Article

The nature of Mawsonites (Ediacara fauna)

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Abstract

Some representatives of the Ediacara fauna have been later re-interpreted as pseudofossils. This was recently also done for Mawsonites, which was re-interpreted as a sedimentary structure formed due to interaction of a sand volcano (or water-escape structure) and a biomat. This sedimentary genesis appears not to be tenable, as several physical processes should have been involved, none of which is known from modern times or the geological past. It is concluded that Mawsonites must be considered as a fossil, either an imprint or a true fossil. This interpretation may be tested on one or more fragments of the structure.

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... Such structures can include gas or fluid escape structures (Callow et al., 2011;Van Loon and Maulik, 2011;Taj et al., 2014;Menon et al., 2015Menon et al., , 2016Tu et al., 2016), liquefaction or overload features (Kahle, 2009), as well as microbial constructions (Grazhdankin and Gerdes, 2007;Ivantsov et al., 2014;Bobrovskiy et al., 2018). This led various authors to reinterpret many discoidal structures originally interpreted as fossils, as inorganic or microbially-induced sedimentary structures (Seilacher et al., 2005;Van Loon, 2008;Hagadorn and Miller, 2011;Menon et al., 2015Menon et al., , 2016. ...
... For wordwide occurrences, recent works have shown that a multi-analytical approach is recommended in order to better understand the genesis, preservational pathways and classification of enigmatic structures and putative fossils (Gehling et al., 2000;Tarhan et al., 2010Tarhan et al., , 2015Retallack, 2012a,b;Bykova et al., 2017;Burzynski et al., 2017;Bobrovskiy et al., 2018). Discoidal structures can be originated under a great variety of paleoenvironmental conditions, and many previously described taxa were reinterpreted in the past decade as inorganic or microbial structures (Van Loon, 2008;Hagadorn and Miller, 2011;Ivantsov et al., 2014;Menon et al., 2015Menon et al., , 2016. As such, the analysis of the depositional context in which they occur, their relationship with MISS (see Gehling and Droser, 2009 for a discussion concerning sedimentary microbial textures developed due to this interaction), as well as their modes of occurrence (epirelief, hyporelief or even endorelief) and internal structures (through detailed petrographic analysis) is crucial to precisely determine their origin. ...
... Concentric ring patterns in recent "discoidal microbial colonies" provide excellent analogs to the concentric zonation of the medusoid fossils (Grazhdankin and Gerdes, 2007). However, some of the discoidal features consisting of lobes radiating out from a central circle (Mawsonites) are still being debated, as to whether they are true Ediacaran fossils (Sun, 1986b;van Loon, 2008) or microbial mat decay related gas domes (Seilacher et al., 2005;Seilacher, 2007). Other examples of pseudo-Ediacaran fossils described in this paper include the purported Ediacaran body fossils Arumberia, which are now considered to be a microbial mat related structure (Mapstone and McIlroy, 2006;Noffke, 2007). ...
... The fossilization potential of Cnidarians is naturally very variable. Whilst calcium-carbonate-producing corals are often preserved in complete fossil coral reefs, medusae that may consist of up to 98% water are rarely preserved as fossils and many 'medusoid' fossils are at the centre of ongoing discussions and reinterpretations, which sometimes reveal inorganic origins of the finds such as scratch circles, sand volcanoes, suction marks and gas escape structures (WeiGuo 1986;vAn Loon 2008;Young & HAgAdorn 2010). Especially problematic are the Ediacaran fossils that were described as medusoids due to their discoidal shape (see e.g.Sprigg 1949;WAde 1972) and their Cnidarian affinity has become more questionable due to lack of diagnostic morphological features (RunnegAr & Fedonkin 1992;MAcGAbhAnn 2007;Young & HAgAdorn 2010). ...
