Wednesday, 23 January 2019

Putative discoidal fossils from the Ediacaran of South America.

Discoidal structures in sedimentary rocks can have a variety of origins, from purely the abiotic, such as fluid escape features or raindrop impressions, through trace fossils and microbial structures to actual body fossils of discoidal organisms. In recent years a range of discoidal structures have been accepted as part of the Ediacaran Fauna, fossils formed by a unique group of animals in the 25 million years before the Cambrian Explosion. These fossils have been extensively studied in Europe, Asia, North America, Africa and Australia, but very little attention has been paid to them in South America, despite a wealth of sedimentary rocks of the appropriate age.

In a paper published in the Journal of South American Earth Sciences on 27 November 2018, Lucas Inglez, Lucas Warren, and Juliana Okubo of the Departamento de Geologia Aplicada at the Universidade Estadual Paulista, Marcello Simões of the Departamento de Zoologia at the Universidade Estadual Paulista, Fernanda Quaglio of the Departamento de Ecologia e Biologia Evolutiva at the Universidade Federal de São Paulo, María Arrouy of the Centro de Investigaciones Geológicas, and Renata Netto of the Programa de Pós-Graduação em Geologia at the Universidade do Vale do Rio dos Sinos, present a review of discoidal structures from Ediacaran strata in South America, based upon mentions in publications on other subjects, abstracts presented at conferences and unpublished masters theses.

Inglez et al. first examine the Puncoviscana Formation of northwestern Argentina. This formation has proved difficult to put a precise age to; zircon grain dating has suggested that it is no more than 530 million years old (a zircon grain in a sedimentary deposit must presumably be older than that deposit, as zircons are formed in igneous environments, and any found in a sedimentary sequence must have originally come from elsewhere), making it Cambrian in age or younger, but it is intruded in several places by igneous and metamorphic rocks dated to between 530 and 540 million years ago (still Cambrian, but these rocks must be younger than the formation hosting them, opening up the possibility that the host rocks are Precambrian in origin). These deposits also contain numerous trace fossils made by bioturbating organisms, which would seem to make a Precambrian origin unlikely, but in addition some impressions in sandstones interpreted as body fossils, including Beltanelloides, a disk-shaped fossil 10-50 mm in diameter with a distinct central depression, which shows similarities to some Ediacaran fossils. Inglez et al. suggest that the Puncoviscana Formation is probably Early Cambrian in origin, but that the presence of Ediacaran-like fossils within these deposits makes them worthy of further investigation.

Centimeter-size discs preserved in positive epirelief, showing central depression, Puncoviscana Formation, Puncoviscana Basin, Argentina. Scale bars is 10 mm. Inglez et al. (2018).

Secondly, Inglez et al. consider the Cerro Negro Formation of southern Buenos Aires Province, Argentina. This series of mudstones and carbonates contains skeletal material interpreted as having come from Cloudina riemkeae, a weakly mineralised organism from the end of the Ediacaran, as well as Late Ediacaran Sphaeromorphous Acritarchs (single-celled organisms interpreted as Algae), such as Synsphaeridium sp., Trachysphaeridium sp. and Leiosphaeridia sp.. A date of 580-590 million years old has been suggested for the Cerro Negro Formation based upon carbon 12/13 ratios, which would indicate that it predates the Ediacaran Fauna, but this seems unlikely. The Cerro Negro Formation also contains numerous Microbially Induced Sedimentary Structures, something closely associated with the preservation of Ediacaran Fauna fossils elsewhere, and numerous examples of Aspidella, disk-like structures interpreted as a holdfasts, which range in size from 6 to 140 mm in diameter. These disks have a depressed central portions surrounded by radial lines, although there overall level of preservation is low.

(B) Tens of discoidal structures with different sizes preserved in positive epirelief in fine sandstones, Cerro Negro Formation, La Providencia Group, Argentina. (C) and (D) Detail of discs from Cerro Negro Formation. (C) Fragmented specimen with detached upper positive feature, showing the tridimensional character of these structures and its internal wrinkled aspect. (D) Radial wrinkles observed in lower surface of discs (negative epirelief) with a well-marked central structure (black arrow). Scale bars are 150mm in (B), and 20mm in (C) and (D). Inglez et al. (2018). 

