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The nature of Mawsonites (Ediacara fauna)
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.
... These features and size of crystals similar to that of those filling the fossils differentiate the extensions from the sediment shears and chips. The regular sculpture on the fossils (radial grooves and extensions), lack of visible upward bending of the surrounding sediment, and absence of distinct vertical or oblique channel or core extending from the bedding surface into the sediment make these fossils different from sand volcanoes and fluid escape structures (van Loon, 2008;Menon et al., 2016). ...
... (2) the formation of dolomite in marine conditions as a result of mixing sea-and freshwater, or as a result of the interaction of sediment with Mg-saturated solutions; (3) secondary dolomitization of buried sediments (Tucker and Wright, 1990;Flügel, 2004); (4) dolomite precipitation in normalmarine environments as a result of bacterial sulfate reduction (Wright and Wacey, 2005;McKenzie and Vasconcelos, 2009). It is the latter model, associating the dolomite precipitation with anoxic environments (Hood and Wallace, 2018;Li et al., 2021), that explains an extraordinary abundance of dolomite in the Precambrian sedimentary sequences (Hood et al., 2011;Wood 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.
... 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. ...
... In the case of fluid or sand escape structures, we would expect to find a huge number of them. In addition, it has been also demonstrated earlier that there was no known physical process might result in a soft-sediment deformation structure with such regular and complex morphology (Loon, 2008). ...
The Ura Uplift section in the Patom Highlands area of Siberia is one of the most complete Neoproterozoic carbonate-siliciclastic succession with excellent chemostratigraphic records (d 13 C and 87 Sr/ 86 Sr). Present study reveals the first finds of Ediacaran macrofossils within thick siliciclastic/carbonate succession of the Chencha and Zherba formations with well-established negative d 13 C anomaly attributed to the Shuram Excursion in this section. The fossils are preserved in glauconitic and quartz sandstone horizons in shallow-marine tide-influenced nearshore environment and share many taphonomic and morphological features with the White Sea-and Nama-type biotas. On the one hand, stratigraphic position of some of fossils within the uppermost Shuram Excursion falls into the time interval 573-567 Ma and therefore is correlative to the Avalon-type fossil assemblage. On the other hand, the new fossil assemblage indicates to the earlier appearance of some Nama-type species, such as erniettomorphs, than it was thought. In addition, the new assemblage is characterised by low-diversity metazoan community dominated by microbial textures. It potentially supports the hypothesis that the Ediacaran shallow water fossil assemblages were significantly less species-rich than their deeper water counterparts. The discovery of the oldest macrofossils in South Siberia both opens a new window to understand their palaeoecology and taphonomy, and demonstrates that palaeontological potential of such a vast region has not entirely been realised. Ó
... 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). ...
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). ...
Deformations formed in unconsolidated sediments are known as soft-sediment deformation structures. Their nature, the time of their genesis, and the state in which the sediments occured during the formation of soft-sediment deforma-tion structures are responsible for controversies regarding the character of these deformations. A defi nition for soft-sediment deformation structures in siliciclastic sediments is therefore proposed. A wide variety of soft-sediment deformations in sediments, with emphasis on deformations in siliciclastic sediments studied by the present author, are described. Their genesis can be understood only if their sedimentary context is con-sidered, so that attention is also paid to the various deformational processes, which are subdivided here into (1) endo-genic processes resulting in endoturbations; (2) gravity-dominated processes resulting in graviturbations, which can be subdivided further into (2a) astroturbations, (2b) praecipiturbations, (2c) instabiloturbations, (2d) compagoturbations and (2e) inclinaturbations; and (3) exogenic processes resulting in exoturbations, which can be further subdivided into (3a) bioturbations – with subcategories (3a') phytoturbations, (3a'') zooturbations and (3a''') anthropoturbations – (3b) glaciturbations, (3c) thermoturbations, (3d) hydroturbations, (3e) chemoturbations, and (3f) eoloturbations. This sub-division forms the basis for a new approach towards their classifi cation. It is found that detailed analysis of soft-sediment deformations can increase the insight into aspects that are of im-portance for applied earth-scientifi c research, and that many more underlying data of purely scientifi c interest can, in specifi c cases, be derived from them than previously assumed. A fi rst assessment of aspects that make soft-sediment deformation structures in clastic sediments relevant for the earth sciences, is therefore provided.
... 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). ...