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Due to their low fossilization potential, Cnidarian medusae have a very sparse fossil record. Many fossils, especially from Ediacaran faunas, have been described as 'medusoid' organisms so far. However, often a confident assignment to a specific class or a comparison with extant taxa is impossible, as diagnostic morphological features are missing or unclear. Here, we introduce new finds of potential fossil hydromedusae from the Upper Jurassic Solnhofen Archipelago and describe them as Palaequorea rygoli g. nov. sp. nov., for which we suggest a modern analogue like the extant genus Aequorea. This study compares 'bona fide' finds of fossil medusae from the Solnhofen Archipelago and elsewhere to the new finds. Following on from the comparison and resulting re-inspection of the holotype, a revised description of Hydrocraspedota mayri shows it to be more comparable to a semaeostome medusa rather than hydromedusa as which it was originally described. Our study highlights how little Cnidarian medusae have obviously changed since the Jurassic and how important it is to use extant material for comparison in order to elucidate the possible identity and morphology of fossil finds. We hope that this example will stimulate the re-examination of some problematica in the fossil record and may lead to a reassessment of further taxa.
... Gutschick & Lamborn, 1975;Bhattacharya and Bhattacharya, 2007;Hertweck et al., 2007), many of them cannot be interpreted with any certainty as being either organic or inorganic in origin. A characteristic example are the different hypotheses regarding a structure in Ediacaran rocks, which has originally been described as a fossil (Mawsonites: Fig. 21G) (Glaessner & Wade, 1966), then as some kind of water-escape structure (Seilacher et al., 2005) and then again as a fossil because the origin as an SSDS could be falsifi ed (Van Loon, 2008a). In other cases, however, it must be concluded that Ediacaran structures originally interpreted as fossils seem to be inorganic in nature, indeed (Pfl üger, 1995;Hagadorn and Bottjer, 1999). ...
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... Concentric ring patterns in recent "discoidal microbial colonies" provide excellent analogs to the concentric zonation of the medusoid fossils (Grazhdankin and Gerdes, 2007). However, some of the discoidal features consisting of lobes radiating out from a central circle (Mawsonites) are still being debated, as to whether they are true Ediacaran fossils (Sun, 1986b;van Loon, 2008) or microbial mat decay related gas domes (Seilacher et al., 2005;Seilacher, 2007). Other examples of pseudo-Ediacaran fossils described in this paper include the purported Ediacaran body fossils Arumberia, which are now considered to be a microbial mat related structure (Mapstone and McIlroy, 2006;Noffke, 2007). ...
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Shallow water late Precambrian and Early Cambrian sequences have yielded not only shallow water trace fossils but also many examples of traces typical of later deep water deposits. Significant colonization of the deep oceans by trace making animals was, however, delayed until the Early Ordovician. There followed a gradual increase in ichnogeneric diversity through much of the Phanerozoic, with an acceleration from the Cretaceous through the Tertiary. The apparent migration of deep water forms from their origins in shallow water out to the deep sea was accompanied by their virtual disappearance from shallow water, thereby providing an early example of "retreat'. -from Authors
Article
Up to the 1950s, the Precambrian was regarded as unrewardingly unfossiliferous, records of fossils being isolated, few in number and dubious. The change came with the discovery by Reg Sprigg of body fossils in latest Proterozoic sediments in South Australia. Although there had been descriptions of isolated fossils (now recognised as Ediacaran) from rocks of this age in the nineteenth century and at the start of the twentieth century, in Newfoundland and Namibia, respectively, the Ediacara finds stimulated researches and now, at the start of the twenty-first century, diversified fossil assemblages are known, all over the world, from the period 600–543 Ma, known formerly as the Vendian and now officially as the Ediacaran. In this account, a brief description of the history of these finds is given, followed by descriptions of the most important provinces [South Australia, Leicestershire, Namibia, Russia (Podolia, the White Sea Coast, Urals and Siberia), Newfoundland (Avalon Peninsula) and Northwest Canada]: then of 27 other known occurrences of this dominantly soft-bodied and perplexing fauna(?)–it seems certain that some, at least of the fossils, are animal fossils, although some, even the greater part, could be a unique form of life, not animals or plants (“Vendobionta”). These descriptions, derived in the course of a literature search lasting over a year, are followed by discussions of important special aspects: trace fossils; geochronology and correlation; geotectonics; glaciation (the “Snowball Earth” concept applied to the Varangian/Laplandian/Marinoan glaciation, which ushered in this last subdivision of the Proterozoic); the evidence for Ediacaran and other life forms existing in the Proterozoic prior to its last, Ediacaran, chronological subdivison; the Vendozoa concept. The last section consists of short summary of conclusions. This text essentially constitutes an objective record of what has been published to 2005 on the Ediacaran System.
