Ediacaran macropaleontology

The Cambrian Explosion is arguably the most extreme example of a biological radiation preserved in the fossil record, and studies of Cambrian Lagerstätten have facilitated the exploration of many facets of this key evolutionary event. As predation was a major ecological driver behind the Explosion-particularly the radiation of biomineralising metazoans-the evidence for shell crushing (durophagy), drilling and puncturing predation in the Cambrian (and possibly the Ediacaran) is considered. Examples of durophagous predation on biomineralised taxa other than trilobites are apparently rare, reflecting predator preference, taphonomic and sampling biases, or simply lack of documentation. The oldest known example of durophagy is shell damage on the problematic taxon Mobergella holsti from the early Cambrian (possibly Terreneuvian) of Sweden. Using functional morphology to identify (or perhaps misidentify) durophagous predators is discussed, with emphasis on the toolkit used by Cambrian arthropods, specifically the radiodontan oral cone and the frontal and gnathobasic appendages of various taxa. Records of drill holes and possible puncture holes in Cambrian shells are mostly on brachiopods, but the lack of prey diversity may represent either a true biological signal or a result of various biases. The oldest drilled Cambrian shells occur in a variety of Terreneuvian-aged taxa, but specimens of the ubiquitous Ediacaran shelly fossil Cloudina also show putative drilling traces. Knowledge on Cambrian shell drillers is sorely lacking and there is little evidence or consensus concerning the taxonomic groups that made the holes, which often leads to the suggestion of an unknown 'soft bodied driller'. Useful methodologies for deciphering the identities and capabilities of shell drillers are outlined. Evidence for puncture holes in Cambrian shelly taxa is rare. Such holes are more jagged than drill holes and possibly made by a Cambrian 'puncher'. The Cambrian arthropod Yohoia may have used its frontal appendages in a jack-knifing manner, similar to Recent stomatopod crustaceans, to strike and puncture shells rapidly. Finally, Cambrian durophagous and shell-drilling predation is considered in the context of escalation-an evolutionary process that, amongst other scenarios, involves predators (and other 'enemies') as the predominant agents of natural selection. The rapid increase in diversity and abundance of biomineralised shells during the early Cambrian is often attributed to escalation: enemies placed selective pressure on prey, forcing phenotypic responses in prey and, by extension, in predator groups over time. Unfortunately, few case studies illustrate long-term patterns in shelly fossil morphologies that may reflect the influence of predation throughout the Cambrian. More studies on phenotypic change in hard-shelled lineages are needed to convincingly illustrate escalation and the responses of prey during the Cambrian.

La biota ediacárica representa uno de los mayores hitos en la historia evolutiva del planeta, pues contiene algunas de las primeras formas de vida animal pluricelular macroscópica bien conocida en el registro fósil y que presenta gran diversidad morfológica e interacciones ecológicas complejas, además de poseer integrantes microscópicos de gran relevancia evolutiva como los posibles primeros animales triploblásticos. Su posición estratigráfica corresponde con el final del Neoproterozoico, tiempo de aparición en el registro fósil de algunos de los primeros animales eumetazoos. La mayoría de los planes corporales de los organismos macroscópicos del Ediacárico no tienen equivalentes modernos e incluyen simetrías triradiales, octoradiales y fractales. La biota ediacárica vio su final al evolucionar los primeros organismos macrodepredadores con exoesqueletos que luego dominarían los ecosistemas del Fanerozoico.