Ediacaran fossils represent a distinct group of large and structurally complex, enigmatic, soft-bodied organisms dominating
the end-Precambrian (Ediacaran) oceans, with an age range from 630 to 542 Ma (Martin et al., 2000; Knoll et al., 2004, 2006).
Microbes constituted the Precambrian biosphere almost entirely, and formed mats on wet sediment surfaces in the absence of
grazers and burrowers. Evidence for microbial mats in the rock record dates back to 3.5 Ga (Altermann et al., 2006; Altermann,
2008) and mats continue to exist today, although largely confined to stressful environments due to metazoan activities.
In the Ediacaran marine succession of the Cerro Negro Formation (Tandilia System, NE
Argentina), abundant microbially induced sedimentary structures indicate general conditions of substrate biostabilization. Numerous discoidal structures in this succession were previously interpreted as moulds of soft-tissue holdfasts of sessile organisms, within the form genus Aspidella. In this study, we performed a detailed re-analysis of some of these features and discuss two alternative hypotheses to explain their genesis: (1) as the result of soft-sediment deformation and fluid injection structures; and (2) as structures of active animal–sediment interaction (i.e. trace fossils). We show that the dome-shaped discs are internally laminated, with a cylindrical to a funnel-shaped vertical tube at their central region. The presence of these downwards vertical extensions and other intricate internal arrangements cannot be explained under the taphonomic spectrum of discoidal fossils, but shows striking similarities to Intriteslike structures and other sand-volcano-like pseudofossils (e.g. Astropolithon). However, some structures are hard to distinguish from vertical dwelling burrows with funnel-shaped apertures and thick-lined walls, commonly produced by suspension- and detritus-feeding invertebrates (e.g. Skolithos isp., Monocraterion isp. and, less likely, Rosselia isp.). Since reliable age constraints are unavailable, and further investigation concerning other palaeobiological indicators is needed, the most parsimonious hypothesis is that of a structure derived from fluid-escape processes. Our study demonstrates the importance of detailed investigation on discoidal structures in either upper Ediacaran or lower Cambrian strata.
The term ‘Ediacara Biota’ (or many variants thereof) is commonly used to refer to certain megascopic fossils of Precambrian and early Palaeozoic age – but what does the term actually mean? What differentiates a non-Ediacaran ‘Ediacaran’ and an Ediacaran ‘Ediacaran’ from an Ediacaran non-‘Ediacaran’? Historically, the term has been used in either a geographic, stratigraphic, taphonomic, or biologic sense. More recent research and new discoveries, however, mean that the term cannot actually be defined on any of these bases, or any combination thereof. Indeed, the term is now used and understood in a manner which is internally inconsistent, and unintentionally implies that these fossils are somehow distinct from other fossil assemblages, which is simply not the case. Continued use of the term is a historical relic, which has led in part to incorrect assumptions that the ‘Ediacara Biota’ can be treated as a single coherent group, has obscured our understanding of the biological change over the Precambrian–Cambrian boundary, and has confused research on the early evolution of the Metazoa. In the future, the term ‘Ediacaran’ should be restricted to purely stratigraphic usage, regardless of affinity, geography, or taphonomy; sufficient terminology also exists where reference to specimens on a geographic, taphonomic, or biologic basis is required. It is therefore time to abandon the term ‘Ediacara Biota’ and to instead treat equally all of the fossils of the Ediacaran System.