Thirdly, Inglez et al. consider the Tagatiya Guazu Formation of northern Paraguay. This is a series of carbonate beds with a range of grainstones with ripple structures, oolites, thrombolites and mud-cracks, that suggests a tidal setting with evaporation and microbial matting. This deposit produces a range of classical Ediacaran fossils, including Cloudina sp., Corumbella sp., and Namacalathus sp., as well as possible trace fossils and occasional discoidal structures. These discs are 6.6-8.6 mm in diameter, with raised edges and centres, but otherwise preserve no ornamentation. Despite the unquestionable Ediacaran origin of these discs, and the fact that they appear in fossil producing strata, Inglez et al. believe that they are more likely to be microbial structures than body traces made by larger organisms.

Discoidal structure exhibiting a small ellipsoidal central boss, Tagatiya Guazu Formation, Itapucumi Group, Paraguay. Scale bar is 20 mm. Inglez et al. (2018). 

Fourthly Inglez et al. examine the Bom Jardim and Santa Bárbara Groups of the Camaquã Basin in southern Brazil. Both these deposits were laid down in a rift basin between 600 and 530 million years ago, and are therefore considered to be Ediacaran to Early Cambrian in age. The Bom Jardim Group comprises a deltaic sequence overlain by a fluvial/lacustrine sequence, while the Santa Bárbara Group is made up largely of fluvial deposits apparently laid down in a broad river basin, but with occasional marine incursions. Both of these related deposits show numerous trace fossils, but also discoidal fossils closely associated with microbial mat structures, which have been interpreted as Aspidella sp., Intrites sp., and Sekwia sp.. Inglez et al. suggest that this combination of Ediacaran body fossils, microbial matting, and diverse trace-makers is very close to what might be expected at the boundary between the Ediacaran and Cambrian Periods, and that these strata therefore merit further investigation.

Several discoidal imprints preserved as positive hyporelief in fine sandstones from the Santa Bárbara Group, Camaquã Basin. Brazil. Scale bar is 10 mm. Inglez et al. (2018). 

The fifth deposit discussed by Inglez et al. is identified as Depositional Sequence II of the Itajaí Basin of southern Brazil, a sequence of slightly deformed deltaic deposits. These strata produce a range of structures related to microbial matting, as well as Acritarchs, some trace fossils, what may be Sponges of Cambrian affinities, and some very rare disc-shaped impressions that have variously been described as Parvancorina sp., Charniodiscus? sp., Cyclomedusa sp. and Aspidella sp.. 

Detail of a single specimen of a disc characterized by two concentric ridges separated by shallow groves, Depositional Sequence II, Itajaí Basin, Brazil. Scale bar is 5 mm. Inglez et al. (2018).

Sixthly, Inglez et al. examine series C of the Camarinha Formation of Paraná State in southern Brazil, a series of sand and siltstones associated with a delta sequence, which contain both trace fossils and discoidal fossils interpreted variously as Cubichnia and Beltanelliformis. These are slightly ovoid, implying a preferred orientation, 4-9 mm in length and associated with microbial mats. 

Elongated unornamented discoidal structures preserved in positive epirelief. Note that the central portion of the discs is slightly depressed, Camarinha Formation, Camarinha Basin, Brazil. Scale bar is 5 mm. Inglez et al. (2018).

Next Inglez et al. look at the Sete Lagoas Formation of the São Francisco Craton in central Brazil. This formation comprises mudstones and other shallow marine sedimentary rock-types, many of which appear to be of microbial origin. This formation contains Acritarchs interpreted to be of Late Ediacaran age, and has been judged to be no more than 557 million years old on the basis of isotopic data. The Sete Lagoas Formation has produced a number of structures interpreted to be pseudofossils, as well as trace fossils and a number of discoidal structures between 20 and 40 mm in diameter, though Inglez et al. interpret these as probably being of microbial origin. 

(B) Several small black-colored discs over thrombolites, Sete Lagoas Formation, Bambuí Group, Brazil. (C) Detail of figure (B) showing a disc with well marked concentric radial ridges and groves. Scale bar is 5 mm. Inglez et al. (2018).

