Sunday 13 January 2019

Reconstructing the Ediacaran Nilpena ecosystem.

The first complex ecosystems appear towards the end of the Ediacaran Period (about 635 to 541 million years ago), in a series of ecosystems known collectively as the Ediacaran Fauna. This fauna (and the period named after it) takes its name from the Ediacara Member of the Adelaide Rift Complex, in South Australia, a succession of sedimentary and volcanic rocks of Cryogenian to Cambrian age which outcrop across the Flinders and Mount Lofty ranges. Within this group, the Ediacara Member of the Rawsley Quartzite Formation produces numerous Ediacaran fossils (which take their name from this strata), laid down in a deep, outer shelf environment, below the influence surface waves, but where they were regularly buried by turbidites, submarine landslips likely to have been caused by storms further up the slope. One of the most spectacular Ediacara Member outcrops found to date is at Nilpena, on the western margin of the Flinders Ranges, where around 300 m³ of Ediacaran seafloor, with accompanying fauna is preserved at a single locality.

In a paper published in the journal Palaeogeography, Palaeoclimatology, Palaeoecology on 8 Setember 2018, Mary Droser of the Earth Science Department at the University of California, Riverside, James Gehling of the South Australian Museum, Lidya Tarhan of the Department of Geology and Geophysics at Yale University, Scott Evans and Christine Hall, also of the Earth Science Department at the University of California, Riverside, Ian Hughes of the Riverside STEM Academy, Emily Hughes of Wesleyan University, Mary Dzaugis of the Museum of Science in Boston, Matthew Dzaugis of the NOAA Climate Program Office, Peter Dzaugis of Holden Massachusetts, and Dennis Rice, also of the South Australian Museum, resent a detailed study of the Nilpena ecosystem.

Droser et al. found the beds of the Ediacara Member at Nilpena can be divided into four facies (a facies is a set of characteristics found in different beds, which can be used to imply similar origins), which repeat in sequence.

Regional (A) and facies-scale (B) stratigraphy of the Ediacara Member at Nilpena. Fossiliferous facies of the Ediacara Member record a range of shallow marine environments. Droser et al. (2018).

At the base of each sequence the  Flat-Laminated to Linguoid-Rippled Sandstone Facies comprises poorly sorted, laminated, silty sandstones which fine upwards (i.e.the average size of the sand particles gets smaller towards the top of the beds). This facies is thought to have been laid down on a rapidly deposited in a delta-front near the wave base, and to have to mark the onset of a transgression (a time of rising sea levels) in which these beds were laid down on what had previously been an air-exposed eroding surface.

Next in the sequence the Oscillation-Rippled Sandstone Facies comprises  thin-bedded, rippled, fine- to coarse-grained sandstones thought to have been below the fair-weather wave base, but above the storm wavebase, and contains fossils of the White Sea Fauna, a collection of simple frond-like organisms that appeared in shallow waters.

A Oscillation-Rippled Sandstone Facies bed flipped (left) and counterpart ripples in pit (right). Droser et al. (2018).

Above this the Planar-Laminated and Rip-Up Sandstone Facies consists of laterally continuous, planar-laminated, fine-grained sandstone beds thought to have formed in the upper part of sub-wave base canyons. The beds of this facies contain fossils of the Avalon Fauna, which is more diverse than the White Sea Fauna and interpreted as having lived at greater depth.

Finally the Channelized Sandstone and Sand-Breccia Facies comprises lenticular, discontinuous bodies of medium- to coarse-grained sandstone in which thick ball-and-pillow beds grade up into massive sandstone beds, thought to have formed in the lower part of sub-wave base canyons. The beds of this facies contain fossils associated with the Nama Assemblage, a more complex assemblage associated with shallower waters, thought to have appeared towards during the last 10 million years of the Ediacaran Period (although this represents a deep water environment, its place in the succession suggests it was the last facies to be laid down in an environment where the waters were getting deeper, and it appears to contain material washed down from further upslope).

Only two fossils are found in all of these facies, Dickinsonia, a bilaterally symmetrical fossil generally interpreted as a motile animal, and Aspidella, a holdfast (stem) probably produced by a range of stalked animals.

Dickinsonia draped over Funisia (arrows). Droser et al. (2018).

All of the beds producing fossils are wholly or partially covered by textured organic surfaces, organosedimentary textures produced by by Bacterial and Algal mats, suggesting that the Ediacaran macrofauna were part of ecologically rich communities of organisms. These range from simple biofilms made by microbes to the impressions of densely packed macroscopic, eukaryotic tubular organisms, such as the macrofossil Funisia, and upright organism variously interpreted as a Sponge a Tube-Worm or and Algae. These dense mats of organisms are thought to have stabilised the sea-floor in even quite high-energy environments, allowing the apparently more delicate members of the Ediacara Fauna to settle here.

