Organic-walled microfossils from the Ediacaran Sete Lagoas Formation, Bambuí Group, Southeast Brazil: taxonomic and biostratigraphic analyses

Abstract

This work presents a detailed taxonomic study on organic-walled microfossils from the Ediacaran Sete Lagoas Formation (Bambuí Group) at the Barreiro section in the Januária area of the São Francisco basin, Brazil. Seven species are described, including Siphonophycus robustum (Schopf, 1968), Ghoshia januarensis new species, Leiosphaeridia crassa (Naumova, 1949), Leiosphaeridia jacutica (Timofeev, 1966), Leiosphaeridia minutissima (Naumova, 1949), Leiosphaeridia tenuissima Eisenack, 1958, and Germinosphaera bispinosa Mikhailova, 1986. These taxa are recovered for the first time in the Sete Lagoas Formation. They occur abundantly in the lower portion of the studied section, but only Ghoshia januarensis is present in the upper part of the studied section, probably due to environmental or taphonomic changes. Leiosphaeridia species, particularly Leiosphaeridia minutissima, dominate the organic-walled microfossil assemblage. Although most taxa described here have long stratigraphic ranges, they are consistent with a terminal Ediacaran age as inferred from detrital zircon data and tubular fossils (e.g., Cloudina and Corumbella) from the Sete Lagoas Formation.

UUID: http://zoobank.org/7f92b900-0176-4da6-93a3-fd51edb22cbf

Non-technical Summary

Seven species occur in shallow-marine limestone of the Sete Lagoas Formation, Bambuí Group, in Januária, Brazil, including Siphonophycus robustum, Leiosphaeridia crassa, Leiosphaeridia jacutica, Leiosphaeridia minutissima, Leiosphaeridia tenuissima, Germinosphaera bispinosa, and a new species named Ghoshia januarensis. In the lower part of the studied section, these occurrences are common, but only Ghoshia januarensis is found in the upper part. This is likely due to changes in the environment or preservation conditions. The Leiosphaeridia species, especially Leiosphaeridia minutissima, dominates the assemblage of organic-walled microfossils. While most described taxa have long stratigraphic ranges, they are consistent with a terminal Ediacaran age, as indicated by detrital zircon data and tubular fossils (e.g., Cloudina and Corumbella) from the Sete Lagoas Formation.

Introduction

The present work provides an updated taxonomic description and biostratigraphic analysis of organic-walled microfossils from the Sete Lagoas Formation, Bambuí Group, from the countryside of Januária Municipality, Minas Gerais State, Brazil. The studied area is an ancient quarry area with an exposure of nearly 70 m of a continuous succession of mixed carbonate and fine-grained siliciclastic rocks. In addition, the Januária area has been the focus of the chronostratigraphic investigation of the Sete Lagoas Formation since Warren et al. (2014) reported Ediacaran tubular fossils such as Cloudina sp. and Corumbella werneri Hahn et al., 1982 from the lower Sete Lagoas Formation in the Januária area.

Several articles on organic-walled microfossils from the Sete Lagoas Formation have been previously published (Sommer, 1971; Simonetti and Fairchild, 1989, 2000; Fairchild et al., 1996, 2012; Sanchez and Fairchild, 2018), describing dozens of species from this unit (Table 1), although Sanchez and Fairchild (2018) invalidated one species, Bambuites erichsenii Sommer, 1971. It is important to underscore that the investigation conducted by Fairchild et al. (1996) centered on the analysis of silicified carbonaceous microfossils in petrographic thin sections from the Sete Lagoas Formation in the western portion of the São Francisco craton. Specimens illustrated by Fairchild et al. (1996) were tentatively compared with seven species, alongside several taxa left under open nomenclature. This study provides a detailed taxonomic description with an analysis of the diversity, abundance, and stratigraphic occurrence of organic-walled microfossils from the Sete Lagoas Formation in the Barreiro section in the Januária area.

Table 1. List of microfossils from the Sete Lagoas Formation, Bambuí Group, published previously and in this study. Articles: 1 = Sommer (1971); 2 = Marchese (1974); 3 = Simonetti and Fairchild (1989); 4 = Fairchild et al. (1996); 5 = Simonetti and Fairchild (2000); 6 = Fairchild et al. (2012); 7 = Warren et al. (2014); 8 = Perrella Júnior et al. (2017); 9 = Sanchez and Fairchild (2018); 10 = Denezine et al. (2022); 11 = this study.

The remarkable diversity of organic-walled microfossils in the Ediacaran Period (Knoll, 1994; Vidal and Moczydłowska-Vidal, 1997; Huntley et al., 2006) is hypothesized to be associated with the ecological rise of animals (Peterson and Butterfield, 2005), followed by the advent of skeletonized animals as evidenced by fossil cloudinids and corumbellids (Germs, 1972; Hahn et al., 1982; Hua et al., 2005; Walde et al., 2015; Adôrno et al., 2017). The transition from the Ediacaran to the Cambrian Period is marked by a significant turnover of acritarch species (Anderson et al., 2017; Grazhdankin et al., 2020; Morais et al., 2021). The Cambrian is characterized by a diversification of acanthomorphs species compared with the sphaeromorph dominance in the late Ediacaran (Gaucher and Sprechmann, 2009). Acanthomorphic acritarchs define four acritarch assemblage zones to recognize the lower Cambrian on the East European Platform (Moczydłowska, 1991): Asteridium tornatum–Comasphaeridium velvetum Assemblage Zone, Skiagia ornata–Fimbriaglomerella membranacea Assemblage Zone, Heliosphaeridium dissimilare–Skiagia ciliosa Assemblage Zone, and Volkovia dentifera–Liepaina plana Assemblage Zone. These biostratigraphic units have been used to correlate Cambrian successions around the world (Zheng et al., 2020). Therefore, a systematic study of organic-walled microfossils from the Sete Lagoas Formation not only provides a tool for biostratigraphic correlation but also offers useful data to improve the understanding of Ediacaran evolution.