Finaly Inglez et al. look at the Jaibaras Group of Ceará State in northeastern Brazil. Here a series of siltstones and sandstones attributed to the Ediacaran Pacujá Formation have previously been reported to contain a variety of fossils similar to those of the White Sea Edaicaran Fauna of Russia and Australia, including Charniodiscus, Cyclomedusa, Ediacaria, Kimberella, Medusinites, Palaeophragmodictya, Parvancorina and Pectinifrons, as well as some trace fossils, and a large number of discoidal fossils. However, examination of these strata by Inglez et al. led them to raise questions about the discoidal fossils. Firstly they note an absence of accurate dating for the supposedly Ediacaran beds, which appear very similar to those of the Silurian Ipu Formation that outcrop in the same area. Furthermore, they note that examination of the 'discoidal' fossils revealed that they are also circular in vertical section, cutting through bedding planes to form spheres, strongly suggesting that they are diagenetic features (features that formed within the sediment after it as buried) rather than true fossils.

(D) Several decimeter size discoidal features densely distributed in the bedding planes of coarse sandstones of the Ipu Formation. Note the concentric internal pattern, commonly marked by two main rings of darker colour. Discs sometimes over cross each other (black squares). Scale bar is 5 mm. (E) Detail of concentric discoidal structure of the Ipu Formation, preserved in a weathered surface. (F) Similar discoidal features seen in cross section in the same outcrop illustrated in (D) and (E).  Inglez et al. (2018).

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Monday, 21 January 2019

Eruption on Mount Shindake, Kuchinoerabu Island, Japan.

The Mount Shindake volcano on Kuchinoerabu Island erupted on Thursday 17 January 2018, producing an ash column that rose to about 6 km above the island. The eruption occurred at about 9.20 am local time, and through hot rocks some way from the crater, causing about 80 of the islands 109 residents to seek temporary refuge in a shelter, though the risk was later judged to have passed and people returned to their homes. This is the latest in a series of eruptions on the volcano that began in October last year.

An ash cloud over Mount Shindake on 17 January 2019. Japan Meteorological Agency.

Japan has a complex tectonic environment with four plates underlying parts of the Islands; the Pacific in the east and the Othorsk in the North, there are the Philippine Plate to the south and the Eurasian Plate to the West. All of these plates are moving in different directions, and some subducting beneath the islands, leaning to a complex tectonic situation where earthquakes and volcanoes are common.

The movement of the Pacific and Philippine Plates beneath eastern Honshu. Laurent Jolivet/Institut des Sciences de la Terre d'Orléans/Sciences de la Terre et de l'Environnement.

Kuchinoerabujima lies at the northeast end of the Ryukyu Island Arc, which sits on top of the boundary between the Eurasian and Philippine Plates. The Philippine Plate is being subducted beneath the Eurasian Plate, in the Ryukyo Trench, to the Southeast of the Islands. As it is drawn into the interior of the Earth, the tectonic plate is partially melted by the heat of the Earth's interior, and liquid magma rises up through the overlying Eurasian Plate to form the volcanoes of the Ryukyu Islands and Kyūshū.

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Magnitude 5.8 Earthquake off the coast of Oaxaca State, Mexico.

The United States Geological Survey recorded a Magnitude 5.8 Earthquake at a depth of 10.0 km, approximately 60 km off the south coast of Oaxaca State, Mexico, slightly before midday local time (slightly before 6.00 am) on Tuesday 21 January 2018. This even was felt across much of southern Oaxaca, but there are no reports of any damage or casualties.

The approximate location of the 21 January 2019 Oaxaca Earthquake. USGS.

Mexico is located on the southernmost part of the North American Plate. To the south, along the Middle American Trench, which lies off the southern coast off Mexico, the Cocos Plate is being subducted under the North American Plate, passing under southern Mexico as it sinks into the Earth. This is not a smooth process, and the plates frequently stick together then break apart as the pressure builds up, causing Earthquakes on the process.

The Cocos Plate is thought to have formed about 23 million years ago, when the Farallon Plate, an ancient tectonic plate underlying the East Pacific, split in two, forming the Cocos Plate to the north and the Nazca Plate to the south. Then, roughly 10 million years ago, the northwesternmost part of the Cocos Plate split of to form the Rivera Plate, south of Beja California.

