The following document compiles the short contributions to the IMECT (International Meeting on the Ediacaran System and the Ediacaran-Cambrian Transition) meeting, celebrated in October 17-24th, 2019, in Guadalupe (Villuercas-Ibores-Jara Geopark). These abstracts have not been peer-reviewed and were only revised from a formal point of view by the Organizing Committee.
El siguiente documento recopila los resúmenes cortos presentados en el IMECT (International Meeting on the Ediacaran System and the Ediacaran-Cambrian Transition), celebrado del 17 al 24 de octubre de 2019 en Guadalupe (Geoparque Villuercas-Ibores-Jara). Estos resúmenes no han sido sometidos a revisión por pares y sólo fueron revisados por el Comité Organizador por lo que respecta a aspectos formales.
The terminal Precambrian Era experienced numerous integrated changes in lithosphere-atmosphere-hydrosphere-biosphere, such as changes in crustal coverage, deep water oxygenation and the evolution of macroscopic biota. Deep water oxygenation and increased nutrient availability were key drivers in the development of Ediacara biota and acanthomorphic acritarchs. The distinct Ediacaran animal fossils disappeared before the onset of the Cambrian and paved the way for new more complex animals. Evolution of bioturbation activity is one of such development that has been recorded since latest Ediacaran times and had become more prominent in the Cambrian. The present study is focused on the significant aspects of the spatio-temporal variability in bioturbation features and its connection with the depositional environment.
To address this issue, six well-preserved ichnogenera have been identified and reviewed, which are
The Navalpino anticline of the Central Iberian Zone provides a unique case study of well-exposed late Ediacaran reefs. Drone images show their spatial arrangement as tens of cloudinid-microbial patch, biohermal and complex reefs, encased in a dominantly shaly succession. The latter was deposited on a perturbed block-platform placed in a Cadomian retro-arc basin fringing West Gondwana. Reefs display the traditional core/flank/inter-reef subdivision and developed in generally low-energy, clear-water substrates, between normal and storm wave base. Cores consist of millimetre-sized irregular mesoclots of dense microbial microsparite, locally interrupted by laminated (stromatolitic) fabrics, containing abundant
The Pyrenees is an Alpine intracontinental fold and thrust belt that resulted from the convergence between the Iberian and European plates from Late Cretaceous to Oligocene times. In the Pyrenees, rocks ranging in age from late Neoproterozoic to Mississippian form an elongated strip in the backbone of the chain, unconformably overlain by Mesozoic and Cenozoic rocks. This pre-Variscan basement lies geographically disconnected from neighbouring outcrops that also belong to the eastern branch of the Ibero-Armorican Arc, such as the Catalan Coastal Range to the South, the Mouthoumet and Montagne Noire (southern French Massif Central) to the North, and Sardinia to the East.
The lower part of this pre-Variscan succession is made up by a pre-Upper Ordovician succession, more than 3000 m thick, which crops out extensively in the central and eastern Pyrenees and contains the Ediacaran-Cambrian transition. However, precise location of this boundary has been hampered by the monotonous character of an important part of the succession, the lack of a well-preserved fossiliferous record and the complex macrostructure. The scarcity of biostratigraphic data in the Ediacaran-Lower Ordovician succession has been partly compensated by reporting of geochronological U/Pb dating of zircon of interbedded metavolcanic rocks. This, together with detailed field work, allow to propose recently a new stratigraphic framework for this lower part of the pre-Variscan sequence, from bottom to top: The Canaveilles Group, with the Nyer, Olette and Pic de la Clape formations and the Jujols Group, with the Err, Valcebollère and Serdinya formations. An Ediacaran-early Cambrian age can be proposed for the Canaveilles Group and a broad Cambrian-earliest Ordovician age for the entire Jujols Group. The Ediacaran-Cambrian transition should be located within the Pic de la Clape Formation, which includes a succession of genetically related volcanic pulsations linked to episodes of carbonate production. Geochronological data also constrain the depositional age of the Ediacaran succession in the Eastern Pyrenees, from ca. 570 to 542 Ma, although the oldest ages obtained from metavolcanic rocks should be regarded as a minimum because of a thick series cropping out below these rocks.
The taphonomy of Ediacaran age soft bodied fossils has been extensively studied because global soft fossil preservation is highly unusual at all other times in Earth history. Ediacaran fossils are typically the results of impressions made by soft and flexible organisms in siliciclastic sediments. This mode of preservation is almost entirely absent from the fossil record from the end of the Cambrian Period onwards.
The microbial mats aid in rapid and early lithification (e.g. through trapping a binding), stabilise soft tissue, mediate pyritisation and diagenesis, and may even be moulding the organism in life. Fossil bedding surfaces covered in micro-bial mats have a wrinkled texture with a range of characters that are distinctive such as pustules, over-steepened ridges, contortion, tea-ring and folding of the microbialite, suspended sediments grains, and microscale concentrations of mine-rals such as mica flakes in the upper layers and pyrite grains beneath. These microbial mats are pervasive on bedding planes at the Flinders Ranges and White Sea fossil sites. The range of their textures and structures however is poorly documented and this new analysis shows that there is a spectrum of structures that grade in to each other, ranging from densely packed bundles of fibres with large tangled pustules to sparsely arranged filamentous structures as the mat first colonises a bedding surface. In the intergradation between these two extremes are a huge range of complex structures formed as the filaments start to overlap and interact, of which the most common are cross hatching fibrous structures as the microbial mat begins to knit toge-ther. The
(overleaf) A. SAM P35655
The Ediacara biota (571-539 Ma) represents the oldest complex large organisms in the fossil record, providing a bridge between largely microbial ecosystems of the Precambrian and the animal dominated world of the Phanerozoic. However, the nature of most Ediacaran organisms remains unclear, not least due to their enigmatic non-actualistic preservation. We show that Flinders-style fossilization of Ediacaran organisms was promoted by unusually prolonged conservation of organic matter, coupled with differences in rheological behaviour of the over- and underlying sediments. In contrast to accepted models, cementation of overlying sand was not critical for fossil preservation, which is supported by the absence of cement in unweathered, unmetamorphosed White Sea specimens and observations of soft sediment deformation in both White Sea and South Australian specimens. This model has been confirmed by laboratory simulations, which produced Ediacaran-like negative hyporelief impressions at the base of sand layers. The rheological model implies that Ediacaran fossils do not necessarily reflect the external shape of the organism. In many cases, the impressions repeat the morphology of a soft external or internal organic ‘skeleton’. This mechanism provides new constraints on biological interpretations of the Ediacara biota.
current address: University of Leeds, School of Earth and Environment, Leeds, LS2 9JT, UK
The late Ediacaran witnessed an increase in metazoan diversity and ecological complexity, marking the inception of the Cambrian Explosion. To constrain the drivers of this diversification, we combine local redox and nutrient data for two shelf transects, with a complete inventory of biotic diversity and distribution from the Nama Group, Namibia (~550 to ~538 Million years ago; Ma). Unstable marine redox conditions characterise all water depths in inner to outer ramp from ~550 to 547 Ma when the first skeletal metazoans appeared, but a marked deepening of the redoxcline and a reduced frequency of anoxic incursions onto the inner to mid-ramp is recorded from ~547 Ma onwards, with full ventilation of the outer ramp by ~542 Ma. Phosphorus speciation data show that initial anoxic ferruginous conditions promoted the drawdown of bioavailable P, with productivity constrained by limited P recycling back to the water column. A long-term decrease in nutrient delivery from continental weathering, coupled with a possible decrease in upwelling, led to the gradual ventilation of the Nama Group basins. This in turn decreased anoxic recycling of bioavailable phosphorus to the water column, promoting the development of stable oxic conditions and the radiation of new mobile taxa.
Ediacaran benthic marine ecosystems were dominated by extensive microbial mats that effectively sealed the sediment from the water column. Ediacaran ichnofaunas are characterized by non-penetrative, non-specialized horizontal trails interpreted as produced by vagile bilaterian metazoans that exploited organic matter concentrated within microbial mats. However, there is growing evidence that this situation started to change in the terminal Ediacaran with the appearance of other style of animal-substrate interactions. The Ediacaran-Fortunian Nama Group of Namibia, with its thick, laterally continuous, virtually undeformed and relatively well-dated shallow-marine successions, is proving to be a key unit to delineate these evolutionary innovations. In particular, the recently introduced ichnotaxon
When the animals arose and diversified continues to be controversial. Here I critically review the stratigraphic and other evidence in the Ediacaran to Cambrian interval to attempt to establish a robust timescale for this critical evolutionary time.
