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Impressions of soft-bodied Ediacaran megafossils are common in deep-water slope deposits of the June beds at Sekwi Brook in the Mackenzie Mountains of NW Canada. Two taphonomic assemblages can be recognized. Soles of turbidite beds contain numerous impressions of simple (Aspidella) and tentaculate (Hiemalora, Eoporpita) discs. A specimen of the frond Primocandelabrum is attached to an Aspidella-like holdfast, but most holdfast discs lack any impressions of the leafy fronds to which they were attached, reflecting Fermeuse-style preservation of the basal level of the community. Epifaunal fronds (Beothukis, Charnia, Charniodiscus) and benthic recliners (Fractofusus) were most commonly preserved intrastratally on horizontal parting surfaces within turbidite and contourite beds, reflecting a deep-water example of Nama-style preservation of higher levels in the community. A well-preserved specimen of Namalia significantly extends the known age and environmental range of erniettomorphs into deep-water aphotic settings. Infaunal bilaterian burrows are absent from the June beds despite favorable beds for their preservation. The June beds assemblage is broadly similar in age and environment to deep-water Avalonian assemblages in Newfoundland and England, and like them contains mainly rangeomorph and arboreomorph fossils and apparently lacks dickinsoniomorphs and other clades typical of younger and shallower Ediacaran assemblages. Fossil data presently available imply that the classically deep- and shallow-water taxa of the Ediacara biota had different evolutionary origins and histories, with sessile rangeomorphs and arboreomorphs appearing in deep-water settings approximately 580 million years ago and spreading into shallow-water settings by 555 Ma but dickinsoniomorphs and other iconic clades restricted to shallow-water settings from their first known appearance at 555 Ma until their disappearance prior to the end of the Ediacaran.
Metazoans (multicellular animals) evolved during the Ediacaran Period as shown by the record of their imprints, carbonaceous compressions, trace fossils, and organic bodies and skeletal fossils. Initial evolutionary experiments produced unusual bodies that are poorly understood or conceived of as non-metazoan. It is accepted that sponges, ctenophorans, cnidarians, placozoans, and bilaterians were members of the Ediacaran fauna, many of which have uncertain affinities. The fossil Sabellidites cambriensis Yanishevsky, 1926, derived from the terminal Ediacaran strata, is the earliest known organically preserved animal that belonged to a newly evolving fauna, which replaced the Ediacara-type metazoans. Morphologically simple soft-bodied tubular fossils, such as S. cambriensis, and biomineralized, as contemporaneous Sinotubulites sp., are not easy to recognize phylogenetically because many unrelated organisms developed encasing tubes independently. Therefore, in addition to morphologic information, evidence derived from the microstructure of the organic wall and its biochemistry may be vital to resolving fossil origins and phylogenetic relationships. Here we present morphological, microstructural and biogeochemical studies on S. cambriensis using various microscopic and spectroscopic techniques, which provide new evidence that supports its siboglinid, annelidan affinity. The late Ediacaran age of Sabellidites fossil constrains the minimum age of siboglinids and the timing of the divergence of including them annelids by fossil record and this could be tested using molecular clock estimates. The fine microstructure of the organic tube in Sabellidites is multi-layered and has discrete layers composed of differently orientated and perfectly shaped fibers embedded in an amorphous matrix. The highly ordered and specific pattern of fiber alignment (i.e., the texture of organic matter) is similar to that of representatives of the family Siboglinidae. The biogeochemistry of the organic matter that comprised the tube, which was inferred from its properties, composition, and microstructure, is consistent with chitin and proteins as in siboglinids.
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.
Here we reconstruct a new tubular, serially divided organism with a bilateral morphology from the Ediacaran of South Australia. The organism, Plexus ricei new genus new species, was a broadly curving tube that resided on the Ediacaran seafloor. Plexus ricei individuals range in size from 5 to 80 cm long and 5 to 20 mm wide, and are comprised of two main components: a rigid median tubular structure and a fragile outer tubular wall. Plexus ricei is preserved as an external mold on bed soles, and as a counterpart cast on bed tops in sandstones interpreted to represent deposition between storm and fairweather wave-base. The phylogenetic affinities of P. ricei are uncertain; P. ricei symmetry implies a bilaterian origin, but a lack of defined anterior and posterior ends precludes definitive assignment.