Bedding-plane assemblages of Ediacaran fossils from Mistaken Point, Newfoundland, are among the oldest known records of complex multicellular life on Earth (dated to ~565 Ma). The in situ preservation of these sessile but otherwise deeply enigmatic organisms means that statistical analyses of specimen positions can be used to illuminate their underlying ecological dynamics, including the interactions between taxa. Fossil assemblages on Mistaken Point D and E surfaces were mapped to millimeter accuracy using differentiated GPS. Spatial correlations between 10 well-defined taxa (Bradgatia Thectardis) were identified using Bayesian network inference (BNI), and then described and analyzed using spatial point-process analysis. BNI found that the E-surface community had a complex web of interactions and associations between taxa, with all but one taxon (Thectardis) interacting with at least one other. The unique spatial distribution of Thectardis supports previous, morphology-based arguments for its fundamentally distinct nature. BNI revealed that the D-surface community showed no interspecific interactions or associations, a pattern consistent with a homogeneous environment. On the E surface, all six of the abundant taxonomic groups (Fractofusus, Bradgatia, Charniid, Charniodiscus, Thectardis, and Plumeropriscum) were found to have a unique set of interactions with other taxa, reflecting a broad range of underlying ecological responses. Four instances of habitat associations were detected between taxa, of which two (Charniodiscus–Plumeropriscum and Plumeropriscum– Fractofusus) led to weak competition for resources. One case of preemptive competition between Charniid and Lobate Discs was detected. There were no instances of interspecific facilitation. Ivesheadiomorph interactions mirror those of Fractofusus and Charniodiscus, identifying them as a form-taxonomic grouping of degradationally homogenized taphomorphs. The absence of increased fossil abundance in proximity to these taphomorphs argues against scavenging or saprophytic behaviors dominating the E-surface community.

Ediacara-type fossils are found in a diverse array of preservational styles, implying that multiple taphonomic mechanisms might have been responsible for their preservational expression. For many Ediacara fossils, the ‘‘death mask’’ model has been invoked as the primary taphonomic pathway. The key to this preservational regime is the replication or sealing of sediments around the degrading organisms by microbially induced precipitation of authigenic pyrite, leading
toward fossil preservation along bedding planes. Nama-style preservation, on the other hand, captures Ediacaran organisms as molds and three-dimensional casts within coarse-grained mass flow beds, and has been previously regarded as showing little or no evidence of a microbial preservational influence. To further understand these two seemingly distinct taphonomic pathways, we investigated the three-dimensionally preserved Ediacaran fossil Pteridinium simplex from mass flow deposits of the upper Kliphoek Member, Dabis Formation, Kuibis Subgroup, southern Namibia. Our analysis, using a combination of petrographic and micro-analytical methods, shows that Pteridinium simplex vanes are replicated with minor pyrite, but are most often represented by open voids that can be filled with secondary carbonate material; clay minerals are also found in association with the vanes, but their origin remains unresolved. The scarcity of pyrite and the development of voids are likely related to oxidative weathering and it is possible that microbial activities and authigenic pyrite may have contributed to the preservation of Pteridinium simplex; however, any microbes growing on P. simplex
vanes within mass flow deposits were unlikely to have formed thick mats as envisioned in the death mask model. Differential weathering of replicating minerals and precipitation of secondary minerals greatly facilitate fossil collection and morphological characterization by allowing Pteridinium simplex vanes to be parted from the massive hosting sandstone.

Ediacara fossils often exhibit enigmatic taphonomy that complicates morphological characterization and ecological and phylogenetic interpretation; such is the case with Pteridinium simplex from the late Ediacaran Aar Member in southern Namibia. P. simplex is often preserved as three-dimensional (3D) casts and molds in coarse-grained quartzites, making detailed morphological characterization difficult. In addition, P. simplex is often transported, distorted, and embedded in gutter fills or channel deposits, further obscuring its morphologies. By utilizing microfocus X-ray computed tomography (microCT) techniques, we are able to trace individual specimens and their vanes in order to digitally restore the 3D morphology of this enigmatic fossil. Our analysis shows that P. simplex has a very flexible integument that can be bent, folded, twisted, stretched, and torn, indicating a certain degree of elasticity. In the analyzed specimens, we find no evidence for vane identity change or penetrative growth that were previously used as evidence to support a fully endobenthic lifestyle of P. simplex; instead, evidence is consistent with the traditional interpretation of a semi-endobenthic or epibenthic lifestyle. This interpretation needs to be further tested through microCT analysis of P. simplex specimens preserved in situ rather than transported ones. The elastic integument of P. simplex is inconsistent with a phylogenetic affinity with xenophyophore protists; instead, its physical property is consistent with the presence of collagen, chitin, and cellulose, an inference that would provide constraints on the phylogenetic affinity of P. simplex.