The evidence for a deep (i.e. pre-Ediacaran) origin of the crown group animals continues to weaken, and birth-modelling animals support this contention. As a large clade, animals are likely to have diversified very quickly, and there can be little support for a “phylogenetic fuse” type model for their origins. The evidence of the fossil record is in particular incompatible with a deep bilaterian origin, and although bilaterians may well have begun to radiate
Ediacaran rangeomorphs are the first substantially macroscopic organisms to appear in the fossil record, but their underlying biology remains problematic. Although demonstrably heterotrophic, their current interpretation as osmotrophic consumers of dissolved organic carbon (DOC) is incompatible with the inertial (>
The palaeontological record of the Ediacaran-Cambrian (E-C) transition, particularly of the Terreneuvian, remains poorly documented in Laurentia. Ediacaran fossils are relatively rare in the Great Basin despite extensive exposures of Proterozoic strata. In addition, except in the Northern Territories of Canada and possibly in the Great Basin (California and Nevada), all described first appearance data of Cambrian skeletonized faunas are post-trilobitic. As a consequence, no detailed biostratigraphic subdivisions have been proposed for the “pre-trilobitic” interval of Laurentia, and constraining the E-C boundary interval has remained difficult in most areas of this major palaeocontinent. Beside stratigraphic difficulties, such uncertainties impact our understanding of the triggers of this critical interval in Earth History. Proposed geodynamic triggers include the break up of Rodinia, generally estimated to end at about 555-550 Ma, but which rift-associated transform-faults might have remained episodically active till 525 Ma in the southwestern palaeomargin of Laurentia. In this context, the E-C succession in northwestern Sonora, which was located on the western margin of Laurentia, is unique. It records a thick, significant volcano-sedimentary episode interrupting a thick mixed carbonate-siliciclastic succession, and attesting for a still intense geodynamic activity in this area. Nevertheless, the exact duration and context of this episode remain unknown, as a result of which, the chronostratigraphy of the E-C -succession remains unresolved despite abundant studies. Indeed, Ediacaran tubular fossils (possibly including the youngest cloudinids worldwide),
Furongian biostratigraphy is typically dominated by trilobite biozonation schemes. However, the middle to upper Cambrian of subsurface Saskatchewan, Canada, is devoid of a diagnostic shelly macrofauna, except for a brachiopod assemblage from southwestern Saskatchewan (Robson
Carbonaceous paraconodonts from the upper Cambrian Deadwood Formation of Saskatchewan, Canada. Scale bar = 100 µm.
The Deadwood paraconodont assemblage includes the first occurrences of
As the most abundant and diverse animal phylum, eurarthropods have been major components of animal ecosystems for over 500 million years. Euarthropod fossils have also been key for examining the dynamics of the rapid early radiation of animals during the Cambrian Explosion. This event is documented by the Cambrian fossil record, however Precambrian ancestors have long been sought. Here we use the early fossil record of euarthropods as a model to explore the quality of fossil data as it relates to the Cambrian Explosion. Numerous types of fossil preservation, including soft-bodied macrofossils from Burgess Shale-Type (BST) localities, biomineralised exoskeletons, microfossils, and trace fossils are compared and contrasted across the Ediacaran-Cambrian boundary to constrain when euarthropods first evolved.
BSTs provide the most complete metazoan example of phylum-level anatomical construction in the euarthropod stem lineage during the Cambrian from 518 million years ago (Ma). The stem lineage includes non-biomineralized groups such as Radiodonta (e.g.
The presence of ichnofossils in the Ediacaran is already consolidated with the records especially of horizontal trails, but with low ichnodiversity and ichnodisparity with simpler morphology. The Ediacaran ichnofossils occupied both shallower marine environments in offshore-transition and offshore areas where their abundance and diversity reach the maximum, and to a lesser extent deep-marine environments in shelf areas. Ichnotaxobases are significant and variable morphological features of a trace that are directly connected to their behaviors. There are five main Ichnotaxobases: General form, Wall and lining, Fill, branching and spreiten. The Corumbá Group it is localized in Mato Grosso do Sul State in Brazil, surfaces in the southern portion of the Paraguay Fold Belt, in which the Guaicurus Formation corresponds to its top followed by the Tamengo, Bocaina, Cerradinho and Cadiueus formations. The Tamengo Formation age 542.37 ± 0.32 Ma have carbonaceous facies intercalated with packages of pelites, recorded with metazoan macrofossils, such as
Funding Projects: FAPESP (2016/ 06114-6); (2017/25433-8).
Ediacaran rangeomorphs possess a unique branching architecture and are amongst the most spatially and temporally widespread members of the Ediacaran macrobiota. Despite being plausibly interpreted as total-group metazoans, many aspects of their biology, as well as their unique phylogenetic placement, remain elusive. Historically, this has meant that rangeomorphs have not been considered in attempts to rationalise between different patterns of character acquisition in early-diverging metazoan clades. We reassess the anatomy, development and phylogenetic affinities of Rangeomorpha by examining a subset of taxa sampling the range of diversity within this group.
We report the developmental patterns of three rangeomorphs;
Finally, we present the results of a phylogenetic analysis concerning the affinities of Rangeomorpha and conclude they are crown-group animals, belonging to the stem-group Eumetazoa. These new data allow us to make inference about the evolution of eumetazoan character states and extends the fossil record of the Eumetazoa beyond 571 million years ago.
The Iberian Massif presents an extensive record of Ediacaran-Cambrian sedimentary rocks deposited along the African margin of Gondwana. They were involved in the Variscan Orogen, where they appear in different units both in autochthonous domains and included in large allochthonous complexes. These successions were originally formed in several E-W locations along the margin of Gondwana, and were later approached during dextral Variscan convergence. Consequently, the Iberian sedimentary successions bring an excellent opportunity to investigate the tectonic setting and Nd isotopic sources of the Ediacaran-Cambrian transition, and also the original palaeogeographic position of the Iberian terranes along the palaeomargin of Gondwana. In the NW of the Iberian Massif, the Basal Allochthonous Units of the Malpica-Tui Complex have been interpreted as an external section of the margin of Gondwana. The Basal Units represent a terrane with continental affinity composed of two metasedimentary sequences, different in age and composition. The lower metasedimentary sequence consists of a thick pile of metagreywackes deposited in the latest Neoproterozoic, while the upper succession is formed mainly by mica schists with a Furongian depositional age. Located under the allochthonous complexes, the Central Iberian Zone (CIZ) represents an autochthonous terrane interpreted as an inner section of the Gondwanan margin. CIZ presents a wide range of pre-Ordovician shales and sandstones distributed in two units: (i) the Lower Unit is mainly composed of monotonous successions of sandstones and shales, whose sedimentation took place over the Neoproterozoic; while (ii) the Upper Unit presents a greater lithological variety with a dominantly pelitic character, whose sedimentary facies suggest a more complex evolution of the sedimentary basins. Within the Upper Unit, the Pusa Group is mainly composed of shales deposited in early Cambrian times.
The Ediacaran greywacke successions from the Basal Allochthonous Units and the Central Iberian Zone share geochemical features typical for an active margin setting as the most probable depositional environment. Discrimination diagrams for the tectonic setting using trace elements point to a sedimentation influenced by an evolved volcanic arc, probably built over an extended and thinned continental basement. By contrast, the Cambrian siliciclastic successions show a greater recycled character and present similar compositions to those observed in common passive margins. Their highly homogeneous geochemical patterns confirm their recycled character and the greater influence of the sedimentary processes over the final chemical composition. These geochemical features suggest the development of the Ediacaran sedimentary series as related to the opening and early evolution of a peri-Gondwanan back-arc basin. The geochemical information provided for the siliciclastic rocks confirms the relatively closeness of the sedimentary basins to source areas, which is also consistent with the immaturity observed in the Ediacaran rocks. The widening of this back-arc basin over Cambrian times, led to a greater contribution from nearby cratonic areas. This stage in the evolution of the peri-Gondwanan realm also coincides with a decrease of the magmatic activity in the volcanic arc, which favoured a more stable depositional setting with a greater control of the sedimentary processes over the final outcome. Therefore, the geochemical features of the siliciclastic rocks from both domains reveal a change in the tectonic setting over the Ediacaran-Cambrian transition, which preceded the development of a Cambro-Ordovician passive margin.
Sm-Nd isotope composition of the Ediacaran-Cambrian siliciclastic sequences in the Malpica-Tui Complex provides a single population of Nd model ages (1743-2223 Ma) and εNdi values (from -13.1 to -8.1). These rather old Nd model ages and high negative εNdi values observed in the NW Iberian Basal Allochthonous Units account for a dominant contribution from continental crustal sources, which suggests an original proximity to the West African craton during the latest Neoproterozoic and Early Palaeozoic. However, the Ediacaran and Cambrian sequences in the CIZ present εNdi values (-1.7 and -4.7 respectively) and TDM (1288 and 1516 Ma, respectively), which support a general higher contribution from juvenile isotope sources. Moreover slightly older Cambrian Nd isotopic sources suggest greater proximity to the mainland, probably in relation to the suggested widening of the peri-Gondwanan back-arc setting. The contrasting Nd model ages for coeval sedimentary sequences in the NW Iberian Basal Units and the CIZ, likely suggests a different lateral location along the Gondwanan margin. Considering the proximity of the Basal Allochthonous Units to the West Africa craton, the younger Nd model ages observed in the autochthonous CIZ successions place the latter sedimentary basins in a more eastern location, probably in a section of the Gondwana margin located between the West Africa craton and the Sahara Metacraton, with a larger input of siliciclastic material eroded from younger continental source areas.
Ecdysozoa unites eight phyla of protostome animals: Priapulida, Kinorhyncha, Loricifera (Scalidophora); Nematoda, Nematomorpha (Nematoida); Tardigrada, Onychophora and Euarthropoda (Panarthropoda). Ecdysozoa includes the most diverse and abundant animal phyla (Nematoda and Arthropoda), as well as depauperate lineages (e.g. Priapulida). The ecdysozoan fossil record extends to the early Cambrian, but the precise timing for the origin of this lineage is still debated. To investigate ecdysozoan origins, we assembled a large phylogenomic dataset (up to 228 genes, 66 ecdysozoan and 14 outgrip taxa). For each major ecdysozoan linage, taxa sapling was designed to allow dating the basal split within that lineage. We then applied Bayesian methods to infer a time tree calibrated using a set of 55 fossils. We explored different analytical conditions (e.g. the impact of different calibration strategies and different assumptions about the rate of evolution). Our results corroborate an Ediacaran origin of Ecdysozoa and many major constituent lineages (Scalidophora, Nematoida+Panarthropoda, Panarthropoda and Euarthropoda). We found Nematoida to cross the Cambrian-Ecdysozoan boundary, while Onychophora, Nematoda and Nematomorpha emerged to be Phanerozoic in age.
