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strata in South America, based upon mentions in publications on other subjects, abstracts presented at conferences and unpublished masters theses.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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