The position of the Cocos, Nazca and Rivera Plates. MCEER/University at Buffalo.
In a paper published in the Journal of Geophysical Research, in 2012, a team led by Igor Stubailo of the Department of Earth and Space Sciences at the University of California Los Angeles, published a model of the subduction zone beneath Mexico using data from seismic monitoring stations belonging to the Mesoamerican Seismic Experiment, the Network of Autonomously Recording Seismographs, the USArray, Mapping the Rivera Subduction Zone and the Mexican Servicio Sismologico Nacional.
The seismic monitoring stations were able to monitor not just Earthquakes in Mexico, but also Earthquakes in other parts of the world, monitoring the rate at which compression waves from these quakes moved through the rocks beneath Mexico, and how the structure of the rocks altered the movement of these waves.
Based upon the results from these monitoring stations, Stubailo et al. came to the conclusion that the Cocos Plate was split into two beneath Mexico, and that the two plates are subducting at different angles, one steep and one shallow. Since the rate at which a plate melts reflects its depth within the Earth, the steeper angled plate melts much closer to the subduction zone than the shallower angled plate, splitting the Trans-Mexican Volcanic Belt into sections above the different segments of the Cocos Plate, and causing it to apparently curve away from the subduction zone.
 Top the new model of the Cocos Plate beneath Mexico, split into two sections (A & B) subducting at differing angles. (C) Represents the Rivera Plate, subducting at a steeper angle than either section of the Cocos Plate. The Split between the two has been named the Orozco Fracture Zone (OFZ) which is shown extended across the Cocos Plate; in theory this might in future split the Cocos Plate into two segments (though not on any human timescale). Bottom Left, the position of the segments on a map of Mexico. Darker area is the Trans-Mexican Volcanic Belt, orange circles are volcanoes, brown triangles are seismic monitoring stations, yellow stars are major cities. Bottom Right, an alternative model showing the subducting plate twisted but not split. This did not fit the data. Stubailo et al. (2012).
Witness accounts of Earthquakes can help geologists to understand these events, and the structures that cause them. The international non-profit organisation Earthquake Report is interested in hearing from people who may have felt this event; if you felt this quake then you can report it to Earthquake Report here.
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Using the Ordovician Tafilalt Lagerstätte of southeastern Morocco as a model for the preservation of Ediacaran fossils, and what this tells us about the Earth's earliest animals.

A hundred and fifty years ago Charles Darwin was puzzled by the abrupt appearance of animals in the fossil record during the Cambrian Explosion, rather than a longer, more gradual evolution from simpler organisms. Today, the widespread fossils of the Ediacaran Fauna are recognised as being animals which predate the Cambrian Explosion, but essentially the same mystery remains; these organisms appear abruptly in the fossil record about 570 million years ago as fully developed communities of large, albeit mysterious, animals. Molecular clock dating methods, which attempt to determine the time at which different organisms diverged by measuring the (presumably fairly constant) rate at which non-coding DNA mutates, have suggested that the last common ancestor of all living animals probably lived in the Cryogenian (720–635 million years ago) or even in the Tonian (1000–720 million years ago), significantly earlier than the appearance of the Ediacaran Fauna, which in turn suggests that, like the fossils of the Earliest Cambrian, the Ediacaran fossils do not represent the earliest animals, but rather a part of an older story that happened to get preserved. If the Ediacaran fossils do not represent the earliest animals, but rather a group of animals selectively preserved by a particular set of conditions, then it becomes important to understand those conditions in order to understand why those particular organisms were preserved, and how they are likely to relate to the total fauna alive at the time.

In a paper published in the journal Palaeogeography, Palaeoclimatology, Palaeoecology on 9 November 2018, Breandán Anraoi MacGabhann of Earth and Ocean Sciences at the National University of Ireland Galway, and the Department of Geography at the University of Limerick, James Schiffbauer of the Department of Geological Sciences and X-ray Microanalysis Core Facility at the University of Missouri, James Hagadorn of the Department of Earth Sciences at the Denver Museum of Nature & Science, Peter Van Roy of the Department of Geology and Soil Science at Ghent University, Edward Lynch, also of Earth and Ocean Sciences at the National University of Ireland Galway, and of the Geological Survey of Sweden, Liam Morrison, again of Earth and Ocean Sciences at the National University of Ireland Galway, and John Murray, once again of Earth and Ocean Sciences at the National University of Ireland Galway, and of the Irish Centre for Research in Applied Geosciences, develop a model for the mode of preservation seen in the Ediacara Fauna based upon the preservation of Eldonids in the Ordovician Tafilalt Lagerstätte of southeastern Morocco as a model.