Article
The medusoid fossils that were conventionally referred to Cyclomedusa Sprigg 1947 are among the commonest elements of the late Precambrian Ediacara metazoan assemblage in South Australia. Specimens showing varying degrees of similarity have been reported worldwide from Precambrian occurrences.This study indicates that the fossils traditionally named Cyclomedusa are a morphological plexus consisting of heterogeneous medusae. The type species C. davidi Sprigg and C. radiata Sprigg are considered as a single species because of differences due to preservation. C. davidi can be compared with a living hydrozoan medusa Aequorea in general configuration and an affinity with the family Aequoreidae is suggested for the typical Cyclomedusa. Spriggia Southcott 1949, which was once regarded as a synonym of Cyclomedusa, is now re-interpreted as a float-bearing chondrophore (hydrozoan colonial medusa). The newly described S. wadeae sp. nov. provides remarkable evidence for the amazingly conservative evolutionary history of the family Porpitidae. ‘C’. plana Glaessner and Wade, 1966 may be also chondrophoran in affinity but its configuration remains uncertain.A critical review on the previously supposed Cyclomedusa and other medusoids from the upper Precambrian in southern Liaoning, China demonstrates that they are Cyclomedusa-like pseudofossils made by upwards escaping gas bubbles and water currents. Therefore they can not be used for stratigraphic correlation. This example suggests the need for extreme caution in studies of various Cyclomedusa-like circular structures.
Article
The non-availability of biomineralized skeletons and low levels of predation led Vendian evolution along strange avenues. The Ediacara-type Vendobionta appear to represent a kingdom, in which foliate shapes, large sizes and the necessary compartmentalization were achieved by quilting of the skin rather than by multicellularity. Psammocorallia, in contrast, are interpreted as coelenterates that constructed an internal sand skeleton. Both were immobile soft-bottom dwellers that had high population densities, and both became preserved by obrutional accidents; thus they render "fossil snap shots', in which the original distributional patterns, age structures and standing biomasses of populations are accurately recorded. -Author
Article
Ediacara metazoan assemblage, Ediacara Range, South Australia. The size and morphological complexity of this species are apparently of the scyphozoan grade. The characteristic three major zones and deeply cleft marginal lappets of this fossil form are similar to those of the living Coronatae but pedalia are not developed. This new species sheds significant light on the previously enigmatic Mawsonites Glaessner & Wade, and provides the basis for the erection of the Family Mawsonitidae under ?Order Coronatae. The present study confirms the fairly advanced evolutionary grade reached by medusae during Late Precambrian time (the Ediacaran or Ediacarian Period).
Article
Sinuous structures preserved in ripple troughs are recorded from strata of different ages, ranging from 1100 Ma to Late Riphean, of the Vindhyan Supergroup of central India. These have been previously interpreted as trace fossils, namely Cochlichnus anguineus Hitchcock. Detailed examination of these features precludes their categorization as trace fossils. These structures are probably related to synaeresis. Final outflow of water from ripple troughs during emergence may have played an important role in the initiation and distribution of these interpreted shrinkage cracks and the meandering structures possibly reflect the principal direction of tension. Identification of such dubiotraces in Proterozoic rocks might sharpen our awareness regarding the exact time of metazoan evolution.