The Cambrian GSSP (ca. 540 Ma) is marked by the appearance of complex trace fossil from the
The Fortunian is then characterised by surficial to shallow-tier, diverse assemblages, whereas the Cambrian Stage 2 displays shallow- to deep-tier, moderately to poorly diverse assemblages. Ichnologic analysis through the CIF indicates a temporal increase in burrow size, BI, BPBI and depth of penetration (Gougeon
The Ediacaran-Cambrian transition is the place of striking changes in Earth ecosystems, with a diversification of life recorded by a complexification in animal behaviors. In a series of classic papers, Crimes (
Spiral trace fossils are poorly reported from the Ediacaran-Cambrian transition and remain problematic (
The Brioverian from central Brittany, NW France, is a thick siliciclastic sedimentary succession that has been intensely deformed due to the successive Cadomian and Variscan orogenetic events. Hence, its stratigraphy, basin architecture and fossil content have been poorly understood, and the position of its uppermost limit within the Ediacaran or the Cambrian is still a pending question. However, recent investigations reported insights of life in the form of simple trace and body fossils in shallow-marine, tidally influenced settings (Gougeon
Looping, circling and spiraling trace fossils from the Brioverian. Scale bars are 1 cm. 1.
An Ediacaran double spiral from Australia closely relates in shape and size to the holotype of
Trace fossils of subsurface burrowing provide the earliest evidence for bilaterian animals, mesodermal musculature, the through gut, a centralised nervous system, and complex behaviour (Brasier
Avalonian Ediacaran successions (those from Newfoundland, Canada, and Charnwood Forest, UK; c. 575-560 Ma) host the oldest known communities of complex multicellular organisms, including some of the best candidate early animals. Rangeomorphs dominate these communities, and are the most geographically and environmentally widespread of the Ediacaran clades. Although the gross morphology of rangeomorph taxa is broadly similar, some species bear morphological structures that are unique among these groups. For example, the recently discovered “brushes” from Charnwood Forest bear a unique external sheath-like structure, while some multifoliate taxa, including
Diversity and species richness are critical parameters in studies of modern and fossil ecology. However, where these measures are influenced by changing opinions on classification, morphological disparity is not. Here we present an analysis of the disparity of rangeomorph taxa, across 20 communities, using metric and non-metric dimensional scaling and multivariate analysis. Our analyses are coded to include intraspecific variation, which has recently been recognised to be considerable in some rangeomorphs taxa. Our disparity framework provides a tool with which we then test how species with unique features (e.g. the “brushes”) fit within the rest of the known rangeomorph taxa. Finally, quantifying morphological disparity in rangeomorphs allows their levels of disparity to be compared to that of extant groups, and provides a quantitative assessment of the biotic composition of different communities.
Mistaken Point Ecological Reserve UNESCO World Heritage Site (MPER) is an internationally recognised locality for fossils of the Ediacaran macrobiota, hosting the oldest known examples of these organisms, including candidate metazoan body and trace fossils. Recent construction of a chronostratigraphic framework for the Conception and St. John’s Groups sedimentary succession within MPER (see Matthews et al. abstract for this meeting) offers an unparalleled opportunity to constrain the tempo of the rise and diversification of the MPER biotas.
Here we present a compilation of fossil occurrence data spanning 80 discrete fossil-bearing horizons within the MPER succession. These data yield stratigraphic ranges for all formally described macrofossil taxa within the MPER succession, facilitating recognition and interrogation of potential evolutionary signals. Plotting these data against both lithostratigraphy and time reveals likely facies controls on the occurrence of certain taxa, and provides temporal constraint on the appearance of key groups.
Peak taxonomic diversity is recognised within the upper Mistaken Point and lower Trepassey Formations. Heterogeneity in both population and species abundance is observed between and within palaeocommunities. Uniterminal rangeomorphs appear within the MPER stratigraphic succession several million years before multiterminal forms. Comparison of our occurrence data with similar stratigraphic range data we have collected from the Bonavista Peninsula (~200km to the NW) provides support to the suggestion that the MPER patterns are real evolutionary signals.
Together, our combined stratigraphic, palaeontological and geochronological approach offers a holistic record of evolution during the mid-late Ediacaran Period, and enables the development of testable hypotheses regarding the evolutionary trajectory of rangeomorphs and associated Avalonian taxa.
So far, our understanding of the life evolution crossing the Ediacaran-Cambrian interval was mainly based on fossil assemblages from platform and inner-ramp facies, while much less is known from deeper facies such as slopes and basins. Here a fossil assemblage is described from the chert of the Liuchapo Formation in Liujiata Village, Hunan Province, which represents a slope-to-basinal setting at the terminal Ediacaran and earliest Cambrian.
The base of the outcrop starts with 10 m of dolostones intercalated with thin chert layers and chert nodules (Dengying Formation). This is overlain by an approximately 50 m chert succession (Liuchapo Formation), which is only interfered by shale and dolostone beds at around 30 m above the Dengying-Liuchapo contact (thereafter abbreviated as DLB). At the top of the Liuchapo Formation, chert gradually turns to the silicified black shale of the Niutitang Formation. According to a previous U-Pb radiometric dating of tuffaceous beds from an adjacent correlative outcrop, the base of the Liuchapo Formation is only slightly older than 541.0 ± 1.0 Ma (Chen
The cherty layers and nodules in the Dengying Formation and the whole Liuchapo Formation were sampled and analyzed. Despite fragmented organic remains from the chert layers of the Dengying Formation, unequivocal
By studying the structure and preservation of the massively preserved fossils using optical microscope, SEM and Raman spectroscopy, we found that In the fabric, where multiple sorts of fossils are densely packed, the fossils never cross each other. Interruption even never happens between different sorts of fossils. All fossils are preserved with clay minerals in different types of cherts and dolostones, while their analogues in siliciclastic and muddy limestone were also reported to be preserved in such a composition. This is hardly a character of trace fossils, in which the burrow infill is expected to be varied in different sedimentary facies. Incongruent with the backfilling model, the sediments in the interspaces between the bowl-shaped units of The investigated fossil assemblage is located in a slope-basinal setting. According to the current oxygenation models for the Ediacaran-Cambrian ocean, the distal deep water was generally euxinic and ferruginous (e.g. Li
For these reasons, we interpret
Assessing changes in ichnodiversity through time is becoming an important tool to calibrate evolutionary radiations. However, the soundness of these studies necessarily depends on a well-supported ichnotaxonomy. In particular, reconstructing ichnodiversity trajectories during the Cambrian explosion is based on compilations of global datasets, which in turn involve multiple ichnotaxonomic decisions, such as careful evaluation of preservational variants and assessment of potential synonyms. In addition, the contrasting philosophical approaches of splitters and lumpers permeate any taxonomic work.
The taxonomic status of
Documenting the multiple preservational variants of Cambrian
The Wuliu-Zengjiayan section, global standard stratotype Section of Cambrian Miaolingian Series and Wuliuan Stage, yields abundant trilobites, acritarchs and brachiopods in Jianhe County, Guizhou Province. Brachiopod fossils are assigned to two subphyla, containing 11 genera and 13 species. The brachiopods are mainly distributed in the lower, middle and upper parts of the Kaili formation at the Wuliu-Zengjiayan section, especially in the middle part. Based on their stratigraphic ranges, the Kaili Formation is divided into three brachiopod assemblages, from bottom to top (Fig. 1).
The division and establishment of the stratigraphic ranges of the brachiopods included in the Kaili Formation at the Wuliu-Zengjiayan section, global standard stratotype Section of the Cambrian Miaolingian Series and Wuliuan Stage, is an essential supplement to the biostratigraphic division of trilobites and acritarchs.
—Correlation chart of the brachiopod assemblages and the trilobite zones of the Kaili Formation at Balang, Jianhe, Guizhou, South China; modified from Mao (2017), Mao et al. (2017), Zhao Stratigraphic distribution of brachiopods in the Kaili Formation at Wuliu-Zengjiayan section, China. Specimens collected from the Kaili Formation in Balang village, Jianhe, Guizhou, South Chnia. Scale bars = 2 mm, except F = 5 mm (A-E, G). A.
This study was supported by the Ministry of Science and Technology of China (Grant No.41772021 to Yang, Zhao)
The first biomineralizing metazoans inhabited late Ediacaran carbonate settings and represent a relatively diverse assemblage, including at least tree genera:
Material from the Ibor Group, Villarta de los Montes, central Spain, sheds light on the mode of growth of
It has been assumed that both
Geologically speaking, the Marwar Supergroup (Neoproterozoic to Cambrian) is a significant entity of the western Rajasthan, India. It has been subdivided into four major groups, from bottom to top: the Jodhpur, Bilara, Hanseran and Nagaur groups (Pareek,
Lithostratigraphy of Marwar Super Group showing revised stratigraphy of Jodhpur Group of Jodhpur city (modified from Sarkar Outcrops of the Malani Igneous suite of rocks surrounding Mehrangarh Fort and Jaswant palace (left). B. Contact between the Malani Igneous suite and the Jodhpur Group marked by a basal conglomerate (centre). C. Contact of rhyolite of MIS and Umaid Bhawan Formation of Jodhpur Group (right). Outcrops of the Sursagar Formation in an open quarry (left). B. Contact between sandstones of the Sursagar Formation (lower) and sandstones of the Motisar Formation (upper) from the Jodhpur Group (centre). C. Wave ripples on sandstones of the Sursagar Formation (right).