Diverse interpretations of Ediacaran organisms arise not only from their enigmatic body plans, but also from confusion surrounding the sedimentary environments they inhabited and the processes responsible for their preservation. Excavation of Ediacaran bedding surfaces of the Rawnsley Quartzite in South Australia has provided the opportunity to study the community structure of the Ediacara biota, as well as the autecology of individual organisms. Analysis of two bedding surfaces preserving large numbers of Parvancorina illustrates that individuals display a preferred, unidirectional orientation aligned with current, as indicated by the identified current proxies: tool marks, overfolded edges of Dickinsonia, felled fronds and drag structures generated by uprooted frond holdfasts. Taphonomic and morphological evidence suggests that the preferred orientations of Parvancorina individuals are not the result of passive current alignment, but represent a rheotactic response at some stage during their life cycle. These results illustrate a previously unrecognized life mode for an Ediacaran organism and arguably the oldest known example of rheotaxis in the fossil record. The morphology and previously suggested phylogenetic affinities of Parvancorina are also re-evaluated. Apart from possessing a bilaterally symmetrical body, there are no unequivocal morphological characters to support placement of Parvancorina within the Euarthropoda or even the Bilateria. Body fossil evidence that shows the ability of Ediacaran organisms to physically respond to external stimuli or move within their environment is generally rare 1. The only broadly accepted evidence of motility and associated behaviours in the Ediacara biota is represented by trace fossils from Canada, China, Namibia, Russia, and South Australia 2-6. Previous attempts to compare Ediacara disc-shaped fossils (e.g., Aspidella) with free-swimming medusoids have been discounted in light of evidence that these were either attachment discs for frond-like organisms 7 or a motile, benthic animal of cnidarian grade 8. Serial imprints left by flat organisms such as Dickinsonia and Yorgia have been previously interpreted as multiple decayed organisms or touchdown impressions made by current-shuffled individuals, but they are likely evidence of periodic creeping to feed via adsorption of mat nutrients 9-13. This last interpretation is based on examples that show an external mould of a body fossil at the end of a serial set of faint, similar-sized casts, aligned such that the presumed anterior end of the organism is most distant from the chain of body imprints. There is also evidence of grazing and locomotory traces associated with body fossils of Kimberella 14,15 , and even signs of possible tactophobic behaviour in Dickinsonia 11. Here we report new information on the problematic Ediacaran organism Parvancorina minchami from two bed assemblages of the Rawnsley Quartzite in South Australia, suggesting that this taxon was capable of performing rheotaxis-oriented movement or positioning in response to a water current-during at least part of its life cycle.

Farm Aar in Namibia contains a sequence of rocks from the late Ediacaran
Nama Group. This site has produced a large number of fossils of Ernietta plateauensis and Pteridinium simplex. These two forms are incompletely characterised in the literature. Pteridinium was an elongate, three-vaned organism. It had a rubbery, semi-rigid consistency evidenced by the fact that it will bend, but does not fold or tear except at distinct points of weakness. It is tri-symmetric around the central axis. There is a distinctive offset symmetry along this axis where the vanes meet. The vanes are relatively thin and made up of parallel tubes, which are arranged in double-layers. This double-layer of tubes is also found in Ernietta. Ernietta was a sac-shaped organism with the same offset symmetry along a suture at the base of the sac, where sets of parallel tubes meet each other. Both of these organisms are found in Nama-style preservation – a taphonomy in which soft-bodied organisms are preserved three-dimensionally in sands and muds. Pteridinium is preserved within event beds, above scours where massive sand has been deposited and within waning (but still upper flow regime) flows depositing sand laminae. Ernietta is also preserved in transported deposits, but can be found on Aar in original life position, where fossils have been buried more gradually by mud. It is likely that the three-dimensional form of these organisms was preserved by the early lithification of surrounding sediment as a result of the precipitation of pyrite in association with bacterial metabolism. Both forms are also associated with membrane-like and bag-like structures. Farm Aar also bears domal structures – sedimentary structures that may represent a new form of biologically influenced soft-sediment deformation. The dual-layer of tubes making up the body walls of Ernietta and Pteridinium represents a synapomorphy unique to the Erniettomorpha. These two forms are the first-named members of this clade, which also includes the Namibian form Swartpuntia germsi and the South Australian form Phyllozoon hanseni. Swartpuntia represents a highly derived, late-appearing erniettomorph, while Phyllozoon may or may not represent a trace fossil form of Pteridinium. In either case, it is interesting in being preserved in a contrasting taphonomic setting – 'Flinders-style' preservation – to most other known erniettomorphs. The closest relatives to the erniettomorphs within the Ediacara biota are likely to be the rangeomorphs and dickinsoniomorphs. The key differences between the erniettomorphs and dickinsoniomorphs centre on the dual-layer of tubes, and the semi-rigid construction of erniettomorphs, which contrasts with the tendency of dickinsoniomorphs to fold and to be found draping small-scale sea-bed topography. Erniettomorphs were most likely osmotrophic organisms. Their anatomy results in body tissue being mostly structural, which may have placed a phyletic constraint against the development of surface area/volume ratios comparable to modern osmotrophs. They were second only to the rangeomorphs in terms of diversity within the Ediacara biota, producing global cosmopolitan taxa such as Pteridinium and novel body-plans such as Ernietta and Swartpuntia right to the end of the Ediacaran.