Article
Based on own laboratory experience and a literature review, techniques for processing of different sedimentary lithologies and microscopical investigation in Neoproterozoic-Cambrian sequences are described. Emphasis is placed on etching and sequential sectioning techniques in cherts, phosphorites and limestones, widespread lithologies during this time interval in West Gondwana and elsewhere. Based on the hypothesis that ancestral metazoans in the Late Precambrian are likely to be expected as plankton or meiobenthos preserved in deeper water environments, prospective lithologies and possible paleoenvironments are indicated where future search should be centered.
Article
The Neoproterozoic interval (1000-540 Ma) contains ample evidence for a series of glacial intervals. These include the 750-700 Ma Sturtian glaciation, the 625-580 Ma Marinoan-Vendian glaciation and the 600-550 Ma Sinian glaciation. Paleomagnetic evidence has suggested that many of these glaciations occurred at tropical latitudes (less-than-or-equal-to 25-degrees) and this led to a number of theories that attempt to explain the occurrence of these anomalously low latitude glaciations (e.g., an increase in the, axial tilt of the earth, an equatorial low-orbit ice-ring, rapid equator to pole continental drift, incorrect identification of impact deposits as glacial deposits or secondary magnetizations misidentified as primary). New paleomagnetic data for Laurentia, China, Baltica and parts of Gondwana are combined with a reanalysis of previously published data to demonstrate that the Neoproterozoic glaciations may well all have occurred above 25-degrees latitude. Climate models using a juvenile Sun of slightly lower luminosity, lower CO2 levels and coupling to Milankovitch cycles suggest, that ice sheets could extend to within +/- 25-degrees of the Neoproterozoic equator. Thus, the new paleomagnetic data and climate models offer an alternative explanation for the Neoproterozoic glaciations that is consistent with the waxing and waning of intermediate latitude ice sheets to form the conformable sequences of warm climate-cold climate strata.
Article
A fossil assemblage of soft-bodied megascopic metazoans possessing faunistic, ecological and taphonomic affinities to known classical Ediacaran assemblages has been discovered from the Proterozoic Vindhyan Basin of India. The assemblage is represented by nine coelenterate genera (Tribachidium, Eoporita, Kaisalia, Cyclomedusa, Ediacaria, Nimbia, Paliella, Medusinites and Hiemalora), one arthropod genus Spriggina and a few unnamed possible new forms belonging to sponge and coelenterate. These fossils show facies-controlled temporal distribution forming two fossil zones: F1 and F2 located, respectively, in the Lakheri and Sirbu Formations (Bhander Group). A majority of them is common to the Ediacara assemblages of several continents, thereby suggesting their global biogeographic distribution. F1 and F2 in the local stratigraphy are separated by a thick stromatolitic carbonate facies devoid of metazoan remains. The Vindhyan Ediacara and host rock sequences reveal energetic (wave-tide-storm induced), shallow marine and near-shore environments of deposition of siliciclastic terrigenous facies. The carbonate facies parting suggests interruption of typical Ediacara environments by a broad spell of lime-rich quieter water settings favoring selective growth of metaphytes. The Vindhyan fossils show both Nemiana (higher relief forms without finer features in sandstones, e.g. Cyclomedusa and Ediacaria) and Beltanelliformis (lower relief forms with finer features in shales, e.g. Kaisalia and Hiemalora) types of preservation.