Elainabella deepspringensis new genus new species is a one-mm-wide, non-biomineralized, three-dimensionally preserved fossil with segmented branches and apparent cellular structure. A single specimen was recovered from an interval of black shale within the Ediacaran portion of the Esmeralda Member of the Deep Spring Formation at Mt. Dunfee in Esmeralda County, Nevada. We interpret the fossil to be the thallus of a multicellular alga of uncertain division. EDS spectral analysis indicates that the exceptional preservation is not due to phosphatization or pyritization. Rather, it appears to be a case of Burgess Shale-type preservation, involving the kerogenization of non-mineralizing organisms. The fossil-bearing shale is closely associated with stromatolites, and we suggest that E. deepspringensis may have been an epibiont on stromatolites or other firm substrates. This is the first multicellular alga and the first occurrence of Burgess Shale-type preservation reported from the Ediacaran of Laurentia.
When each of the Avalon-, Ediacara-, and Nama-type fossil assemblages are tracked through geological time, there appear to be changes in species composition and diversity, almost synchronized between different sedimentary environments, allowing a subdivision of the late Ediacaran into the Redkinian, Belomorian and Kotlinian geological time intervals. The Redkinian (580–559 Ma) is characterized by first appearance of both eumetazoan traces and macroscopic organisms (frondomorphs and vendobionts) in a form of Avalon-type communities in the inner shelf environment, whereas coeval Ediacara-type communities remained depauperate. The Belomorian (559–550 Ma) is marked by the advent of eumetazoan burrowing activity in the inner shelf, diversification of frondomorphs, migration of vendobionts from the inner shelf into higher energy environments, and appearance of tribrachiomorphs and bilateralomorphs. Ediacaran organisms formed distinctive ecological associations that coexisted in the low-energy inner shelf (Avalon-type communities), in the wave- and current-agitated shoreface (Ediacara-type communities), and in the high-energy distributary systems (Nama-type communities). The Kotlinian (550–540 Ma) witnessed an expansion of the burrowing activity into wave- and current-agitated shoreface, disappearance of vendobionts, tribrachiomorphs and bilateralomorphs in wave- and current-agitated shoreface, together with a drop in frondomorph diversity. High-energy distributary channel systems of prodeltas served as refugia for Nama-type communities that survived until the end of the Ediacaran and disappeared when the burrowing activity reached high-energy environments. This pattern is interpreted as an expression of ecosystem engineering by eumetazoans, with the Ediacaran organisms being progressively outcompeted by bilaterians.
Ediacara fan-shaped sets of paired scratches Kimberichnus teruzzii from the Ediacara Member of the Rawnsley Quartzite, South Australia, and the White Sea region of Russia, represent the earliest known evidence in the fossil record of feeding traces associated with the responsible bilaterian organism. These feeding patterns exclude arthropod makers and point to the systematic feeding excavation of seafloor microbial mats by large bilaterians of molluscan grade. Since the scratch traces were made into microbial mats, animals could crawl over previous traces without disturbing them. The trace maker is identified as Kimberella quadrata, whose death masks co-occur with the mat excavation traces in both Russia and South Australia. The co-occurrence of animals and their systematic feeding traces in the record of the Ediacara biota supports previous trace fossil evidence that bilaterians existed globally before the Cambrian explosion of life in the ocean.
Here we describe large, complex trace fossils in the late Ediacaran Omkyk Member of the Zaris Formation, Nama Group, southern Namibia. The horizontal trace fossils are preserved on a number of talus blocks from a bedding plane of a cm-thick sandstone lens from a single stratigraphic horizon less than 100 m below an ash bed dated at 547.3 ± 0.7 Ma. The forms consist of overlapping U-shaped spreiten elements with parallel limbs surrounded by an outer tube. Individual U-shaped elements are 0.2 to 1 cm in diameter, the outer tube is less than 3 mm in diameter, and the forms as a whole range from 5 to 30 cm long and 3 to 10 cm wide. The specimens commonly show a change in direction and change in diameter. The morphology of these trace fossils is comparable to backfill structures, particularly specimens of Paleozoic Zoophycos from shallow water environments. Here we interpret these horizontal spreiten-burrows to record the grazing of the trace-maker on or below a textured organic surface. The identification of large late Ediacaran trace fossils is consistent with recent reports of backfilled horizontal burrows below the Precambrian–Cambrian boundary and is suggestive of the appearance of complex feeding habits prior to the Cambrian trace fossil explosion.