During the Ediacaran the Clymene Ocean separated the Laurentia,
Amazonia, and Río Apa cratons from several landmasses to the west forming the proto-Gondwana supercontinent. However, no clear evidence about the existence of Ediacaran epeiric seas over
those landmasses has been found. Here we report and discuss the
discovery of the Ediacaran guide fossil Cloudina sp. associated with other metazoan body and trace fossils in the Bambuí Group (central eastern Brazil). The Ediacaran age of the Bambuí Group and the paleogeographic position of Cloudina-bearing successions in Brazil, Antarctica, Namibia, and Argentina suggest a scenario of ocean connectivity among coeval intracratonic basins of South America, Africa, and Antarctica at the end of Neoproterozoic time. The new finding epitomizes one of the most important paleontological discoveries ever made in South America, helping to solve an old paleogeographic puzzle
of the Gondwana supercontinent.

The early evolution of metazoans has been reconstructed by studies on exceptionally preserved molds in siliciclastic rocks from the Ediacaran Period. However, there remains considerable controversy regarding the formation mechanisms of this unusual 'Ediacaran-style' preservation. Proposed hypotheses usually include early authigenesis of minerals, but evidence for this is scarce. In a recently discovered deposit of Ediacaran biota in Brazil, we show that the classic moldic preservation is related to clay mineral authigenesis. Specifically, these clays originated from the alteration of original pyroclastic sediments, likely enhanced by microbial activity, leading to early illitization and morphological templating of the fossiliferous surfaces at a micrometric scale. Such high-fidelity preservation was made possible by rapid burial during volcanic events and the in-situ templating of tissue by clays via microbially-mediated mineralization. This newly described Lagerstätte demonstrates that a number of minerals can facilitate preservation, and that perhaps 'Ediacaran-style' preservation result from different processes leading to the same broad style of preservation. The typical moldic preservation of soft-bodied organisms in the Ediacaran Period (635-539 Ma) 1-4 , in particular the positive epirelief type where the fossils protrude up from bedding surfaces 4 , has few parallels in other periods of geologic time, hence the term "Ediacaran-style preservation. " One explanation points to rheological differences amongst the sediments 5 , a hypothesis that has received recent support 4. Gehling 1 also proposed the so-called 'death-mask' model for this preservation, in which microbial mats stabilized the sediments and produced a veneer of authigenic pyrite (FeS 2) that replicated the morphology of the organisms. Evidence in favor of this hypothesis comes from the presence of iron oxide and pyrite concentrations along fossiliferous bedding planes in certain localities 3,6. Other factors may include episodic event deposition 7 , early cementation by amorphous silica (SiO 2) 2,8 , and burial by volcanic ash-the so-called "Conception-style" preservation 9. It is similarly possible that multiple mechanisms could produce this type of moldic fossilization and/or that taphonomic factors varied according to the geologic setting. In this regard, the Itajaí Basin 10-12 (ca. 563 Ma, Brazil) appears to be a promising locality in which to study the possible mechanisms of preservation operating during the Ediacaran Period. This geological unit is relatively unmetamorphosed, has similar ages to the Avalon OPEN

Ediacaran fossil communities consist of the oldest macroscopic eukaryotic organisms. Increased size (height) is hypothesized to be driven by competition for water-column resources, leading to vertical/epifaunal tiering and morphological innovations such as stems.  Using spatial analyses, we find no correlation between tiering and resource competition, and that stemmed organisms are not tiered.  Instead, we find height is correlated to greater offspring dispersal, demonstrating the importance of colonization potential over resource competition.