All these successions are overlain by the Quaternary JCF. The basal conglomerate consists dominantly of pebbles of rhyolite, subordinate amount of vein quartz, jasper and granite pebbles set in a fine- to medium-grained quartz-felspathic matrix. The clast/matrix ratio suggests that it is ranging from clast- to matrix-supported, oligomictic to rarely polymictic in nature, and the deposits were sedimented as channel lags to fluvial fan-apron deposits. The coarsening upward UBF is represented by claystone at the base followed by silty shale, fine- to medium and coarse-grained sandstone and pebbly sandstone at the top. Dominantly, festoon cross-bedding bearing UBF indicates a deltaic environment. The SF is characterized by fine- and medium-grained sandstones typically of quartz arenitic nature. It shows a prolific development of wavy ripple marks, scour and fill, rib and furrow structures with Ediacaran fauna (Kumar & Pandey,
The Conception and St. John’s Groups of south eastern Newfoundland, Canada, contain some of the oldest known examples of the Ediacaran macrobiota. Mistaken Point Ecological Reserve UNESCO World Heritage Site (MPER) is internationally recognised for such fossils, hosting both the oldest known palaeocommunities of large, architecturally complex life anywhere in the world, and the oldest evidence of metazoan-style locomotion. Understanding the causes of the emergence, evolution and extinction of the Ediacaran macrobiota, and their relationship to environmental change, requires the construction of an accurate, highly-resolved chrono-stratigraphic framework upon which biostratigraphic, taphonomic, sedimentological, and environmental data can be integrated. The MPER sedimentary succession, including more than 1000m of fossil-bearing strata alongside numerous datable tuffaceous horizons, is ideally suited to this endeavour.
We here present six U-Pb ID-TIMS radio-isotopic ages derived from zircons within volcaniclastic beds of the Conception and St. John’s Groups at MPER. Detailed sedimentological analysis reveals the depositional processes associated with the dated tuffites, contextualising our understanding of both the ages yielded, and the taphonomy of associated Ediacaran macrofossils. Remapping of the MPER stratigraphic succession enables us to present an updated lithostratigraphy in which to place these new ages.
Our data provide high-precision age constraints on horizons including the oldest architecturally complex macrofossils in the MPER succession from the upper Drook Formation; the youngest MPER rangeomorph fossils from the Fermeuse Formation; the famously diverse fossil assemblage of the ‘E’ Surface; and exceptionally preserved specimens on the ‘Brasier’ Surface in the Briscal Formation. We compare and contrast our ages to previously published datasets, and use our new ages to construct an age model for the MPER sedimentary succession, that is consistent with deposition within a back-arc basin. The chronostratigraphic framework we present permits the evaluation of regional palaeobiological datasets relative to both lithostratigraphy and time, broadening our understanding of the rise of early animals.
A study of the terminal Neoproterozoic-early Cambrian palaeobiology in the India Block is focused on the molecular fossil record from oil shales in Salt Range, Pujab Basin, northern Pakistan. Biomarkers from oil shales have been extracted at the top of the Salt Range Formation, East Salt Range, Pujab Basin, northern Pakistan, and analyzed using gas chromatography-mass spectrometry. The biomarker assemblage indicates distinctive and significant algae and bacteria contribution similar to other evaporitic basins during the time. Dominance of C29 sterane signifies that green algae were important primary producers, and similar to the Precambrian Huqf Group oils and solid bitumens from the Sichuan Basin, South China. The community of algae and other taxa, thrived in a thin oxygenated layer in the upper water column. However, the presence of an underlying euxinic water column is demonstrated by the presence of gammacerane. Excellent preservation of the biomarkers and organic-walled microfossils is indicated by sterane and triterpane molecular ratios with preferences for biological stereoisomers. The entire biomarker assemblage is indicative of a restricted, salinity-stratified depositional environment, possibly including the presence of extensive, anoxygenic microbial mats. Some of the observed characteristics mirror those of other coeval successions in Oman, India and South China, but the particular combination of features appears to be unique to this setting.
Reproductive traits are key to an organism’s capacity to adapt and evolve, but little is known about the influence of reproductive modes on selection pressures within the Ediacaran macrobiota. Previous analyses of ~580-560 Ma Avalonian palaeocommunities from Newfoundland (Canada) and Charnwood Forest (UK) found that inter-specific resource competition was rare, and weak where present. Independent spatial analyses predicted stoloniferous reproduction within some Avalonian rangeomorph populations, which has seemingly been corroborated by fossil evidence suggesting that filamentous stolons facilitate persistent connectivity even between large specimens. These latter results offer a possible explanation for the observed low resource competition, whereby physical connection of proportions of Avalonian populations via stolons enabled resource sharing, thus reducing intra-specific competition. In this study we test this hypothesis by investigating the relationship between reproductive mode and intra-specific competition for 12 Avalonian palaeocommunities.
Spatial point process analyses (SPPA) were used to identify taxon dispersal patterns within palaeocommunities, with analyses of population size-distributions used to constrain methods of reproductive behaviour. Assessment of whether taxa utilised waterborne and/or tethered/stoloniferous reproduction was performed by analysis of the directionality of dispersal clusters using K-measures, calibrated against models of water-dispersal mechanisms in extant taxa. We independently assessed our inferred reproductive behaviours using the Coefficient of Dispersal to identify significant intra-specific morphological variation, which can distinguish between asexual and sexual reproductive strategies. Together these methods enabled reconstruction of the reproductive modes of 13 taxa. The presence and strength of intra-specific competition within the communities was determined using SPPA, which found only rare instances of such competition. Carrying capacity was rarely reached in the studied populations, suggesting reduced selection pressures within these Avalonian communities. Combining these analyses enables us to use General Linear Model regressions to investigate the extent to which asexual reproduction influenced intra-specific competition within Avalonian communities.
Organic-walled microfossils from the early-middle Ediacaran Doushantuo Formation studied in several sections in the Yangtze Gorges area, South China, display lavishly ornamented cyst-like vesicles of high taxonomic diversity. Microfossils are diagenetically silicified in chert nodules and were studied in thin sections. Seven species of
The Bocaina Formation crops out in the state of Mato Grosso do Sul, SW Brazil, beneath the black limestones and shales of the Tamengo Formation, where the important
Funding Project: FAPESP (2016/06114-6); (2017/22099-0)
Molecular phylogenies suggest that polycystine radiolarians (the ones secreting aesthetically pleasant and delicate siliceous skeletons) are one of the oldest lineages amongst the Rhizarian supergroup of unicellular heterotrophic eukaryotes (Sierra
The presence of polycystine radiolarians in the Middle Cambrian was firmly established 20 years ago, based on a well-preserved and diverse microfauna extracted from Templetonian - Undillan strata of Australia (Won & Below,
However, the record of early Cambrian radiolaria and their morphological and anatomical appearance is a matter of ongoing controversy (see Aitchison
Nevertheless, three recent papers presenting radiolarians from Lower Cambrian strata of South China have revived the debate by questioning the archaeospicularian-type skeleton structure for the oldest polycystine radiolarians (Zhang & Feng,
Cambrian radiolarians extracted from the Shashkunar pelagic sedimentary sequence that crops out at the Ak-Kaya section of the Gorny Altai (southern part of western Siberia) provides useful insights to this debate. The presence of polycystine radiolarians was initially reported nearly 20 years ago (Obut & Iwata,
A Russian-French collaborative project, initiated in 2010, allowed re-examination of the radiolarian-bearing strata cropping out at the Ak-Kaya section. Results established the presence of archeontactiniid radiolarians in an 8 m-thick interval of bedded cherts situated at the upper part of the Shashkunar Formation (Pouille
We have recently obtained additional material from the siliceous mudstone beds situated in the upper siliceous interval of the Ak-Kaya section. Although the preservation of the extracted radiolarians is average to poor, it is possible to observe spherical forms devoid of any internal spicule or outer spines, made of a fairy thick shell wall composed of irregularly interwoven threadlike elements. Some of the specimens recall the echidninid genus
The novelty of our results consist in the fact that extensive processing of the trilobite-bearing limestones with dilute acetic acid yielded several tens of fairly well-preserved radiolarian specimens, as well as diverse forms of siliceous sponge spicules and the SSF species
In summary, all radiolarians extracted from both the upper siliceous interval and the trilobite-bearing limestones belong respectively to the echidninid and archeoentactinian lineages of Archaeospicularia. The absence of any palaeospiculids in our material is noteworthy. They are early Botoman in age (early Cambrian Stage 4) and they represent the oldest radiolarians known so far. Our results confirm the hypothesis that the earliest polycystine radiolarians were characterized by spicular skeletons and archaeospicularian type of body plans and that they built their shells by a three-dimensionally interwoven meshwork of spines and bars.
O. Obut’s research was supported by the Ministry of Education and Science of the Russian Federation grant n° 14.Y26.31.0018. T. Danelian thanks the Région Hauts-de-France and the Ministère de l’Enseignement Supérieur et de la Recherche (CPER Climibio), and the European Fund for Regional Economic Development for their financial support.
Most Precambrian sediment-hosted barites are usually formed by diagenetic, hydrothermal exhalation, or methane-seepage processes. However, seafloor barite precipitates and void-filling barite cements in basal Ediacaran cap carbonates have been interpreted as sedimentary and early diagenetic in origin. Barite crystals that precipitated syndepositionally or authigenically from porewaters in direct communication with seawater can potentially offer insights into Neoproterozoic ocean geochemistry in the aftermath of the terminal Cryogenian (or Marinoan) Snowball Earth glaciation.