These mat structures are far more widespread than in Phanerozoic (Cambrian onwards) strata, with thin microbial structures separating even thin laminations in the Oscillation-Rippled Sandstone Facies. In the Planar-Laminated and Rip-Up Sandstone and Channelized Sandstone and Sand-Breccia facies these commonly occur as what appear to be rip-up structures, with individual clasts ranging from fully planar structures to mostly planar with raised lips to three-dimensional casts and molds, suggesting that sections of matting have been torn up elsewhere and washed down the slope.

Most fossil assemblages contain collections of material, such as shell or bone, that have built up over time, as these materials tend to accumulate in environments where other organic matter decays. Even deposits that preserve abundant specimens with soft tissue intact, such as the Messel Shale or Crato Formation, are thought to be assemblages built up over time as animals fall into toxic, anoxic environments, where they cannot be broken down by other organisms. However the soft-bodied nature of the Nilpena fossils (and Ediacaran fossils in general) suggests this is unlikely to be the case, and that the preserved fossils are likely to represent a reasonable reflection of the animals alive at one time, preserved by rapid burial (although the course nature of the sands in which many of these fossils are preserved will have tended to prevent the preservation of smaller organisms).

This can tell us something about the biology of the organisms preserved. For example, in most of the beds containing Dickinsonia these are of mixed sizes, suggesting that these organisms had continuous reproduction and lived in communities with individuals of mixed ages, rather than periodic reproduction, in which members of a species all reproduce at the same time, and cohorts tend to contain specimens of identical size and age. However, this is only possible to observe because of the large number of specimens from different beds present; one Oscillation-Rippled Sandstone Facies bed was found to contain numerous large (i.e. more than 30 cm) Dickinsonia specimens but no smaller ones. This is interpreted as specimens that have been sorted by the action of currents, with the smaller specimens deposited elsewhere, something which is supported by the deformed nature of many of these specimens, which suggests they are not preserved in life position.

Idealized cartoons and photograph of environmental disturbance observed in the Oscillation-Rippled Sandstone Facies. (A) Bed representing a channel-fill deposit with transported Dickinsonia, which contains (B) highly deformed Dickinsonia. Droser et al. (2018).

The tubular Funisia is the most common fossil in both the Oscillation-Rippled Sandstone Facies and the Planar-Laminated and Rip-Up Sandstone Facies. These fossils are almost always preserved in positive relief, unlike most of the Nilpena fossils, suggesting they were fluid filled in life and quickly collapsed when they died, enabling sand to enter their body cavities. Funisia forms densely packed beds, reaching over 1000 individuals per square meter, with specimens reaching up to 30 mm in length and 8 mm in width, with individuals on the same bedding plane typically all of similar size. Many beds contain nothing but densely packed Funisia and the stems of Aspidella, suggesting that once this association was established it was able to exclude other organisms by covering the substrate. However, one Oscillation-Rippled Sandstone bed was found in which Funisia was present without Aspidella, and had apparently been toppled then colonised by other organisms, particularly Dickinsonia, providing a rare example of ecological succession within the Nilpena beds.

Examples of Ediacara Member beds and fossils. (A) Funisia-dominated surfaces, with arrow indicating well preserved Funisia. (B) Aspidella (top left arrow), on a Funisia (right, arrows) dominated surface. Droser et al. (2018).

The tri-radial Tribrachidium is also abundant on the Oscillation-Rippled Sandstone Facies  beds, with two beds dominated by this fossil, where it makes up 60% and 80% of the fossils present, respectively. The beds that produce Tribrachidium seem to be ecologically diverse, with Ediacaran fossils such as Aspidella, Coronacollina, Dickinsonia, Rugoconites, Parvancorina, and Spriggina, as well as Algae and trace fossils such as Plexus and Aulozoon. All of these beds are accompanied by Algal matting, but the two beds where Tribrachidium is dominant show different types of matting, with one showing thick, continuous matting and the other thin, discontinuous mats. Together this is taken to indicate that Tribrachidium was an ecological generalist. Furthermore, all of the Tribrachidium containing beds contain individuals of the same size, suggesting that Tribrachidium reproduced periodically, i.e. all at the same time.

Bed with different taxa (here, an Aulozoon (arrow), along with a Tribrachidium), and a textured organic surface cover consisting of discrete patches of 'micropucker'. Droser et al. (2018).

Several beds are also dominated by a 'Bundle of Filaments' fossil, which has also been seen at other Ediacaran fossil producing sites in South China, and which has been interpreted as an Algae. Interestingly one of these beds produces numerous small fossils of taxa such as Dickinsonia and Spriggina, which may indicate that these Algal beds served as a sort of nursery for young, motile Ediacarans.

Bundle of Filaments fossil from Nilpena. Droser et al. (2018).

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