The Ediacaran Fauna is found in a variety of environments, including shales, coarser clastic siltstones and sandstones, volcaniclastic sediments and carbonates, with fossils preserved by phosphatisation, pyritisation, carbonaceous compression, aluminosilicification, or as molds and casts, with some biomineralised specimens known from the very end of the period. However the vast majority of specimens found are preserved by classic Ediacara-style preservation, which is to say as molds or casts in sandstones or other course sediments.

These classic Ediacara-style fossils can be split into three main groups, based upon their style of preservation. Firstly there are gravity-cast fossils, which are preserved in negative relief on the top part of a bed, with a corresponding cast in the overlying bed, in which the underlying bed was able to retain a mold of the animal after it decayed, which was then infilled by sediments from above. Secondly, death-mask fossils occur as similar fossils in the opposite orientation, with a mold on the upper bed infilled by a cast from the lower bed, suggesting that the overlying bed has preserved a mold after the animal has decayed, which has then been infilled by material from the lower bed pushed upwards by pressure. Finally there are endorelife fossils, in which a mold is preserved in both upper and lower beds, and infilled by other material.

The most widely accepted explanation for this is currently the microbial mat model proposed by Gehling (1999), which proposes that the fossils were not preserved due to any innate property of the organisms themselves, but due to the properties of the microbial mats widespread in the shallow seas of the time, which reduced sulphur as they decayed, leading to the precipitation of iron pyrites on perservational surfaces, which retained the shape of the Ediacara Fauna animals. No such iron pyrites layers are known today, but many of these beds do have layers of iron oxides which might reflect the original deposition of iron pyrites.

Gehling (1999) model for the microbial ‘death mask’ taphonomy of fossils from the Ediacaran of South Australia. (A) The living benthic community; shown are Dickinsonia, a resistant form; Phyllozoon, a non-resistant form; and Charniodiscus, a frond with holdfast. (B) Burial of the community. (C) Decay and compaction of Phyllozoon, with sediment moving down to fill the space left by the organic tissues. A new microbial mat forms on the surface of the burying bed, sealing the pore waters from seawater oxygen. (D) Decay of the microbial mat by sulfate reduction causes the precipitation of iron sulfides (shown in yellow) in the sole of the burying bed. Decay of Charniodiscus holdfast, with sediment moving down to fill the space. (E) Sediment moves up from below to fill the space left by the decomposition of Dickinsonia. (F) Present day. Telodiagenetic oxidation of the pyrite sole veneer to hematite (shown in red), leaving fossils preserved in positive epirelief and both shallow and deep positive hyporelief. MacGabhann et al. (2018).

Although this classic Ediacara-style preservation is closely associated with the Ediacaran Fauna, and is generally assumed to have ended with the Cambrian Explosion, after which there were numerous bioturbating organisms around to disrupt preservation of this type, there are in fact numerous examples of apparent classic Ediacara-style preservation in the Phanerozoic fossil record. These include possible Colonial Hydrozoans from the Devonian of New York and the Carboniferous of Kentucky and Oklahoma, putative Jellyfish from the Ordovician or Silurian of Sweden and Silurian of the Yukon, more plausible Jellyfish from the Cambrian of Wisconsin, New York and Quebec, the enigmatic Protonympha salicifolia from the Devonian of New York, and Priscapennamarina angusta from the Cambrian of China.

One group that MacGabhann et al. consider to be of particular interest are the Eldonids, a group of enigmatic Palaeozoic organisms, which have a disk-shaped body with a flat lower surface, a convex upper surface and a coiled internal organ interpreted as a peri-alimentary coelom surrounding the alimentary canal. These organs are known from the Cambrian Chengjiang and Burgess Shale deposits, from which their anatomy is well known, but also from a variety of Cambrian to Devonian deposits where they are preserved Ediacaran-style as molds in course sandstone deposits.

In order to better understand the processes that led to the fossilisation of these apparently soft-bodied organisms in course sandstones, MacGabhann et al. examined a collection of 233 Eldonids from the Late Ordovician Tafilalt Lagerstätte of southeastern Morocco, as well as additional Eldonid material from the Upper Devonian Genesee and West Falls Groups of New York.