THE PUNCOVISCANA ICHNOFAUNA OF NORTHWEST ARGENTINA: THE COLONIZATION OF THE DEEP SEA AND RECONSTRUCTION OF PALEOENVIRONMENTS AND PALEOECOSYSTEMS OF THE PRECAMBRIAN-CAMBRIAN TRANSITION. The paleoecological, paleoenvironmental and evolutionary significance of the Puncoviscana ichnofauna is re-evaluated in the light of new ideas regarding the anactualistic nature of Precambrian-Cambrian ecosystems and the role of microbial communities in ecologic structure. The ichnofauna includes a wide variety of ichnotaxa, such as Circulichnis, Cochlichnus, Helminthopsis, Helminthoidichnites, Nereites, Oldhamia, Palaeophycus and Treptichnus, as well as several types of arthropod trackways. Structures considered in previous studies as graphoglyptids (Agrichnia), such as Protopaleodictyon and Squamodictyon, are reinterpreted here as wrinkle marks. Current depositional models for the Puncoviscana Formation envisage accumulation from turbidity currents in submarine fans. However, preliminary sedimentologic studies suggest a more complex paleoenvironmental situation, including not only deep-marine, flysch-type deposits, but also shallow-water settings affected by wave action. Within this framework, the Oldhamia association is interpreted as formed in deep water, while the Nereites association corresponds to shallow-marine environments. Nearly all the ichnofossils are oriented parallel to the bedding plane, displaying restriction to two-dimensional biotopes,and therefore they do not disturb the primary sedimentary fabric. Bedding plane trace fossils mostly reflect very shallow grazing by mobile, bilaterian metazoans. The only exception is Treptichnus, which represents a shallow infaunal, three-dimensional burrow system that records a feeding strategy of underground mining. Its presence reveals an early attempt of colonization of the infaunal ecospace during the early Phanerozoic. The Puncoviscana ichnofauna reflects, at least in part, lifestyles related to microbial mats that protected the sediment from erosion. The presence of wrinkled surfaces and palimpsest ripples suggests that stabilization by microbial binding was a major factor in Neoproterozoic to Early Cambrian ecosystems. Close association of the ichnogenus Oldhamia with these structures supports Seilacher's hypothesis that this icnotaxon records a feeding strategy of undermat mining. Grazing trails commonly are associated to microbial mats also. Behavioral patterns represented by several Oldhamia ichnospecies in deep Lower Cambrian deposits reflects that feeding strategies associated with microbial mats persisted in the deep sea after the agronomic revolution. This fact suggests a gradual closure of a taphonomic window during the Proterozoic-Cambrian transition and is consistent with the recognition of Ediacara-type body fossils in Cambrian strata. Presence of guided meanders (Nereites saltensis) in Cambrian shallow-water environments is consistent with the onshore-offshore model. Integration of ichnologic and isotopic data suggests a Vendian to Nemakit-Daldynian age for the Puncoviscana Formation.
The ichnogenus Curvolithus Fritsch, 1908, originally described from the Ordovician of the Prague Basin, typically comprises ribbonlike or tonguelike, flattened, endostratal traces with three rounded lobes on the upper surface. However, considerable confusion persists regarding the ichnotaxonomic status and diagnostic features of its ichnospecies. The type specimens of this ichnotaxon, overlooked in most subsequent reports, are redescribed herein. Curvolithus multiplex Fritsch, 1908, the type species, is retained for specimens with a trilobate upper surface and a quadralobate lower surface, in contrast to the criteria adopted by subsequent authors. The other ichnospecies originally proposed from the type locality, C. gregarius Fritsch, 1908, actually consists of a series of grouped parallel scratch marks forming ridges and should be removed from Curvolithus. Subsequently, four ichnospecies were defined: C.? davidis Webby 1970; C. annulatus Badve and Ghare 1978; C. aequus Walter et al. 1989; and C. manitouensis Maples and Suttner 1990. Curvolithus? davidis shows the typical trilobation of Curvolithus apparently in its lower surface, but the morphology of the upper surface is uncertain. Accordingly, it does not warrant ichnospecific assessment, and is regarded as a nomen dubium. The nature of the annulations on the trilobate upper surface of C. annulatus is unclear, and this ichnospecies is also best considered as a nomen dubium. Curvolithus aequus has a bilobate lower surface and probably represents washed out specimens of Didymaulichnus. Finally, C. manitouensis comprises specimens with a smooth, trilobate upper surface and a smooth, quadralobate lower surface, and is best regarded as a junior synonym of C. multiplex. Curvolithus multiplex has been used incorrectly for Curvolithus with a trilobate upper surface and a trilobate to unilobate lower surface. The new ichnospecies, Curvolithus simplex, is proposed herein for such traces. Curvolithus is interpreted as a locomotion trace (Repichnia) of endostratal carnivores, possibly gastropods, flatworms, or nemerteans. Curvolithus is a component of the Cruziana ichnofacies in shallow-marine facies, either of normal salinity or slightly brackish, in the latter case typically associated with fan deltas.
Occurrences of Psammichnites are of particular interest not only because of their unique biostratigraphic attributes underlying the oldest Cambrian trilobites in the Montagne Noire (southern France), but also because of their environmental significance. They are compared with other Lower Cambrian Psammichnites found elsewhere, particularly in siliciclastic-dominant platforms of south-western Europe, in which this ichnogenus occurs from late Tommotian to middle Atdabanian (Siberian chart) or late Cordubian to middle Ovetian (Iberian scale) times.