Rangeomorphs from the Ediacaran of Avalonia are among the oldest known complex macrofossils and our understanding of their ecology, ontogeny and phylogenetic relationships relies on accurate and consistent classification. There are a number of disparate classification
schemes for this group, which dominantly rely on a combination of their branching characters and shape metrics. Using multivariate statistical analyses and the diverse stemmed, multifoliate rangeomorphs in Charnwood Forest (UK), we assess the taxonomic usefulness of the suite of characters currently in use. These techniques allow us to successfully discriminate taxonomic groupings without a priori assumptions or weighting of characters and to document a hitherto unrecognized level of variation within single taxonomic groups. Variation within the currently defined genus Primocandelabrum is too great to be realistically assigned to different species and may instead reflect
primary character diversity, ontogenetic changes in character state or ecophenotypic variability. Its recognition cautions against generic-level diagnoses based on single differences in character state and will be crucial in understanding the mode of growth of these enigmatic organisms.

Ediacara-type fossils are found in a diverse array of preservational styles, implying that multiple taphonomic mechanisms might have been responsible for their preservational expression. For many Ediacara fossils, the “death mask” model has been invoked as the primary taphonomic pathway. The key to this preservational regime is the replication or sealing of sediments around the degrading organisms by microbially induced precipitation of authigenic pyrite, leading toward fossil preservation along bedding planes. Nama-style preservation, on the other hand, captures Ediacaran organisms as molds and three-dimensional casts within coarse-grained mass flow beds, and has been previously regarded as showing little or no evidence of a microbial preservational influence. To further understand these two seemingly distinct taphonomic pathways, we investigated the three-dimensionally preserved Ediacaran fossilPteridinium simplexfrom mass flow deposits of the upper Kliphoek Member, Dabis Formatio...

Fósseis de cloudinas e corumbellas encontrados no norte do estado indicam que um mar raso cobria partes da América do Sul e da África há cerca de 550 milhões de anos. Com pouco menos de 70 mil habitantes, o município de Januária, no norte de Minas Gerais, é conhecido hoje por suas cachoeiras, grutas calcárias e cachaças artesanais, cujas virtudes derivam, segundo os produtores, do clima e da umidade natural do solo local, bom para o cultivo de cana-de-açúcar destinada à fabricação da aguardente. Sua posição geográfica estratégica, na margem esquerda de quem sobe o grande São Francisco, chamado de opará (rio-mar) pelos antigos índios da região, fez com que fosse um importante porto e entreposto comercial na época colonial. Vestígios de um passado muito mais remoto, quase imemorial e também marcado por uma relação íntima com as águas, acabam de vir à tona em pedreiras ainda ativas nos arredores da cidade.

ChinaRetallack (this issue) argues that Shibantan trace fossilsdescribed by Chen et al. (2013) were not made by bilaterian ani-mals, but instead by a motile grex phase of cellular slime molds,such as Dictyostelium discoideum, a terrestrial eukaryote. Based onthis reinterpretation, he further indicates that the Shibantan tracesare terrestrial and the hosting limestone could be paleosols. Theslime mold interpretation is interesting, but can be falsified on theground of petrographic observations. The Shibantan trace makerswere capable of excavating and displacing sediments (Figs. 3H, 5A-C of Chen et al., 2013), constructing persistent tunnels by splittingclay-rich microbial mats (Fig. 2C and D of Chen et al., 2013), andtruncating and deforming microbial laminae (Fig. 6F of Chen et al.,2013). Although some slime molds can leave sub-millimeter-sizedsurface trails on agar gel, to the best of our knowledge no slimemolds are known to be able to displace and excavate intraclasticsediments, to construct persistent millimeter-sized tunnels withinsediments, and to truncate sedimentary laminae. Below we brieflyrespond to Retallack’s specific comments.