The fossils show no signs of any biomineralisation, and are preserved as molds and casts in sediments with grain sizes between 100 μm and 5 mm. The majority of these specimens are preserved on bedding planes as gravity-casts, with a minority of death-mask fossils and endorelife casts. Many of the fossils are overlapping, with the lower fossil preserved as a gravity-cast with an endorelife fossil above.

 Fossil Eldonids from Tafilalt (A)–(G) and New York (H), (I) preserved as molds and casts in siliciclastic sediments. All scale bars 50 mm. (A) Three overlapping negative epirelief molds of Discophyllum peltatum. (B) Endorelief cast of Discophyllum peltatum with a conspicuously reddened surface. (C) Two overlapping endorelief casts of Discophyllum peltatum preserving the coiled sac with negative relief from the surface and with a dark mineral coating. (D) Negative hyporelief mold of Discophyllum peltatum preserving the coiled sac as a reflective sheen on the dorsal surface. (E) Two overlapping positive hyporelief casts of Discophyllum peltatum, with the overlapped portion of the upper specimen (arrowed) seen preserved in endorelief in the adjacent (right) image. (F) Endorelief cast of an un-named eldonid with a conspicuously darkened surface. (G). Two overlapping positive hyporelief casts of Discophyllum peltatum, one of which is folded. (H) Positive hyporelief cast of Paropsonema cryptophya. (I) Positive hyporelief and partial endorelief specimen of Paropsonema cryptophya, rolled up in a cigar-like shape. MacGabhann et al. (2018).

Importantly, more detail is preserved in Eldonid specimens from the Burgess Shale than from Tafilalt; the Tafilalt fossils preserve the coiled sac, typically as a reflective sheen or dark mineral coating, but this lacks the three-layered structure seen in Burgess Shale fossils, in which the sac is preserved as an inner coiled tube and surrounded by an outer membrane, and is further divided into proximal, medial, and distal segments along its length. Not do the Tafilalt specimens show other structures seen in the Burgess Shale Eldonids, such as internal lobes, radial fibres, a central ring, and circumoral tentacles.

Normally such loss of details is disappointing to palaeontologists, as it makes it more difficult to understand the biology and relationships of fossils, but in this case MacGabhann et al. see it as an opportunity, as it is apparently shows preferential preservation of certain tissues over others in Ediacaran-style fossils.

Eldonia ludwigi from the Burgess Shale. (A) Eldonia ludwigi showing the tripartite lengthwise division of the coiled sac into proximal, medial, and distal portions, with an internal coiled tube. Branched circumoral tentacles surround the oral aperture, while in the inner area, radial fibres extend from a central ring. Internal lobes are also commonly preserved in the outer area of the fossil, often with superimposed radial strands. (B) Eldonia ludwigi showing the tripartite lengthwise division of the coiled sac, with the coiled tube visible in the distal portion, and the circumoral tentacles. Radial fibres are also seen diverging from a central ring, and are clearly deflected to allow the proximal and distal terminations of the sac to reach the ventral aperture. The bifurcating internal lobes are also clearly visible in the outer part of the fossil. (C) Eldonia ludwigi showing the proximal, medial, and distal portions of the coiled sac, the circumoral tentacles, the internal lobes, the radial fibres, and the central ring. (D) Eldonia ludwigi, with superimposed Trilobite, showing the proximal, medial, and distal portions of the coiled sac, with the medial portion clearly surrounded by an outer membrane and containing an internal tube. Circumoral tentacles are also present, and the internal lobes and radial fibres are clear, though the central ring is obscured by the Trilobite. (E) Magnified view of the specimen in (D) showing the coiled sac surrounded by the membrane and containing the tube. Scale bars 20mm for (A)–(D), 5mm for (E). MacGabhann et al. (2018).

The Tafilalt fossils show the preservation of certain tissues, preserved when they were in contact with the sediment or microbial mat surface. MacGabhann et al. suggest that this may be indicative of Fe²⁺ (reduced iron) ions onto negatively charged functional groups in organic biopolymers of high molecular weight, something found in biological materials such as chitin, collagen, cellulose, and peptidoglycan. This can only happen in a reducing environment (i.e. one lacking oxygen), but these commonly occur in sediments even a short distance beneath the surface, as microbes attacking decaying carcasses use up the available oxygen more quickly than it can be replenished via the available pore-spaces.