Wrinkle structures are a class of oddly textured sedimentary structures that are common in Proterozoic-Cambrian marine siliciclastic strata, but uncommon in post-Ordovician subtidal marine facies. Despite a long history of study, there is little agreement on how these structures form, or their role in larger-scale sedimentologic and paleontologic contexts. Based on similarities with morphologic and sedimentologic characteristics of modern microbially dominated communities, it appears that ancient wrinkle structures could have been formed by microbial mats. Microbial genesis for wrinkle structures not only helps explain their ubiquity during the late Neoproterozoic, but also their distinct paleoenvironmental and temporal shifts related to increasing vertically-oriented metazoan bioturbation. The Vendian-Cambrian transition offers a unique opportunity to examine the influence of suspect-microbial communities on the siliciclastic sedimentary record and their relationship with the earliest burrowing, grazing, and locomotive activities of metazoans. Based on outcrops in the Great Basin, USA, metazoans appear to have been moving on, in, and under suspect microbially-bound sediment, and exhibit features that suggest active and passive epifaunal and infaunal sediment ingestion. Together with more intricate horizontal burrow networks, this style of horizontal bioturbation is common in shallow settings of the Vendian-Cambrian transition, and is hypothesized to reflect highly specialized approaches to exploiting mat-mediated organic-rich sediment layers. Post-Ordovician restriction of such bedding-parallel burrowing behavior to deeper settings mirrors a shift of suspect-microbial structures to stressed or deep-sea settings. This restriction suggests replacement (rather than progressive evolution) of metazoans with horizontally specialized ecological strategies by more vertically-oriented bioturbating organisms.
—The bulk of Neoproterozoic trace fossils can be interpreted as horizontal creeping trails produced by minute vermiform,organisms moving,on or just beneath the seafloor or under algal mats. We have investigated the formation of trails by living cnidarians and platyhelminths that creep by cilia on mucus ribbons. These relatively simple metazoans,produce trails that are similar in size and morphology to some Neoproterozoic traces, owing to the entrainment of sediment with- in their mucus trails. Thus a mucociliary locomotory system provides sufficient means to form some types of Neoproterozoic traces. It follows that the body architectures of the Neoproterozoic trace-makers may have been quite simple, though complex bodyplans are, of course, not ruled out. Thus, the use of Neoproterozoic trace fossils to constrain the time of origin of bilaterians or of any crown-group bilaterian taxon remains questionable. Allen G. Collins, Jere H. Lipps, and James W, Valentine. Department of Integrative Biology and Museum
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.
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.
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.
An Ediacaran assemblage dominated by an unidentified species of CyclomedusaSprigg 1947, along with species of Ediacaria?Sprigg 1947, BeltanellaSprigg 1947, HiemaloraFedonkin 1982, and Nimbia?Fedonkin 1980, is described for the first time from the Innerelv Member of the Stappogiedde Formation exposed in coastal outcrops west of Tanafjorden on Digermul Peninsula, in northeastern Finnmark, northern Norway. The fossil assemblage is dominated by discoidal forms which share certain affinities with the cosmopolitan genera. Cyclomedusaand Ediacaria. However, our specimens differ from these and other discoidal Ediacaran fossils in the absence of radial sculpture. This, along with a basically concentric organization, are characteristics shared with Kullingiafrom the Dividal Group of northern Scandinavia, the White Sea, Podolia, and northwestern Canada, along with undescribed discoidal remains from the Charnian Supergroup, Charnwood Forest, Leicestershire, England, and the Conception Group, Avalon Peninsula, Newfoundland.Our discovery of an Ediacaran-type assemblage within the Middle Innerelv Member provides support for previous suggestions of a late Vendian age for this sequence. This general conclusion is consistent with the occurrence of early Cambrian taxa, including the trace fossil Phycodes, and the problematical forms Vendotaenia and Sabellidites, in basal portions of the Lower Breivik Formation, within the same stratigraphie section. The lowest formally-proposed faunal zone in northern Scandinavia is the Kullingia Zone, based on the occurrence of the fossil medusoid Kullingia concentrica in Member III (Middle Sandstone C) of the Dividal Group, northern Scandinavia. Kullingiais a distinctly chambered form that was probably pelagic. In contrast, Cyclomedusa,and related genera of the so-called Cyclomedusaplexus, comprise an informal grouping of intergrading, probably benthic, taxa that possess radial and/or concentric organization. In light of the intergradational nature of taxa, present difficulties in taxonomic interpretation and correlation, and the abundance of cyclomedusoids in many Ediacaran assemblages, we suggest that the concept of the Kullingia Zone, as originally defined for northern Scandanavia, be broadened to include the common form genera of the Cyclomedusa plexus, inclusive of the occurrences in the Innerelv Member described herein. It is our hope that additional fieldwork will provide a basis for more refined taxonomic evaluations and biozonation.
