sea-level curves, suggesting regional tectonic control. Rough estimates of subsidence rates give an average value of 126 m/ma. However, much higher values for the Aalenian (230 m/ma), particularly the Late Aalenian (700 m/ma), indicate a distinct increase in subsidence rate towards the Early Bajocian mid-Cimmerian tectonic event. These high subsidence rates suggest that the sediments of the Shemshak Formation of the eastern Alborz formed in a (young) rift basin.     

安徽淮南早寒武世蠕虫化石的发现

Ｐａｌａｅｏｓｃｏｌｅｘｈｕａｉｎａｎｅｎｓｉｓｓｐ．ｎｏｖ．是我国华北地区寒武纪地层中首次发现的蠕虫化石，也是我国寒武纪地层中所发现的蠕虫化石的最高层位。     

Antiquity and evolutionary status of bacterial sulfate reduction: sulfur isotope evidence

The presently available sedimentary sulfur isotope record for the Precambrian seems to allow the following conclusions: (1) In the Early Archaean, sedimentary delta 34S patterns attributable to bacteriogenic sulfate reduction are generally absent. In particular, the delta 34S spread observed in the Isua banded iron formation (3.7 x 10(9) yr) is extremely narrow and coincides completely with the respective spreads yielded by contemporaneous rocks of assumed mantle derivation. Incipient minor differentiation of the isotope pattersn notably of Archaean sulfates may be accounted for by photosynthetic sulfur bacteria rather than by sulfate reducers. (2) Isotopic evidence of dissimilatory sulfate reduction is first observed in the upper Archaean of the Aldan Shield, Siberia (approximately 3.0 x 10(9) yr) and in the Michipicoten and Woman River banded iron formations of Canada (2.75 x 10(9) yr). This narrows down the possible time of appearance of sulfate respirers to the interval 2.8--3.1 x 10(9) yr. (3) Various lines of evidence indicate that photosynthesis is older than sulfate respiration, the SO4(2-) Utilized by the first sulfate reducers deriving most probably from oxidation of reduced sulfur compounds by photosynthetic sulfur bacteria. Sulfate respiration must, in turn, have antedated oxygen respiration as O2-respiring multicellular eucaryotes appear late in the Precambrian. (4) With the bulk of sulfate in the Archaean oceans probably produced by photosynthetic sulfur bacteria, the accumulation of SO4(2-) in the ancient seas must have preceded the buildup of appreciable steady state levels of free oxygen. Hence, the occurrence of sulfate evaporites in Archaean sediments does not necessarily provide testimony of oxidation weathering on the ancient continents and, consequently, of the existence of an atmospheric oxygen reservoir.     

How far did feedback between biodiversity and early diagenesis affect the nature of Early Palaeozoic sea floors?

Latest Precambrian to Early Palaeozoic biosphere evolution triggered changes in early diagenesis and carbonate precipitation which fed back to biodiversity through colonization of hard substrates. Progressive increase in the depth and intensity of bioturbation and bio‐irrigation lowered the zone of early carbonate cementation in the uppermost sediment column. This firstly led to a decline in the abundance of the flat‐pebble conglomerates which had been a common feature of Cambrian and Early Ordovician successions, replaced by the peak and subsequent decline in the Palaeozoic abundance of submarine hardgrounds. The availability of very widespread lithified sea floors in shallow subtidal settings during the Ordovician promoted a rapid expansion in sclerobiont diversity and contributed to the Great Ordovician Biodiversification Event.