In this study, we investigated void-filling barite cements in the basal Ediacaran cap dolostone of the Sete Lagoas Formation, Bambuí Group, central Brazil. Our goal was to investigate the origin and diagenetic history of the Sete Lagoas barite by addressing the following questions: (i) whether the barite precipitated from porewaters in direct connection with seawater and hence sulfate was ultimately sourced from seawater, (ii) whether barite cements experienced any subsequent hydrothermal alteration, and (iii) if hydrothermal alteration did occur, what is the nature of the hydrothermal fluids.
Sete Lagoas barite occurs as veins and as major void-filling cement in multiple horizons (
Mesoscopic and microscopic features of a barite layer. A. Void-filling barite in the outcrop, commonly forming rosettes, growing inward from both the top and bottom of the vug. B. Dolomitic matrix (top) and radiating bladed barite crystals (bottom). C. Detail of B showing radiating bladed crystals of barite. Plane polarized light microscopy (PPL). D. Enlarged view of (C), showing trails of intergranular and intragranular secondary (?) fluid inclusions perpendicular to the growth direction of barite crystals. PPL.
Sulfur isotope compositions of coexisting barite and carbonate-associated sulfate or CAS (see location of the isotope analyses in
X-ray CT three-dimensional reconstruction of a Sete Lagoas sample that contains a layer of void-filling barite crystals
(bottom half of the specimen) and a layer of the host rock (top half of the specimen). Barite is segmented as green color, micritic matrix
in the barite layer as pink color, and laminated dolostone in the host rock as blue color. A-B and C-D are two different views of the same
specimen, with A and D showing only the barite component of the specimen. δ34SCAS and δ34Sbarite values are marked on the specimen. The specimen is oriented in stratigraphic up direction.
Carbonaceous material (CM) in barite crystals and CM in fluid inclusions trapped in barite crystals share a similar thermal history, with a peak temperature in the range of 206-257°C. CM was not detected in the host rock, possibly due to pervasive dolomitization, thermodegradation or the fine-grained nature of the host rock. Temperatures and salinity, evaluated by fluid inclusion study, are both higher than seawater. We suggest that hydrothermal activity may have modified the original composition of the Sete Lagoas barite. This activity may be linked to MVT mineralization in the study area. Thus, the Sete Lagoas barite may have been subsequently modified by hydrothermal activities, after its precipitation from seawater or porewaters in communication with seawater. From this perspective, a thorough diagenetic assessment must be done before the Sete Lagoas barite can be used to infer seawater geochemistry and environment in the aftermath of Marinoan Snowball Earth.
This research was sponsored by FAPESP (grant 2015/07391-0 and 2017/19023-1) and NASA Exobiology and Evolutionary Biology Program (80NSSC18K1086).
The timing of origin and early evolution of animals is bound up with whether we interpret the Cambrian Explosion to reflect an explosive diversification of animals or an explosive increase in fossils, relative to the foregoing Neoproterozoic. There can be no doubt that animal phyla diverged prior to their appearance in the stratigraphic record but the same palaeontological and phylogenetic evidence underpins competing views that lie at and between the extremes of crown animals emerging in the deep Neoproterozoic versus limited entirely to the Phanerozoic. However, fossil evidence alone cannot establish a timescale of animal evolution, the only viable methodology for which remains the molecular clock. It is perhaps surprising that most scenarios for the emergence of animal diversity remain calibrated to the fossil record but this likely reflects that lack of confidence which stems from the implied extent of early animal evolution unrepresented by the fossil record – but also their lack of consistency, with some studies differing by as much as hundreds of millions of years. This lack of consistency between molecular clock studies is not surprising since divergence time methods are now diverse and use palaeontological data in different ways. Here we attempt to benchmark molecular clock methods using a common suite of molecular and morphological data, living and fossil taxa, and phylogenetic hypotheses. In comparison to conventional node calibration, we have undertaken a morphological clock analysis including both living and fossil taxa, a combined molecular and morphological Total Evidence Dating (TED) analysis, and an analysis calibrated using the Fossilised Birth-Death (FBD) integrated diversification and fossil sampling model. In particular, it has been anticipated that methods which incorporate more palaeontological data and are guided by the stratigraphic distribution of included fossil species, such as TED and FBD, might result in evolutionary timescales that are in closer agreement with the fossil record. However, in their application to the timing of origin of animals, this expectation is not met. While the results from application of these and other methods differ in their precision, they are agreed that animals diverged long before their appearance in the fossil record.
The Ediacaran Period witnessed the early diversifications of marine eukaryotes after the terminal Cryogenian global glaciation. These stepwise evolutionary events include a rapid radiation of microscopic acritarchs and multicellular algae in the early-mid Ediacaran, the succeeding diversification of macroscopic Ediacaran biota in the mid-late Ediacaran, and the appearance of biomineralized skeletal organisms in the Ediacaran-Cambrian transition interval (Narbonne
But questions concerning when and how the first Ediacaran eukaryotes began to thrive and dominate the marine ecosystem still remain. Acanthomorphic acritarchs have been recovered from around the world, including Ediacaran successions in southern Norway, Svalbard, Siberia, South China, central and southern Australia, Lesser Himalaya, East European Platform, Vindhyan Basin, and northern Mongolia (e.g., Spjeldnaes,
Our palaeontological investigation shows that at the Jiulongwan section in the Yangtze Gorges area, South China, the lowest occurrence horizon of the Ediacaran eukaryotic microfossils is only 2.8 m above the top of cap dolostone (Zhou
These findings are consistent with the results of a very comprehensive palaeontological study of the Doushantuo acanthomorphic acritarchs recently published by Liu & Moczydłowska (
The present available microfossil data indicate that eukaryotic life recovered and flourished soon after the termination of the Marinoan glaciation. In the Yangtze Gorges area, two zircon U-Pb ages of 635.2 ± 0.6 Ma and 632.5 ± 0.5 Ma have been reported for ash beds of 2.3 m and 9.5 m above the base of the Doushantuo Formation, respectively (Condon
It is worth noting that the occurrence of eukaryotic fossils above the glacial deposits is very likely to be controlled by particular taphonomic window. Microfossils from the Doushantuo Formation are all permineralized, and most of them are found from chert nodules or phosphorites. Since acanthomorphic acritarchs were discovered from the lowest chert nodule horizon at the Jiulongwan section, the possibility that the absence of microfossils below this horizon is due to the lack of suitable preservational condition cannot be completely excluded.
The FAD of acanthomorphic acritarchs has already been suggested as a potential criterion for the stage-level subdivision of the Ediacaran System for it represents a significant evolutionary event (e.g., Liu
This study was supported by the Strategic Priority Research Program (B) of Chinese Academy of Sciences (XDB18000000), National Key R & D Program of China (2017YFC0603101), and National Natural Science Foundation of China (41672027).
On the edge of the Neoproterozoic Era, the Earth experienced diverse and consequential changes in the lithosphere, biosphere, atmosphere, and hydrosphere. It is marked by major carbon cycle perturbations, transient oxygenation of the deep ocean, and transformative biological innovations (Erwin
Here we demonstrate the new occurrence of the Cambrian-type skeletal fossil
Neoproterozoic successions in western Shandong of North China contain fine-grained siliciclastic rocks which have the potential to preserve organic-walled microfossils. Unfortunately, their abundance and diversity of organic-walled microfossils have long been overlooked and the exact depositional age of these units has not been well constrained. In this study, we have -systematically investigated organic-walled microfossils, using low-manipulation maceration techniques, from the Tongjiazhuang Formation, Tumen Group in western Shandon, North China. A well-preserved and diversified microfossil assemblage, characterized by abundant smooth-walled sphaeromorphic acritarchs and cyanobacterium-like filamentous forms and relatively low abundance of more complex acritarchs, is obtained from the Tongjiazhuang Formation. A total of 40 microfossil taxa belonging to 20 morphological genera have been identified, most of which are described for the first time from the Tongjiazhuang Formation of the Tumen Group in western Shandong, including
Some of the best outcrops of Iberia to study the Ediacaran-Cambrian boundary are located in the Ossa-Morena Zone. In the Crato-Campo Maior region (SW Iberia, Portugal), this stratigraphic boundary is marked by an angular unconformity (Gonçalves,
This deformation event is previous to the intrusion of c. 526-525 Ma granitic rocks (Barquete and Barreiros plutons; Pereira
Ediacaran-Cambrian stratigraphic section of the Ossa-Morena Zone (Crato-Campo Maior region, Portugal). A- Volcaniclastic conglo-merate is interbedded with sandy tuff beds (Early Cambrian Freixo-Segóvia Volcanic-Sedimentary Com-plex); B- Deformed metapelite (Edia-caran Série Negra Group) is cross-cut by a granitic rock (lower Cam-brian Barreiros pluton).
This study is included in the research activity plan of the Instituto de Ciências da Terra (Universidade de Évora), Lithosphere Dynamics research group, with financial support from the Fundação para a Ciência e Tecnologia (Portugal), and of the project CGL2016-76438-P (Spanish MEIC).