Article
Fossil Medusoid genera resembling Ediacaria (Sprigg 1947) and Hiemalora (Fedonkin 1982) and having distinctive Ediacaran affinity have been discovered in a shale horizon occurring at the base of the Bhander Group, the uppermost unit of the Vindhyan Supergroup of central India. This is the first record of unequivocal occurrence of Ediacara-like fossils in a Proterozoic basin of the Peninsular India. This finding substantially extends the previously known biogeographic range of the Ediacaran elements to Peninsular India and further enhances their biostratigraphpic potential for correlation of the upper Vindhyans with some Ediacaran horizons of Canada, Australia, South Africa and Russian Platforms. With this extension, the representatives of the genus Ediacaria can be regarded as having a global distribution in places now occupying both lower and higher latitudes. The genus Hiemalora, which appeared to be endemic to the Russian block, also has wide biogeographic coverage. These fossils assign an Ediacaran (550–543Ma) age for the host Lakheri Limestone and suggest that the Lakheri unit was deposited within 6 million years of the Precambrian–Cambrian boundary. They also support and refine the traditional view of the Late Neoproterozoic age for the lower Bhander Group. These fossils provide positive stratigraphic clues for locating the Precambiran–Cambrian boundary strata in the overlying Lakheri–Sirbu segment of the Vindhyan sequence. They also indicate a depositional environment typical of a muddy shallow shelf setting above storm wave-base.
Article
The Ribeira belt in SE Brazil is a Neoproterozoic to Early Palaeozoic orogen, whose architecture and history is not yet fully understood. The depositional age of many of the sedimentary sequences in the Ribeira Belt remains unconstrained, and with debate concerning their depositional environment and tectonic setting. In this paper we present SHRIMP zircon U/Pb age constraints for one such problematic unit in the Ribeira Belt – the Iporanga Formation – and discuss the significance of this age with regards to the timing of Neoproterozoic glacial events in southeast Brazil. Using a felsic volcanic unit immediately under the Iporanga Formation and granite cobbles from breccias in its basal parts a reconnaissance SHRIMP U/Pb zircon maximum depositional age of 580 Ma is assigned for the base of this unit. This age is marginally younger than the 625–605 Ma ages for intrusions into the Lajeado and Ribeira subgroups, with which the Iporanga Formation is in tectonic contact. This indicates that the Lajeado and Ribeira subgroups are not stratigraphically equivalent to the Iporanga Formation, as thought previously by some workers. The maximum depositional age of 580 Ma also places a maximum time constraint on the tectonic juxtaposition of the Iporanga Formation with other supracrustal units, and on the greenschist facies metamorphism and isoclinal folding that affected it. The potential glacial origin for the Iporanga Formation, if correct, would place it in the late Ediacaran — provisionally equivalent to the Gaskiers glaciation.
Article
The assembly of the Gondwana supercontinent during the waning stages of the Proterozoic provides a tectonic backdrop for the myriad biological, climatological, tectonic and geochemical changes leading up to, and including, the Cambrian radiation. A polyphase assembly of Gondwana during the East Africa, Brasiliano, Kuungan and Damaran orogenies resulted in an extensive mountain chain which delivered nutrients into a shifting oceanic realm. An analysis of key evolutionary events during this time period reveals the following (a) several fauna show well established endemism that may be rooted in a cryptic evolutionary pulse (c). 580 Ma (b) the margins of the Mirovian and Mawson Oceans formed the locus of radiation for the Ediacaran fauna (c) the margins of the Iapetan and Mirovian oceans form the olenellid trilobite realm (d) the margins of the Mawson and Paleo-Asian oceans are the birthplace of the so-called Gondwana Province fauna (e) evolutionary events associated with the Cambrian radiation were likely driven by internal (biological) changes, but radiation was enhanced and ecosystems became more complex because of the geochemical, ecological and tectonic changes occurring during Ediacaran–Cambrian periods.
Article
After describing the hazards of studying fossilized soft-bodied faunas, the paper goes on to present a review of the faunal diversity of Ediacaran faunas and the faunal composition and wider implications of Burgess Shale-type faunas. The late Precambrian and early Cambrian faunas are compared and contrasted. -A.W.Hall
Article
The genus Chondroplon, known from only 2 specimens worldwide, is re-interpreted as a deformed Dickinsonia.
