Australia: The Land Where Time Began
Pre-Marinoan Limestones from South Australia Containing Possible Animal-Body Fossils
During the Neoproterozoic there were 2 periods of glaciation, at about 710 Ma the Sturtian Glaciation and at about 635 Ma the Marinoan Glaciation. A succession of limestones and shales in South Australia, deposited at a time when Australia was at tropical latitudes, separate layers of rocks that were deposited during the glaciations. The authors1 describe fossils, that were weakly calcified, being of various shapes, such as anvil, ring, wishbone and slabs with perforations, recovered from stromatolitic limestones in the Trezona Formation that was deposited before the Marinoan Glaciation. The fossils from the Trezona Formation are about 90 My older than any other known calcified fossils of their size. According to the authors1 as it was not possible to extract from the calcite matrix in which they were embedded, and the traditional method using X-ray based tomography could not be used, they resorted to tracing cross-sections of individual fossils by grinding serially and scanning each fossil at 50.8 μm resolution, then constructing 3D digital models of the fossils. Their results showed a population of asymmetrical ellipsoidal organisms that had a network of canals in their interior leading to circular apertures in the surface of the organisms. The authors1 suggest that several characteristics of these fossils, that were reef-dwelling organisms, are best explained by considering the organisms to be sponge-grade metazoans.
In the central Flinders Ranges stromatolite flake breccia, and bioclast packstone filling the spaces between the stromatolites heads, with synoptic relief of up to 1 m, is one of the commonest facies associations in the Trezona Formation. Bioclast packstones have been found to be present as dune-cross-stratified channel fill between larger stromatolite bioherms, as well as overbank deposits with a sheet-like form. According to the authors1 they identified a large diversity of bioclasts as they mapped and measured 14 detailed sections through the Trezona Formation. They found clasts that were probably of microbial origin in most packstones studied, that included spalled flakes from adjacent stromatolite laminae and sediment that had been ripped-up and rolled-up, with its cohesion enhanced by microbial mats. Among the many bioclasts found were a large number that were difficult to assign to abiotic roll-up or of a microbial mat origin, many of the bioclasts having morphologies of various shapes such anvils, wishbones, rings and perforated slabs. Unique to the packstones are the red colour, and the calcite composition of these clasts, that have distinctive shapes, and even the packstone clasts that are in Elatina Formation diamictite up to 65 km from the nearest stromatolite reef outcrop of the Trezona Formation, that are not found in situ as layers in other parts of the Trezona Formation that could have been brecciated and transported. Based on this the authors1 suspect that the remnants of a community of organisms that were endemic to the stromatolite-packstone environment are represented by 1-cm-scale red bioclasts.
According to the authors1 the packstone matrix can be seen in thin section to be an interlocking network of equant calcite crystals and peloids (floating circular crystal aggregates) that are coated by a rim of micritic cement that varies in thickness. Bulbous and chambered micritic textural elements are mixed in with the peloids and coccoid clay spheroids, as well as, though rarely, iron oxides and organic matter, and intergrown with calcite spar. The distinctively-shaped bioclasts of centimetre-scale, that are red when present in outcrops, have a coating of uniformly thick micritic cement. Calcite spar, usually finer than the matrix, commonly comprises the bioclast interiors, and from the inner edge of the micritic rim towards the centre the crystals sometimes decrease in size. The authors1 have interpreted the crystal size gradient as heterogeneous cementation and recrystalisation proceeding from the exterior of the clasts inwards. According to the authors1 there is no evidence of diffusive or porosity-following micritisation, and the contact between the micrite coating and the interior of mixed clay-chert-calcite is usually sharp. There are a few examples of calcite spar that is spaced and isolated has been found to have grown from the coating of the rim towards the interior of the clast, possibly filling an aperture or perforation in the clast. When the authors1 compared material from the matrix with that from bioclasts using attenuated total reflectance Fourier transform infrared they found that bioclasts were richer in either iron or manganese, which they suggest possibly explains the red colour. It was found by elemental mapping with environmental scanning electron microscope (E-SEM) energy dispersed X-ray system (EDX) that Fe, Na and K clays formed irregular patches inside the most coarsely recrystallised and tabular-shaped clasts. The best preserved wishbone-shaped clasts, however, contain iron-rich clays that are very finely disseminated and elliptical chert blebs 0.1-0.5 mm in diameter that are either to[?] the bioclast or extend across the outer edge and the micrite coating. The authors1 say they have not been able to document any microstructures or textures that are diagnostic that would allow the bioclasts to be identified as organic walls, that are weakly calcified, of a specific micro-alga or animal, and did not find specific evidence that allowed the interpretation of the chert blebs as being of biogenic silica.