In the Central-Iberian Zone, the earliest deformation event found in the Ediacaran Series (i.e. Schist Greywacke Complex) is not represented in the unconformable overlying Early Ordovician strata. Traditionally there have been distinct views to explain the occurrence of this deformation, whose age is still uncertain. Some authors argue that the earliest deformation was caused by a Late Ediacaran contractional event (i.e. Cadomian; Talavera
In the Cáceres region (Central Extremadura Batholith) the first cleavage observed in the Ediacaran Series has been attributed almost exclusively to Late Paleozoic (i.e. Variscan) deformation events which erased previous structures (Tena-Dávila Ruiz
Recently, it was suggested that the earliest deformation found in the Ediacaran metasedimentary rocks of Cáceres mostly represents the response to an extensional deformation event that conditioned the emplacement at depth of the extensive cortege of coeval Ordovician plutonic bodies (Pereira
Schematic reconstruction of the early-middle Ordovician plutonic-metamorphic belt of the Central Extremadura batholith (Cáceres), resulting in the development of: (A) a migmatitic foliation (Smig) in the Ediacaran Schist Greywacke Complex, which is associated with (B) tonalite layers with Ordovician crystallization age (modified from Pereira
Although both the field relations and the new radiometric ages are indicative of the existence of Ordovician deformation and metamorphism reaching high-grade conditions in the Central-Iberian Zone Ediacaran Series, we must not ultimately exclude the existence of previous structures, leaving some doubts to clarify in the scope of investigations still in progress.
This study is included in the research activity plan of the Instituto de Ciências da Terra (Universidade de Évora), Lithosphere Dynamics research group, with financial support from the Fundação para a Ciência e Tecnologia (Portugal), and of the project PGC2018-096534-B-I00 funded by Ministerio de Ciencia, Innovación y Universidades (Spain).
The nature of the Cambrian explosion has been the subject of heated debate for decades. A popular interpretation poses that the sudden appearance is simply a consequence of the acquisition of hard parts such as jaw elements and thick cuticles during the diversification of bilaterian phyla. However, this may be a perception caused by our clearer understanding of bilaterian morphology and evolution compared to other non-bilaterian animals. A similar pattern of sudden appearance is also observed in the record of sponges; despite the lack of convincing Ediacaran spicules, Early Cambrian microfossil assemblages contain diverse spicule assortments, with even some types that are considered to be diagnostic of extant sponge orders. Furthermore, reports of Ediacaran ctenophores and cnidarians are still received with major skepticism by some authors. Here we explore character state distributions in non-bilaterian animals in an attempt to explain the conflict between molecular clocks and the scant record of early animals. We argue in favour of a taphonomic explanation imposed by early animal bodyplans. Most of the apomorphies accrued along the deepest splits in the animal tree of life are characters with exceptionally low preservation potential, i.e.: cytological and histological innovations. Apomorphies with higher preservation potential do appear early in the fossil record, and support the affiliations of certain controversial fossil taxa. Overall, our results paint a picture of a late Proterozoic world that was probably dominated by small and simple stem members of major metazoan groups: animals that would resemble extant marine invertebrate larvae, placozoans and aspiculate sponges. This study sheds light on the late Proterozoic diversification of animals, and aims to inform future work on exceptional sites such as Doushantuo about what early animal fossils could be like.
The Ossa Morena Zone (OMZ) of SW Iberia records a complex pre-Variscan geodynamic evolution including the following major events: (i) subduction/arc growth (~630-550 Ma); (ii) back-arc opening and spreading (~600-570 Ma); (iii) back-arc closure, polyphase deformation (Cadomian orogeny) and growth of a secondary arc at the N margin of OMZ (~565-535 Ma); and (iv) subduction halt and rapid transition to rifting (~535-530 Ma).
Neoproterozoic subduction/arc development at/near the outer margin of N Gondwana characterizes most Peri-Gondwanan terranes. In the SW Iberia segment no basement exposures occur but an Andean-type context is suggested by Ediacaran mature sandstones and carbonates (Serie Negra succession). Detrital and inherited igneous zircon data link this part of Cadomian Arc to the West African craton. Opening of a back-arc basin is indicated by tholeiitic mafic/ultramafic sequences interpreted as ophiolites, e.g. Calzadilla de los Barros (gabbro protolith dated at ca. 600 Ma). Back-arc basin opening and spreading was probably related to slab-rollback, whereas its inversion may have been connected to either subduction rate increase, a decrease of the subduction angle or both. Inversion was probably accommodated by antithetic subduction of the basin oceanic lithosphere and secondary arc growth (Malcocinado volcanics and associated plutons). Significant sinistral strike-slip components are suggested by juxtaposition of the OMZ to units derived from more easterly N-African areas (Saharan metacraton/Arabian-Nubian shield), currently outcropping north of the OMZ. Formation of a wide foreland basin onto those units, incorporating detritus sourced in the OMZ, and location of the secondary arc rocks close to the northern OMZ boundary support the antithetic polarity of subduction that led to closure of the back-arc basin. Finally, a process of oblique ridge-trench collision is envisaged as responsible for a rapid shift from subduction to rifting (Baja California style), which culminated in opening the Rheic Ocean ca. 495 Ma.
Investigations of the early evolution of sponges are impeded by the dearth of fossil records. Despite of the questionable Ediacaran sponge-like fossils, the earliest convincingh sponge spicules appeared from ca. 535 Ma. However, disarticulated spicules are of little taxonomic value, and almost all the articulated and well preserved sponge fossils were reported after the beginning of Cambrian Age 3, from Burgess-Shale-type preservation. Here we demonstrate three-dimensionally preserved articulated sponge fossils from the lower Cambrian phosphorites in South China (the basal Niutitang Formation) (
Partly articulated hexactinellid sponge embedded in brownish apatite from Fontanarejo (Central Spain). The only spicules are hexactins in different sizes. Main phosphatic sponge layer from the base of the Niutitang formation below the Mo-Ni ore layer in Sancha (China). Detailed preservation of a phosphatized hexactinellid sponge from the main phosphatic sponge layer in Sancha. Spicules are in place and exhibit various sizes, including micro-hexactins.
The Niutitang sponges are pre-served as iso-lated, oval bodies in nodular, carbo-naceous cherty phosphorites (
The taxonomic diversity of the-se fossil com-munities has not yet been fully explored. Two sorts of skeletal frames have been re-cognized in the Niutitang materials: one is composed of hexactines in at least three size-hierarchies; the other is dominated by diactines and few hexactines which show perpendicular arrangement in 3-D space. These fossils are here interpreted as stem group Hexactinellida. Apart from this, some of the disarticulated spicules scattered in the matrix are identical to typical demosponge spicules. Meanwhile,
These observations indicate that the lower Cambrian phosphorites can be a new taphonomic window to study the early evolutionary history of sponges. Further taxonomic and geochemical analyses are required and ongoing to reveal more details.
The Neoproterozoic Era witnessed dramatic environmental changes with global low latitude glaciations in the Cryogenian Period, followed by a significant rise in environmental oxygenation. These events may have set the stage for the evolution of first multicellular organisms in the Ediacaran Period. However, the biogeochemical changes that occurred in post-glacial marine environments need to be constrained in more detail before we can evaluate the implications for the evolution of early life.
To elucidate the environmental conditions following the Marinoan glaciation (ca. 635 Ma) we performed a series of geochemical analyses in carbonate lithologies from the Cryogenian to Edicaran Witvlei Group, Namibia (Prave
This study presents data obtained in post-Marinoan cap dolostones, low Mg dolostones and limestones hosting abundant
The Cadomian basement in central and southern Europe is composed by Late Ediacaran-Cambrian rock complexes that appear dismembered as part of different geotectonic zones along the Variscan Orogen, formed in Devonian and Carboniferous times during the main stages of the Pangea assembly. The stratigraphic record of these ancient series contains geochemical and isotopic clues about the origin and evolution of different basins along the Gondwana margin, whose current location does not correspond in some cases with their original position on the African paleo-margin. The southern branch of the Variscan Orogen, the Iberian Massif, contains an excellent stratigraphic record of Ediacaran and Paleozoic sedimentary series in addition to several sutures generated during the long convergence and collision between Gondwana and Laurussia. The nature of the boundary between an autochthonous domain (Central Iberian Zone) and the allochthonous domains (Ossa-Morena Complex) in the SW of Iberian Massif, is under discussion. Previous interpretations, based on U-Pb geochronology of detrital zircons, have suggested that the Ediacaran and Early Paleozoic sedimentary series from the Central Iberian Zone were deposited in more eastern locations along the Gondwana margin than the equivalent series of the Ossa-Morena Complex. However, whole rock geochemistry and Sm-Nd isotope sources of the contemporary siliciclastic series of the Central Iberian Zone and the Ossa-Morena Complex have not yet been compared, bearing in mind they allow a reliable time-resolved comparison between different crustal sectors. The present research compares the whole rock geochemical data and Sm-Nd isotopic tracers of two analogous Ediacaran siliciclastic series (c. 565 Ma), located on both sides of the Central Iberian Zone – Ossa-Morena Complex boundary. The comparison is established for very close domains of both regions: the southernmost sector of Central Iberian Zone, including only the Lower Unit of the so-called Schist and Greywacke Complex (Lower Alcudian), and the Serie Negra Unit (Black Series) of the northernmost part of the Ossa-Morena Complex, designated as the Obejo-Valsequillo Domain.
