Monday, 21 January 2019

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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