3-D fossil reconstruction
According to the authors1 the 1-cm-scale of the bioclasts is of similar dimensions to that of Namacalathus fossils found in packstones from the Late Ediacaran of Namibia. They constructed 3D models of individual specimens from a single block of packstone to test the hypothesis. They could not use CT scanning and X-ray scanning as the density contrast between the organic material and the matrix were too similar, and they were also unable to use an acid bath technique as the solubilities of the matrix and the calcified fossils it contained were too similar. They resorted to grinding and scanning at 50.8 μm intervals. This allowed the formation of a 3D model they developed a new routine involving converting auto-traces of individual specimens to a point cloud that can be meshed and modelled as a 3D volume. An advantage of the new routine is that interpretation by the researchers is minimised, the model volume being generated entirely based on the colour contrasts in the original digital images.
The individual reconstructions that resulted are not identical, 3D forms varying to some degree, though common features are shared by many of the reconstructions. There is no consistent symmetry of the external or internal morphology. The 2D sections through a particular specimen display anvils, wishbones, horseshoes and slabs with perforations, and these are of identical morphology and scale to those seen in images from outcrops. All the canals connect to recessed apertures, though a possible exception has been found. There is a short appendage, that is tube-shaped, at the base of at least 3 specimens that the authors1 suggest could possibly be a stalk or holdfast. The almost-circular cross-sections of the interior canals suggest that the topological symmetry of these objects is primary, though the organic walls, that are weakly calcified, appear to have been malleable and easily deformed during compaction.
The authors1 suggest that these structures could possibly be casts of filaments of forms that are similar, though the network topology and recessed apertures are not consistent with these structures being casts. Flakes of microbial mats that were ripped up and subsequently mined by benthic macroborers searching for organic matter. If the macroboring suggestion is correct it would explain the texture and possibly the relatively clay-rich nature of the interiors of the clasts as relict bacterial mat, instead of the uniform sparry calcite that would be expected to replace skeletons, though if this was the case it would not be clear how the clasts' exterior walls and the interior walls of the tunnels could be coated with a uniform thickness and texture of micrite. Another problem with this interpretation is that the tunnels found in the Trezona Formation interconnect and always both enter and leave the clast, so not ending in dead-ends as does the tunnels of most endolithic organisms. Borings have not been found penetrating stromatolites or microbialites that were in situ.
From the Upper Devonian to the present endolithic fungi and algae are commonly found on coral reefs, though they tend to bore holes that are much smaller than the 1 mm holes that are found in the Trezona Formation, being 0.1-6 μm in diameter. According to the authors1 the oldest known macroborers are found in the Lower Cambrian Archaeocyathid reefs of Labrador and a characteristic of these borings is that they are straight and normal to the surface of the reef, and there are no branches or interconnections. It has been suggested that these borings, averaging 2 cm long and 1 mm diameter, were made by polychaete or sipunkulid worms. The canals of the Trezona Formation would be about 130 My older than those of the macroborers from Labrador and significantly more complex, and about 70 My older than the Ediacaran organisms that were large cnidarian-grade forms. The authors1 suggest a number of possible biological affinities for these organisms that include microbialites, giant protists, calcareous algae, and metazoans. In the fossil record algal nodules, thrombolites and stromatolites are common, though their structures are linear or columnar, and they don't have regular internal canals or apertures as found in the Trezona Formation. The fauna from the Trezona Formation is present as bioclasts that are broken and transported in channel fill that separates the stromatolite bioherms, though it would have been possible for macro-algae and metazoans, such as sponges, to have thrived as organisms that were sessile near the bathymetric highs that were formed by stromatolites.
Extracellular polymeric substances produced by microbes often facilitate the formation of microbial carbonates leading to stabilisation and preservation of microbial communities. Among these may be included channel and canal systems providing nutrients and oxygen and the removal of wastes, though they are usually 20-40μm wide, and the height of biofilms are generally less than 100 μm. In the Trezona Formation there are no matches for these characteristics.
Unicellular Eukaryotes are generally microscopic and their cell-tissues are usually unspecialised. In the waters of the Bahamas there are giant deep-sea protists that grow to 1-3 cm in diameter, are almost spherical that produce sediment traces that are like those of bilaterians, prompting a comparison with ichnofauna from the Proterozoic. Protists, such as foraminifera, that are calcified, can grow to centimetre-scale and have complex camerate structures with walls surrounding internal spaces. Another type of deep sea protists that grows to large size are the xenophyophores, organisms with internal pores and skeletons constructed of polysaccharides and debris that is available, including foram tests and sponge spicules, that could be calcified during diagenesis. But unlike the Trezona fauna protists lack the internal canal networks and symmetry of growth that is seen in the fossils from the Trezona Formation.
Grypania, macro-algae, are known in the fossil record from about 1.5 Ga. Green algae, such as codiacean and dasycladalean, can be found as simple, curved bioclasts and skeletons that have internal pores. The complex internal anatomy of algae with cylindrical pores of several classes of size, mostly much smaller than 1 mm. Rocks from the Ordovician-Silurian contain the oldest-known calcifying green macro-algae. Walls that are distinctly calcified are formed by some red algae that have been confused with chaetetid sponges from the Palaeozoic. In the Doushantuo Formation of China, of immediate post-Marinoan age, macroscopic, non-calcifying floridophyte red algae have been found. The pores and columns in red algae are generally 30-50 μm in diameter, that is similar to the dimensions of the cells. The authors1 suggest there would be no reason for circulating water in an alga, or maximise the surface away from chemical or radiative gradients used by the alga in its autotrophic metabolism.