The Serie Negra Unit has traditionally been divided into two formations, from bottom to top: the Montemolín Formation, whose maximum depositional age has been estimated at c. 590 Ma, and the Tentudía Formation (c. 565-541 Ma). With the exception of the Mérida Massif, the exposed section of the Serie Negra within the Obejo-Valsequillo Domain belongs to the upper part of the Tentudía Formation, and is composed by metasandstones, volcanogenic metagreywackes, slates and phyllites, black quartzites, metacherts and layers of micaschists and limestones. Major and trace geochemical data of the Serie Negra and Lower Alcudian siliciclastic rocks (mainly metagreywackes) are consistent with an immature character of the samples and a limited alteration in the source area. All the study samples follow similar Post Archean Australian Shale and Upper Continental Crustal patterns, suggesting an igneous provenance. Likewise, their geochemical features suggest an active margin setting as the most probable context for the deposition of both Ediacaran sedimentary series. Nevertheless, the Sm-Nd composition of the Serie Negra shows a range of higher negative εNd(0) and εNd(565) values (-9.8 to -20.5 and -4.0 to -13.9, respectively), relative to the Lower Alcudian values (-6.4 to -8.8 and -1.4 to -3.0, respectively). TDM model ages of the Serie Negra range from Mesoproterozoic to Paleoproterozoic (1421-2040 Ma) and are significantly older than those of the Lower Alcudian series (1256-1334 Ma). These isotopic results would situate the source areas of the Serie Negra in the periphery of a cratonic region, likely in the realm of the West African Craton. This old cratonic isotope sources would be mixed with other younger sources, probably derived from the nearby Ediacaran magmatic arc. However, the different Nd model ages of the Lower Alcudian indicate source areas isotopically younger than those for the Serie Negra, which would suggest deposition at distant eastern sections along the Gondwana margin.
According to these data, the contrasting Sm-Nd isotopic sources make it hard to sustain that the siliciclastic rocks of the Lower Alcudian and Serie Negra were deposited close to one another. The current boundary between the Central Iberian Zone and Ossa-Morena Complex (Obejo-Valsequillo Domain) may represent a zone with significant tectonic transport. A major Variscan thrusting developed in a dextral convergence setting can achieve the juxtaposition of distant terranes, as required for the Ediacaran series along this boundary. A similar approach could be likely investigated in other areas with similar geology along the Variscan Orogen, where there are available data to test the existence of major tectonic boundaries in the absence of clear structural and petrological indicators.
For almost 150 years, megascopic structures in siliciclastic sequences of terminal Precambrian age have been frustratingly difficult to characterize and classify. As with all other areas of human knowledge, progress with exploration, documentation and understanding is growing at a superexponential rate. Nevertheless, there is much to learn from following the evolution of the logic behind the biological interpretations of these enigmatic fossils. In this presentation I review five long-established taxa that represent core components of widely recognized clades that are still difficult to graft onto the tree of life. These “orphan plesions” occupy roles that were once dominated by some famous former Problematica such as archaeocyaths, graptolites and rudist bivalves. In some of those cases, taxonomic enlightenment was brought about the discovery of new characters; in others it required a better knowledge of their living counterparts. Can we use these approaches to rescue the orphans? The five taxa that will be examined in this way are
The distribution of the Ediacaran acritarchs in South China is largely controlled by preservation factors and restricted to the presence of phosphorites and early diagenetic chert nodules, and it shows significant palaeogeographic heterogeneity (Jiang
The microfossils in the Liujing section represent taxonomically identified 25 genera, 44 species, and one undetermined species. The assemblage is dominated by
As the representative species of
The dramatic changes in the fossil record across the Ediacaran–Cambrian transition seemingly capture the initial diversification of bilaterian animals. This radiation is principally recognised by the signal from mineralized skeletal fossils (Budd & Jensen,
A series of new SCF biotas identified from Ediacaran–Cambrian successions on Baltica (Slater
Metazoan SCFs from the early Cambrian of Baltica. A-B. Simple spinose elements; C–D. Priapulid-derived cuticular sclerites; E. Palaeoscolecid plate; F. Sclerite sourced from a Assortment of non-animal SCFs from the earliest Cambrian of Baltica. A. Large acritarch (
The Paracuellos Group is the oldest sedimentary sequence of the Iberian Chains in north-east Spain. The group is subdivided into three formations which are, in stratigraphic order, the Sestrica Formation, known only from drill cores, the Saviñan Formation and the Aluenda Formation (
The Paracuellos Group is exposed in two geographically separated areas where it forms the core of faulted anticlines, the Paracuellos antiform and the Codos antiform in the in the northwestern and central Eastern Iberian Chains, respectively (
The putative
Re-investigation of crack-out material of the Codos Bed, however, has revealed the presence of also molluscan taxa. Poorly preserved composite(?) molds of two helcionellid taxa were recovered, which tentatively have been assigned to
In conclusion, the newly discovered fossils from the Codos Bed confirm that the Paracuellos Group is not in its entirety Neoproterozoic in age but that it is in part early Cambrian. The Ediacaran-Cambrian boundary is stratigraphically below the Aluenda Formation, i.e. within the 300 to 400 m thick sequence of the two underlying formations. Paleontological re-investigation of the sandstones and shales of especially the Saviñán Formation, as well as re-evaluation of the stratigraphic significance of
The evolution of infaunal animals dramatically modified Earth’s surface environments. Bioturbating (sediment-mixing) infaunal animals are preeminent ecosystem engineers that, by physically and chemically reworking seafloor sediment, shape nutrient cycling and the structure of benthic communities on both local and ocean-wide scales. Given their substantial environmental and ecological impact in modern marine settings, constraining the timing of the emergence and radiation of infauna is an essential step in reconstructing the coevolution of ancient life and environments.
The earliest sediment-displacement structures (i.e., trace fossils) created by bilaterian animals, simple curvilinear or meandering furrows, first appear in upper Ediacaran strata and record the advent of the infaunal lifestyle (e.g., Jensen
We present a new trace fossil assemblage, recording complex, Cambrian-style infaunalization, from the upper Ediacaran Dunfee Member of the Deep Spring Formation at Mount Dunfee, Nevada, more than 500 m below the local Ediacaran-Cambrian boundary, as previously defined at the first occurrence of
The Dunfee trace fossils are unique, among Ediacaran assemblages, in their combination of morphological complexity and stratigraphic distance from the Ediacaran-Cambrian boundary. Statistical sedimentological and chemostratigraphic data indicate that the Dunfee assemblage is one of the oldest documented instances of sediment-penetrative infaunalization, likely predating the Ediacaran-Cambrian boundary by at least a million years. Moreover, these Dunfee Member trace fossils occur along the same horizons as classic upper Ediacaran tubular fossils such as
The Ediacaran-Cambrian transition marks one of the most critical geobiological revolutions in Earth history. It is during this period we see not only the first complex macroscopic metazoans in the fossil record, but Earth’s potential first mass extinction as well. The “biotic replacement” hypothesis posits the Ediacara biota were driven to extinction by the evolution of the Cambrian fauna, possibly due to the advent of Cambrian-type ecosystem engineering behaviors such as burrowing. Preliminary micro-CT scans of material from the terminal Ediacaran Spitskopf Formation (Nama Group, southern Namibia) suggest the presence of comparatively large bilaterian burrows, some of which indicate vertical movement. Using CT data and subsequent segmentation, as well as polished thin sections, we identify several unique forms, interpret them in the context of diversity and complexity of bilaterian behaviors, and discuss their potential relevance to broader Ediacaran-Cambrian bioevents.
The Bodoquena-Pantanal Geopark exists since 2009, having a total extension of approximately 40 thousand square kilometers. In this area there are several geosites of different interests, highlighting, in particular, records of support scientific studies regarding the interpretation of the tectono-environmental evolution of the Paraguay Belt (Walde
The region includes an important but fragile ecosystem of broad interest: the swampy Pantanal of Mato Grosso do Sul (
Typical scenery of the Pantanal region (Mato Grosso do Sul). B. Fe-ore mining areas near Corumbá (Santa Cruz Formation, Jacadigo Group).
Potential threats to biodiversity, landscape and cultural heritage in the region of the Geopark (Rolim & Theodorovicz,
The Pyrenean massif (French-Spanish geographical border) mostly resulted from the convergence between the Iberian and European plates during the Alpine orogenesis. It is structurally subdivided into three zones (the North, Axial and South Pyrenean zones) separated by major W-E-trending thrusts. Most of the Ediacaran-Cambrian successions of the Pyrenees crop out in the Axial Zone, which belongs to the Pyrenean Domain of the Variscan Ibero-Armorican Arc. The Axial Zone has undergone a complex geodynamic evolution which led to fossil depleted, Cambrian metasedimentary successions. As a consequence, the relationships of the Cambrian Pyrenean Domain with neighbouring areas of the northwestern Gondwana margin remain mostly based on: (i) lithostratigraphic correlations, (ii) detrital source analysis through U-Pb laser ablation ICP-MS zircon dating of sedimentary and volcano-sedimentary intervals, and (iii) signature and distribution of magmatic episodes. The Salud/Salut Unit (Gerona Province, Spain), a small inlier of pre-Variscan rocks from the South Pyrenean Zone, makes an exception to this general scheme. In this unit, a ‘lower Cambrian’ succession in the vicinity of the Terrades village has provided the only significant and indisputable Cambrian fossil record of the Pyrenean Chain. This succession mostly consists of shales alternating with reefal and perireefal limestones from which archaeocyaths have been intensively studied, whereas associated skeletonized microfossils remain vaguely mentioned. This study was therefore aimed at describing, for the first time, the small shelly fossil (SSF) assemblage of the Terrades outcrops in order to document the palaeobiogeographic affinities of the Cambrian Pyrenean Domain with surrounding zones of the peri-Gondwanan margin. The studied limestones yielded bradoriids, brachiopods, molluscs, tommotiids (
Representatively in Laurentia, the Precambrian-Cambrian boundary has been extensively recognized between the Precambrian basement rock and Cambrian sedimentary cover, which was known as the Great Unconformity and causally linked to the environmental trigger for the Cambrian explosion. A similar unconformity has long been known in the North China Craton (NCC), but its exact stratigraphic location has not been constrained in many successions. Especially reflected in the Huaibei region of the southeastern margin of the NCC, this unconformity has been variously placed beneath or above the Gouhou Formation, which has been interpreted as Tonian, Cryogenian to Ediacaran, or Cambrian in age. To resolve this issue, we carried out an integrated sedimentary petrological, biostratigraphic and detrital zircon geochronological investigation on the Precambrian-Cambrian boundary transitions including the Gouhou Formation and the overlying Houjiashan Formation in the Huaibei region. We confirmed a conglomerate bed within the Gouhou Formation, and the potential Tonian index fossils are preserved in the Lower Member below the conglomerate bed, while all the possible trace fossils, bioturbation-caused mottled structures and the early Cambrian detrital zircons are yielded from the Middle-Upper members above the conglomerate bed. Our data suggest that the Lower Member of the Gouhou Formation is Tonian in age whereas the Middle-Upper members are early Cambrian in age, and the Precambrian-Cambrian boundary lies within the Gouhou Formation and represents a ca. 200-300 Myr depositional gap equivalent to the Great Unconformity. As a result, we propose that the Gouhou Formation be redefined to contain only the lower member, and the middle-ppper members be included in the Houjiashan Formation. Our analysis also confirms that, according to the similar basis in the Huaibei region, the Great Unconformity can be also widely recognized in the Huainan region, southwestern Shandong, eastern Liaoning and southern Jilin provinces along the southeastern margin of the NCC. Additionally, the age distributions of detrital zircons from the Precambrian–Cambrian boundary transitions in Huaibei region provide critical insights into changes of sediment provenance related to the break-up of Rodinia and the final assembly of Gondwana. The recognition of the Great Unconformity in the Huaibei region helps to clarify the tectonic history, basin development, stratigraphic correlation, and paleogeographic reconstruction of the southeastern margin of the NCC during the Tonian-Cambrian periods.