Acritarchs and other organic-walled microfossils occurring in siltstones of the Las Ventanas Formation (Quebrada de Viera and El Perdido members) are systematically described and illustrated. The assemblage includes the following species: Leiosphaeridia tenuissima, Leiosphaeridia minutissima, Lophosphaeridium sp., Soldadophycus bossii, Soldadophycus major, Soldadophycus sp. and Vendotaenia antiqua. The microflora is characterized by low diversity (six species), dominance of L. tenuissima, absence of acanthomorphic acritarchs, and relatively large size of sphaeromorphs, reaching 400 μm in diameter. A number of species are shared with acritarch assemblages preserved in the overlying Arroyo del Soldado Group. Differences between assemblages include the occurrence of abundant Bavlinella faveolata and small size of spheroids in the Arroyo del Soldado Group. The assemblage occurring in Las Ventanas Formation is assigned to the Ediacaran Leiosphere Palynoflora, which spans the interval between the base of the Ediacaran (end of the Marinoan Glaciation, 635 Ma) and the termination of the Gaskiers Glaciation (582 Ma). An early Ediacaran age between 615 and 579 Ma is also supported by available radiometric ages. An extensional setting for Las Ventanas basin is suggested on the basis of the bimodal, synsedimentary volcanism, strong palaeorelief, great thickness of alluvial fan conglomerates and the evolution from continental to open marine environments. Diamictites occurring in the Quebrada de Viera Member are described for the first time, including associated dropstones which suggest a glacial origin. If confirmed, this would be one more example of the association between rifting and glaciation in the Neoproterozoic, coeval with a low-diversity, high-abundance acritarch microflora. A causal relationship between these tectonic, climatic and biologic events is discussed.
After taxonomic revision, trace fossils show a similarly explosive diversification in the Ediacaran–Cambrian transition as metazoan body fossils. In shallow-marine deposits of Ediacaran age, trace fossils are horizontal, simple and rare, and display feeding strategies related to exploitation of microbial matgrounds. Equally notable is the absence of arthropod tracks and sinusoidal nematode trails. This situation changed in the Early Cambrian, when a dramatic increase in the diversity of distinct ichnotaxa is associated was followed by the onset of vertical bioturbation and the disappearance of a matground-based ecology (‘‘agronomic revolution’’). On deep sea bottoms, animals have been present already in the Ediacaran, but ichnofaunas were poorly diverse and dominated by the horizontal burrows of undermat miners. As shown by the ichnogenus Oldhamia, this life style continued to be predominant into the Early, and to a lesser extent, Middle Cambrian. Nevertheless, there was an explosive radiation of behavioral programs during the Early Cambrian. When exactly the bioturbational revolution arrived in the deep sea is uncertain. In any case, the Nereites ichnofacies was firmly established in the Early Ordovician. The rich ichnofauna in the Early Cambrian Guachos Formation of northwest Argentina probably marks a first step in this ecological onshore–offshore shift.
A uranium-lead zircon age for a volcanic ash interstratified with fossil-bearing, shallow marine siliciclastic rocks in the Zimnie Gory section of the White Sea region indicates that a diverse assemblage of body and trace fossils occurred before 555.3 +/- 0.3 million years ago. This age is a minimum for the oldest well-documented triploblastic bilaterian Kimberella. It also makes co-occurring trace fossils the oldest that are reliably dated. This determination of age implies that there is no simple relation between Ediacaran diversity and the carbon isotopic composition of Neoproterozoic seawater.