Article
Seilacher, Adolf 1989 07 15: Vendozoa: Organismic construction in the Proterozoic biosphere. Lethaia, Vol. 22. pp. 229–239. Oslo. ISSN 0024–1164.Ediacara-type impressions of large, but flat and soft-bodied organisms in Late Proterozoic rocks are here interpreted not as ancestors of modern animal phyla, but as foliate pneu constructions, whose quilting patterns had to be accommodated with various modes of growth. In this view Vendozoa represent an evolutionary experiment that failed with the coming of macropbagous predators. True Metazoa are also represented, but in the form of trace fossils rather than body impressions. *Precambrian fossils, evolution, constructional morphology.
Article
Certain worm-like configurations on rocks are recognized as shrinkage-crack infillings. Some genuine Precambrian trace fossils are briefly described. The early Cambrian contains a richer assemblage, including some distinctive and widespread form genera. The study of early trace fossils leads to conclusions not only on facies, but also on the evolution of behaviour and functional morphology in soft-bodied organisms.
Article
At least two of the Cryogenian (Neoproterozoic) glacials, viz. those of the Sturtian and the Marinoan, are said to have been so severe that the entire Earth was covered with ice (Snowball Earth). The most convincing evidence consists of diamicts with some glacial striae and of other glacial signatures (striated surfaces, polished rocks) that have been found in areas that are interpreted on the basis of paleomagnetic data as being positioned, at the time, at low latitudes. The extremely low temperatures must have contributed to entirely frozen oceans. Nevertheless, diamicts of exceptional thickness were formed in a marine environment. This cannot be explained satisfactorily, as icebergs cannot have floated in an entirely frozen ocean. It is suggested that at least a considerable part of the extremely thick Neoproterozoic 'glaciomarine' deposits represent syntectonic mass-flow deposits rather than glacial deposits. The existence of a huge mountain range between Eastern and Western Gondwanaland provided favourable conditions for such deposits.
Article
Madagascar lay in an interesting position in Gondwana, straddling one of the largest orogens that formed as the supercontinent amalgamated. The Malagasy basement preserves a record of the timing and style of this amalgamation, and in addition contains much information as to the palaeogeography of the eastern Mozambique Ocean.Madagascar consists of a number of tectonic units that amalgamated in the Ediacaran–Cambrian. The tectonic units are: The Antongil Block; the Antananarivo Block; the Tsaratanana Sheet and the Bemarivo Belt. In addition to these, there are a number of regions dominated by Neoproterozoic metasedimentary rocks, including the Molo, Betsimisaraka, Vohibory and Androyen regions. In this review I outline these units, discuss their amalgamation history and implications for Neoproterozoic–Cambrian palaeogeography, and highlight a few key questions for future study.
Article
The East Gondwanaland evolved as a result of break up of the Rodinia supercontinent. The late Neoproterozoic-Early Cambrian geochemical events documented in the rocks of the East Gondwanaland, and Siberia suggest variations in the C, S, and Sr isotopic compositions of the contemporary seawater, and systematic distribution of phosphorite, and evaporite deposits. The geochemical records in the Peninsular India, Himalaya, South China, Iran, and Oman regions have been discussed, and used for establishing late Neoproterozoic links of these widely separated sedimentary basins.
Article
Fossils of the Ediacara biota offer our earliest insight into diverse macroscopic life on this planet. In particular, given the diversity and range of exquisite soft-bodied preservation, the potential for unraveling aspects of the paleobiology and paleoecology is great. Clearly, however, there can be a taphonomic overprint that dictates how complete the assemblage is. New diversity data (including richness and evenness) from South Australia is compared to previously published data from Newfoundland and the White Sea and is within the range of values for both modern and Phanerozoic assemblages. However, missing from our current understanding of Ediacaran ecosystems is a full description and interpretation of the many problematica structures and organic surfaces.
Encyclopedia of Geology 4
  • R M Plimer
R.M., Plimer, I.R. (Eds.), Encyclopedia of Geology 4. Elsevier Academic Press, Amsterdam, pp. 371–381.