The Huanglian section, located in the west of Huanglian village, Songtao County, Guizhou Province was the basic section for establishing the Nangaoan Stage and the Duyunian Stage (Peng
The trilobites from the Balang Formation of the Huanglian section are very abundant except the lower part, from the base boundary to 38 m. Two trilobite zones are defined according to the collected data: (i) the
Correlation chart of the trilobite zonation from Duyunian Stages, Qiandong Series (Cambrian Series 2, Stage 4) in Songtao County and Jianhe County. A.
The trilobite biostratigraphy of the Balang Formation at the Huanglian section is exactly the same than that of the Balang Formation at the Jiaobang section, which is the potential Global standard stratotype-section and point (GSSP) for the base of the Cambrian Stage 4 (Zhao
This study is supported by the Ministry of Science and Technology of China (2015FY310100), and the National Natural Science Foundation of China (No. 41662001 to Wu and Zhao; No. 41772021 to Yang and Zhao).
Rising oxygen levels are often regarded as an environmental factor facilitating greater biodiversity. Here we consider the possibility that overall low but permissible oxygen levels, and hence dynamic and heterogeneous oceanic redox conditions, may have been a driving force for the evolution of bilaterian animal mobility in the Ediacaran Period.
Phylogenetic relationships among animals indicate that the earliest animals were not actively mobile and the origin of actively mobile bilaterians post-dated the origin of animals . The fossil record shows that the earliest mobile bilaterians appeared in the late Ediacaran Period (ca. 560-540 Ma) , with an apparent increase in ichnofossil diversity and animal mobility in the terminal Ediacaran Period (ca. 550-540 Ma) when many Ediacara-type macro-organisms went extinct . Available geochemical data indicate that the late Ediacaran Period is characterized by a major expansion of marine anoxia relative to preceding and following periods, thus resulting in temporally dynamic and spatially heterogeneous redox conditions in shallow shelf environments where animals lived. Are these environmental and evolutionary events isolated coincidences or causally related? Previous studies were focused on the possibility that environmental stress such as an expansion of oceanic anoxia may have caused extinctions . But is it possible that, while anoxia may have driven some organisms to extinction, it may have also stimulated evolutionary innovations among others?
The observation that Ediacaran trace fossils tend to be closely associated with (presumably cyanobacterial) microbial mats offers insights into the possible relationship between animal mobility and redox conditions. Such an association could be taken as evidence for an early animal behavior to explore localized oxygen oases within cyanobacterial mats in an ocean characterized by relatively low levels of dissolved O2. Because of the dynamic redox conditions in the water column and in the cyanobacterial mat (due to diurnal photosynthesis cycles), animals exploring these oxygen oases must also be able to respond to such dynamic changes by maneuvering in and out the cyanobacterial mats. We hypothesize that such localized and dynamic O2 oases were an environmental stimulus that drove the evolution of mobility among early benthic bilaterian animals.
Ediacaran-Cambrian tubular fossils have been reported from numerous paleocontinents. The cloudinids, particularly, are of significant evolutionary importance because they contain the oldest recognized biomineralizing metazoans. Here we report a new representative of the cloudinids from the Ediacaran strata of western Mongolia. These phosphatic fossils have collars similar to those in
The “Tsinghsutung Formation” is an important lithostratigraphic unit, fossiliferous and bearing a potential significance to define the base of Cambrian stage 4. It is well exposed in Balang village, Jianhe County, Guizhou Province, and represents a deposit formed in transition slope belt between the Yangtze platform and the Jiangnan basin, and the overlying Balang Formation, and is overlain by the Kaili Formation, and consisting of thin- to medium-bedded limestone with silty mudstone, as well as fine-grained dolomite in the uppermost part. The microfossils after acetic acid treatment were mainly collected from the dark grey medium-bedded argillaceous limestone in the middle to upper part of “Tsinghsutung Formation”. Microfossil brachiopods in the “Tsinghsutung Formation” are abundant not only quantitatively but also in diversity: 4 genera and 4 species constitute 50 % of the whole microfossil content. The tubular skeletal microfossils also are very common, belonging to hyoliths and arthropods, respectively. Three types of sponge spicules have been found, among which hexactines were dominant. But beyond that, some fragments of trilobites, sponge spicules, echinoderms and algae have also been found during the thin section. This microfossils assemblage is different from the macrofossil assemblage of the “Tsinghsutung Formation” of Jianhe dominated by arthropods. Compared with the microfossil assemblage of coeval strata, the fossil associations from the “Tsinghsutung Formation” share many similarities with the Harkless Formation in southern Nevada because of its low biodiversity, while the diversity of the brachiopod is relatively higher, and most of them have
As the simplest animals on the planet, sponges have been widely accepted as a monophyletic clade and among the earliest branching lineages of the animal tree. Therefore, the origin and early evolution of sponges are critical for understanding the rise of animals. Molecular clocks estimate that the initial rise of sponges may occur in the Tonian or Cryogenian periods (850-650 Ma), and majority sponge bodyplans or clades probably diversified during Ediacaran. However, the earliest unambiguous sponges did not appear in fossil records until the early Cambrian, about 535 Ma. Even though putative sponge biomarkers have been recovered from Cryogenian and Ediacaran deposits, these molecular fossils have been questioned. Possible sponge grade body fossils have also been reported from Cryogenian and Ediacaran periods, but none of them could be assigned into crown sponges with confidence. Here we present microscopic sponge gemmule-like fossils from the early Ediacaran Weng’an Biota (609 Ma), Guizhou Province, southwest China. These three-dimensionally phosphatized fossils developed a thick, ornamented envelope with a circular micropyle-like opening, and a membrane-bounded multicellular inner body. They show striking similarities to extant sponge gemmules not only in size and morphology, but also in anatomy and histology. This discovery pushes back the oldest fossil record of sponges by at least 77 million years, which is consistent with the molecular clock estimates. Furthermore, in extending the fossil record of crown sponges, this finding also suggests that the earliest stem metazoans and the last common ancestor of metazoans probably arise before the early Ediacaran.
Previously, early-mid Cambrian organic-walled microfossils were mainly studied and reported from the East European Platform, the Baltoscandian Platform, the southwestern Europe and North America. Many diagnostic morphological forms of organic-walled microfossils had been described. Their biostratigraphic distribution is not only restricted in local region and even known from elsewhere of the world.
The lower Cambrian stratigraphic sequences in the Jiaobang section located at a mountain ridge of eastern Jiaobang village, Jianhe County, eastern Guizhou, South China, include, in ascending order, the Bianmachong, Balang and Tsinghsutung formations with a total of 645m in thickness in the section. 31 morphological genera (including one new genus -
Geographical locality of the study section, and Jiaobang section with lithostratigraphy and relative trilobite biozones.
In addition, at the boundary between the Bianmachong Formation and the overlying Balang Formation, the obvious change of organic-walled microfossil assemblages is also noted, but the gap more far about 24 m in distance from the bottom of the Balang Formation.
The trace fossil record implies that large worm-like animals were in place along with the skeletonizing organisms during the initial stage of the Cambrian explosion. Body fossils of large worms, however, have so far not been found. The large, soft-bodied, worm-like organism,
The middle Ediacaran Shuram excursion is the most pronounced negative carbon isotopic shift in Earth history. Potential equivalents of the Shuram excursion have been reported in many places around the world (Grotzinger
The Shuram excursion generally shows a simple stratigraphic pattern with a sharp shift to negative δ13C values followed by a slow recovery to positive values (Grotzinger
The ~580 Ma Gaskiers glaciation has been used as a palaeoclimatic marker to subdivide the Ediacaran System at a series level (Narbonne
A recent subsidence model analysis of the Ediacaran Johnnie Formation indicated that the Shuram excursion occurred 585-579 Ma and its termination was coincident with the ~580 Ma Gaskiers glaciation (Witkosky & Wernicke,