Ediacaran fossils allow us the opportunity to investigate how the earliest preserved metazoans lived, interacted, and diversified. Such interpretations are complex as these organisms are often morphologically enigmatic and defy ready categorization
within modern taxonomic schemes. Therefore, researchers have an incomplete view of their biological affinities. Pteridinium (Family Petalonamae) has been the focus of taxonomic studies due to its unique quilted, triple-bladed morphology (Seilacher and
Grazhdankin, 2002). While currently there are only two formally recognized species within the genus, P. simplex and P. carolinaensis (Gürich, 1930; St. Jean, 1973), there is still debate as to what characteristics differentiate Pteridinium. Some authors speculate that Pteridinium, as classically defined, only exists in Namibia, whereas others contend that all triple-bladed forms, regardless of location, are morphologically identical and
simply represent specimens deformed during preservation or ecophenotypy reflecting variable environmental factors (McCall, 2006). In this study, material will be examined from two localities, Namibia and North Carolina, as the fossils are thought to represent
two distinct species and, hence, display pronounced morphological differences between them.

The early evolution of metazoans has been reconstructed by studies on exceptionally preserved molds in siliciclastic rocks from the Ediacaran Period. However, there remains considerable controversy regarding the formation mechanisms of this unusual ‘Ediacaran-style’ preservation. Proposed hypotheses usually include early authigenesis of minerals, but evidence for this is scarce. In a recently discovered deposit of Ediacaran biota in Brazil, we show that the classic moldic preservation is related to clay mineral authigenesis. Specifically, these clays originated from the alteration of original pyroclastic sediments, likely enhanced by microbial activity, leading to early illitization and morphological templating of the fossiliferous surfaces at a micrometric scale. Such high-fidelity preservation was made possible by rapid burial during volcanic events and the in-situ templating of tissue by clays via microbially-mediated mineralization. This newly described Lagerstätte demonstrates ...

The early evolution of metazoans has been reconstructed by studies on exceptionally preserved molds in siliciclastic rocks from the Ediacaran Period. However, there remains considerable controversy regarding the formation mechanisms of this unusual 'Ediacaran-style' preservation. Proposed hypotheses usually include early authigenesis of minerals, but evidence for this is scarce. In a recently discovered deposit of Ediacaran biota in Brazil, we show that the classic moldic preservation is related to clay mineral authigenesis. Specifically, these clays originated from the alteration of original pyroclastic sediments, likely enhanced by microbial activity, leading to early illitization and morphological templating of the fossiliferous surfaces at a micrometric scale. Such high-fidelity preservation was made possible by rapid burial during volcanic events and the in-situ templating of tissue by clays via microbially-mediated mineralization. This newly described Lagerstätte demon...

During the Ediacaran the Clymene Ocean separated the Laurentia, Amazonia, and Río Apa cratons from several landmasses to the west forming the proto-Gondwana supercontinent. However, no clear evidence about the existence of Ediacaran epeiric seas over those landmasses has been found. Here we report and discuss the discovery of the Ediacaran guide fossil Cloudina sp. associated with other metazoan body and trace fossils in the Bambuí Group (central eastern Brazil). The Ediacaran age of the Bambuí Group and the paleogeographic position of Cloudina-bearing successions in Brazil, Antarctica, Namibia, and Argentina suggest a scenario of ocean connectivity among coeval intracratonic basins of South America, Africa, and Antarctica at the end of Neoproterozoic time. The new fi nding epitomizes one of the most important paleontological discoveries ever made in South America, helping to solve an old paleogeographic puzzle of the Gondwana supercontinent.

O que é que a Argentina, a Namíbia e o Estado de Minas Gerais
têm em comum? Pouca coisa, se é que alguma. Mas no passado estas massas de terra, tão distantes agora, estiveram  conectadas. E serviram de habitat para alguns dos primeiros seres multicelulares a ocupar o planeta. Em abril do ano passado, um grupo de geólogos perscrutava os arredores da
cidade de Januária, MG, numa manhã de calor escaldante. Lá pelas 10 horas, os olhos treinados dos cientistas perceberam,
encravados na rocha, fósseis de cloudinas e corumbelas, seres vivos que estiveram entre os primeiros organismos a desenvolver esqueletos, e que viveram ali há 550 milhões de anos. Segue...