Organic biopolymers of high molecular weight with bonded Fe²⁺ ions would be likely to retain cohesion as other tissues decayed, and would in addition facilitate the further bonding of aluminosilicate clay minerals and perhaps oxides and oxyhydroxides such as haematite and goethite onto their surfaces, further reinforcing the structure of these tissues.

Simplified and idealised cartoon illustration of Tafilalt Eldonid taphonomy. (A) Eldonids in life. Coiled sac represented by the dark areas at the dorsal surface. (B) Transport. Magnified area shows the potential adsorption of detrital clay minerals to the surfaces of the fossils during transport; these may also adsorb to Bacterial exoenzymes (not illustrated). (C) The specimens are buried, shown left to right: (1) with the dorsal side resting on the top surface of the underlying bed, but partially folded up into the sediment. (2) with the dorsal side resting on the top surface of the underlying bed, and evidence for minor scavenging. (3) with the ventral surface resting on the top surface of the underlying bed. (4) entirely within the bed, dorsal side up. (5) entirely within the bed, dorsal side down. (D) Decay proceeds first by aerobic decay, then Mn (IV) reduction, then Fe (III) reduction. Reactive iron is reduced and mobilised; Fe²⁺ ions produced adsorb to active sites in complex organic biopolymers constituting the outer integument and coiled sac (shown in the magnified area). Red filled circles represent Fe³⁺; green filled circles represent Fe²⁺. Lightening of the colour fill of the Eldonids represents decay. (E) Decay continues by sulfate reduction, with the sulfide produced reacting with adsorbed Fe²⁺ ions to produce iron monosulphides, which subsequently transform to pyrite, in situ on the surfaces of the outer integument and coiled sac (shown in the magnified area). (F) Decay continues: clay minerals (represented by blue hexagons) nucleate around adsorbed Fe²⁺ ions on the surfaces of the coiled sac and outer integument, where these have not reacted with sulfide (shown in zoomed area). (G) Decay is complete, with sediment having progressively moved to fill the space vacated by the decomposition of the organic tissues. (H) Telodiagenetic oxidation of iron sulfides to haematite and goethite or lepidocrocite (represented by dark red filled circles) by reaction with oxygen and water (shown in the magnified area). This is the present-day state of the fossil surface veneer. MacGabhann et al. (2018).

MacGabhann et al. argue that this mode of preservation is consistent with that observed in a variety of Ediacaran deposits, in which the organisms appear to have been preserved in may places where the microbial mats were not.

Locus of authigenic mineralisation. Preservation in positive epirelief requires (i) mineralisation on or above the organism, not just on the mat below, as this could not preserve the upper surface; (ii) the sediment below the organism to be fluid after this mineralisation, otherwise fossil molds could not be cast; and (iii) this mineralisation to occur in a concentrated fashion on the surface of the organism, as disseminated mineralization could not preserve fine details. MacGabhann et al. (2018).

Tafilalt style taphonomy is thought to preserve only parts of organisms made from organic biopolymers of high molecular weight, such as the outer integument and coiled sac of the Eldonids. It can therefore be presumed to preserve only organisms in which such tissues are present. Sponges and Cnidarians, considered to be the oldest living groups of animals, produce such biopolymers, but few produce tissues made largely of these, rather they tend to be scattered through their overall structures.

This being the case, Cnidarians and Sponges would not be preserved by Tafilalt style taphonomy, and would not be found in Ediacaran deposits preserved in the same way. This would imply the organisms preserved in the Ediacara Fauna are not Sponges or Cnidarians, as has often been speculated, but rather more advanced group, with tissue development comparable to that seen in Arthropods or Molluscs. This would also suggest that Cnidarians and Sponges may have been present, even abundant, at the time when the Ediacaran fossils were being preserved, but not to have been preserved themselves. The possibility that Cnidarians and Sponges were not amiable to preservation during the Ediacaran raises the possibility not only that they were present at the time, but that they had emerged somewhat earlier and not entered the fossil record, consistent with the earlier dates suggested by molecular clock dating methods.

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