Pre-Marinoan - possible animal fossils
The interconnected network of canals of 1-mm diameter that enter and exit the fossil via circular apertures found in the Trezona Formation organisms, as well as other characters, are also found in other simple sponge-grade organisms. The authors1 suggest these structures could serve a clear function in the organisms as part of the water canal system and filter feeding apparatus of a simple sponge. Single calcite spars sometimes grow inwards from the micritic rims, the authors1 suggesting this could possibly be through primary orifices such as Ostia. Elliptical blebs of silicon dioxide are found in the least recrystallised wishbone specimens, that cross-cut clast walls and the micritic coating, that the authors1 suggest could possibly be roots of siliceous spicules, but they are yet to find unambiguous whole spicules attached to clasts or floating in the matrix. Palaeophragmodictya, interpreted as a 2D impression of a sponge, of Ediacaran age, in the coarse sandstone of the Rawnsley Quartzite in the Adelaide Rift Complex, South Australia, shares some morphological similarities with Trezona Formation fossils, though the latter are significantly smaller and they don't have the hexactinellid spicule network or the prominent oscular disc. The upper surface of Palaeophragmodictya typically preserves as a series of grooves that intersect of 2-3 cm diameter suggested by the authors1 to be infilled canals.
The authors1 have considered the question of the organisms of the Trezona Formation being primary calcifiers. They suggested that the micrite and texture, of uniform thickness, coating both the exterior surface and the canal walls in the interior may represent cell layers that have been weakly calcified, sandwiching the mesohyl of an organism of sponge grade. Peloids, that don't appear to be part of the organisms from the Trezona Formation, have micrite that is texturally similar, though the thickness is less uniform. The maintenance of circular cross-sections during transport and burial deformation requires a form of partial cementation. The authors1 suggest a calcite precipitation on an organic template lightly mineralising the fossils, such as has been interpreted from Cloudina and Namacalathus of Ediacaran age, preserved as casts of voids filled with calcite. An alternative suggestion has been that a bacterial extracellular polymeric substance could have coated the original organic skeletons after they died. The iron, sodium and potassium clays present in bioclasts from the Trezona Formation has a similar composition to that authigenic minerals that microbial biofilms precipitated during soft tissue replacement. A template for abiotic precipitation of calcite would have been formed by the extracellular substance.
According to the authors1 the asymmetric body plan and the system of interconnected canals are the most powerful arguments for the proposed sponge hypothesis. Arguments that remain ambiguous concern the textural, biocalcification and spicule arguments, though when added to the other evidence they support the hypothesis. As there is a belief that the earliest sponges would have been simple tubes it has been suggested that the complex canal network of the organisms from the Trezona Formation may not be the first sponge-grade organisms.
According to the authors1 the interpretation of the organisms from the Trezona Formation as early sponges is consistent with dates arrived at by molecular clocks for the divergence of sponges and Eumetazoa. Strata below the cap carbonate of the Hadash Formation, of Marinoan age, in Oman, lipid bilayers suggestive of Demosponges have been recovered. Etina Formation equivalent microbialite reefs in the Gammon Ranges, South Australia, and the Lower Little Dal Group reefs, McKenzie Mountains, Canada, that are more than 723 My old, have produced calcified chambered microfossil textures of mm-scale with an affinity to sponges. The organisms in the Trezona Formation have been shown to be older than the oldest known definitive body fossil evidence for animals, and predate the oldest putative sponge spicules that were found in phosphorites of the Doushantou Formation in South China situated above the cap carbonate of Marinoan glacial sediments. These earliest known sponge spicules first appear in the fossil record in the late Early Cambrian. It is indicated by the realisation that sponges of the present are probably paraphyletic that the water canal system and other topologies are ancestral characteristics that are shared with other basal Metazoa, as well as probably with other stem lineages that are now extinct. The authors1 suggest the organisms from the Trezona Formation may be either stem metazoans or from lineages that are now extinct. They concluded that a generalised sponge-grade metazoan is the best explanation of several characteristics, though there are not enough diagnostic synapomorphies the Trezona Formation organisms shared with other clades to demonstrate phylogenetic affinity.
An ecological puzzle is posed by the possibility that the Trezona Formation fauna is the first body fossil evidence for sponges from the time before the Marinoan Glaciation that was predicted by molecular clock and lipid biomarker studies. Diverse assemblages of microfossils were characteristic of the 30 My of the Ediacaran, though there are no known apparent metazoans. 579 Ma the earliest known probable metazoans, in the form of large soft-bodied organisms from siliciclastic environments in fairly deep water have never been found associated with carbonate reefs. Between the Trezona fossils and other known animal fossils there is a large time gap and between these animal fossils and Cloudina and Namacalathus, of cnidarian grade, that in the latest Ediacaran re-inhabited thrombolite reefs about 90 My later.
|Author: M.H.Monroe Email: email@example.com Sources & Further reading|