The phylogenetic relationships among most metazoan phyla remain uncertain. We obtained large numbers of gene sequences from
metazoans, including key understudied taxa. Despite the amount of data and breadth of taxa analyzed, relationships among most
metazoan phyla remained unresolved. In contrast, the same genes robustly resolved phylogenetic relationships within a major
clade of Fungi of approximately the same age as the Metazoa. The differences in resolution within the two kingdoms suggest
that the early history of metazoans was a radiation compressed in time, a finding that is in agreement with paleontological
inferences. Furthermore, simulation analyses as well as studies of other radiations in deep time indicate that, given adequate
sequence data, the lack of resolution in phylogenetic trees is a signature of closely spaced series of cladogenetic events.
The debate on the origin of animals remains topical. In their Perspective, [ Jermiin et al. ][1] discuss the study by [ Rokas et al. ][2] in the same issue, which concludes that the ancestors of living animal phyla arose in two rapid episodes of diversification. Their study goes some way to reconcile the conflicting fossil and molecular data, but Jermiin et al. argue that the debate should perhaps continue.
[1]: http://www.sciencemag.org/cgi/content/full/310/5756/1910
[2]: http://www.sciencemag.org/cgi/content/full/310/5756/1933
In this classic series-generating paleontology/geology book published by Columbia University Press, Mark and Dianna McMenamin explore the evolutionary and paleoecological questions associated with the Cambrian Explosion. This book both names and maps the initial paleogeographic reconstruction of the billion year old supercontinent Rodinia. The observations and interpretations in this book, particularly as regards the timing of the Cambrian Explosion, have stood the test of time. The issues identified herein as most important for understanding the Proterozoic-Cambrian transition, remain so today.
Numerous studies have shown that there is a dramatic increase in the abundance and diversity of trace fossils at about the base of the Cambrian (Seilacher, 1956; Daily, 1972; Crimes, 1975, 1987; Alpert, 1977; Fedonkin, 1980a,b; Brasier, 1982; Urbanek and Rozanov, 1983; Crimes and Anderson, 1985; Fritz and Crimes, 1985; Nowlan et al., 1985; Paczesna, 1986; Narbonne et al., 1987). Accordingly, the possibility of using trace fossils to define or correlate the base of the Cambrian System has been widely discussed (Daily 1972; Alpert, 1977; Fedonkin, 1980a; Crimes and Anderson, 1985; Crimes, 1987; Narbonne et al., 1987).
Beach and laboratory experiments with fresh living jellyfish have shown how casts and impressions can be obtained from both fine and coarse sand. The results are described and cast doubts on the "medusoid' traces from the Late Precambrian. -from Author
The paper offers an alternative interpretation of the Kingdom Vendobionta as a monophyletic sister group to the Eumetazoa. It is hypothesized that the Vendobionta are cnidarian-like organisms that lacked cnidae. Cnidarians are held to have arisen by acquisition of cnidae by symbiosis with a microsporidian. The analysis differs from existing interpretations of the Ediacaran fossils as ancestors of extant cnidarians in that none of these forms is recognized as either polypoid or medusoid. This interpretation requires the erection of a new metazoan phylum, the Vendobionta. -Authors
A uranium-lead zircon age for a volcanic ash interstratified with fossil-bearing, shallow marine siliciclastic rocks in the Zimnie Gory section of the White Sea region indicates that a diverse assemblage of body and trace fossils occurred before 555.3 ± 0.3 million years ago. This age is a minimum for the oldest well-documented triploblastic bilaterian Kimberella. It also makes co-occurring trace fossils the oldest that are reliably dated. This determination of age implies that there is no simple relation between Ediacaran diversity and the carbon isotopic composition of Neoproterozoic seawater.
The strange biota of Neoproterozoic sea bottoms become more understandable if we assume that otherwise soft sediments were sealed by firm and erosion-resistant biomats. This allowed 'mat encrusters' (vendobionts; trilobozoan and other sponges) to get attached to sandy bottoms, and molluscan 'mat scratchers' to scrape off an algal film, as if they were living on rocks. Minute conical 'mat stickers' (Cloudina) probably required a sticky substrate to become stabilized in upright position. Horizontal burrows are interpreted as the works of worm-like 'undermat miners.' Only the latter lifestyle appears to go back to the Mesoproterozoic; the others emerged in Vendian times and virtually disappeared when matgrounds became restricted to hostile environments in the wake of the Cambrian ecological revolution.
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
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.
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.
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
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).
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.
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.
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.
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.
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
The genus Chondroplon, known from only 2 specimens worldwide, is re-interpreted as a deformed Dickinsonia.
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.
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.
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.
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.
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.
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.