Geological setting

The Januária Municipality is located in the central part of the São Francisco craton (Fig. 1). The paleogeographic location of the São Francisco craton during the Ediacaran Period has not been precisely constrained, mainly because of poor biostratigraphic and paleomagnetic data. However, Merdith et al. (2021) placed the São Francisco craton in high latitudes during the Ediacaran Period.

Figure 1. (1) Geological map of the São Francisco basin (red dashed line) in the São Francisco craton, showing its relationship with neighboring Neoproterozoic fold belts. Inset map shows major cratons in the western Gondwana in a Neoproterozoic paleogeographic configuration: A = Amazonian craton; P = Rio de la Plata craton; K = Kalahari craton; WA = West Africa craton; SFC = São Francisco-Congo craton; PC = Paramirim Corridor. Modified from Reis and Alkmim (2015). (2) Stratigraphic position of the studied section in the Bambui Group columnar section. (3) Geological map of the studied area. The purple dot marks the location of the studied section.

The western portion of the São Francisco craton comprises a succession of siliciclastic and carbonate rocks dated between 1.77 Ga and 0.56 Ga (Pimentel et al., 2011; Alvarenga et al., 2012). The Brasília fold belt that bounds the western margin of the São Francisco craton was deformed during the Brasiliano–Pan African orogeny between 790 Ma and 540 Ma (Pimentel and Fuck, 1992). This fold belt borders to the east with the São Francisco craton covered with undeformed Neoproterozoic strata. It consists of a tectonic domain where only the upper 2 km strata are deformed and a domain further west where both the basement and sediment cover are deformed (Alvarenga et al., 2014). A thick interval of Mesoproterozoic and Neoproterozoic sedimentary rocks was deposited along the west portion of the São Francisco craton. These strata are separated into three stratigraphic units, including, in ascending stratigraphic order, the Paranoá Group (Barbosa, 1963), Jequitaí Formation (Oliveira and Leonardos, 1943), and Bambuí Group (Rimann, 1917).

The Sete Lagoas Formation, which is the main focus of this work, represents the basal unit of the Bambuí Group and consists of a sequence of carbonate-dominated sediments in the São Francisco basin. Those sediments are characterized by a low total organic carbon (TOC) content of less than 2% (Uhlein et al., 2019; Caetano-Filho et al., 2021) and relatively low thermal maturity (Reis and Suss, 2016). The δ¹³C_carb and δ¹³C_org excursions indicate a disconnection between the São Francisco basin and the global carbon cycles, which would imply marine isolation and paleogeographic shifts driven by the dynamic changes in marginal orogenic systems (Caetano-Filho et al., 2019, 2021; Guacaneme et al., 2021).

The depositional age of the Bambuí Group has long been a matter of debate. The Bambuí Group was initially considered to be Cretaceous (Liais, 1872 in Couto et al., 1981), but recent studies show that it is probably Ediacaran–Cambrian (Pimentel et al., 2011; Warren et al., 2014; Paula-Santos et al., 2015; Moreira et al., 2020; Sanchez et al., 2021; DaSilva et al., 2022). Geochronological constraints on the Bambuí Group are few and inconclusive. Carbonates of the lower Sete Lagoas Formation yielded Pb–Pb apparent ages of ~740 Ma (Babinski et al., 2007). However, Caxito et al. (2021) analyzed samples from crystal-fan-bearing limestone from the base of the Sete Lagoas Formation and obtained U–Pb ages of 615.4 ± 5.9 Ma if both the crystal fans and matrix were considered together, 608.1 ± 5.1 Ma for crystal fans, and 607.2 ± 6.2 Ma for the matrix. The youngest population of detrital zircons from the Sete Lagoas Formation gave U–Pb ages of ~557 Ma (Paula-Santos et al., 2015), and the youngest population of detrital zircons from the Três Marias Formation gave U–Pb ages of ~620 Ma (Rodrigues, 2008; Pimentel et al., 2011), providing maximum age constraints on the host strata. More recently, a zircon U–Pb age of 520.2 ± 5.3 Ma has been reported from a volcanic ash bed in the Serra da Saudade Formation (Moreira et al., 2020), suggesting that the upper Bambuí Group may belong to Stage 2 of the Cambrian System.

The possible occurrence of Cloudina sp. and Corumbella werneri—tubular fossils typically found in terminal Ediacaran rocks—in the lower Sete Lagoas Formation (Warren et al., 2014; Perrella Júnior et al., 2017) and the putative presence of Treptichnus pedum (Seilacher, 1955)—a trace fossil whose first appearance is used to define the base of the Cambrian System—in the Três Marias Formation (Sanchez et al., 2021) also indicate that perhaps the entire Bambuí Group is Ediacaran–Cambrian, although the conflict with the ~740 Ma Pb–Pb age from the Sete Lagoas Formation (Babinski et al., 2007) remains unresolved.

Materials and methods

The studied Barreiro section is located in the Santa Luzia quarry near the Barreiro Community, western Januária Municipality, Minas Gerais State, Brazil (Fig. 1). The samples were collected from two different mining benches, as well as exposures in the hills where the Santa Luzia quarry is located. The stratigraphic thickness of the Sete Lagoas Formation in the studied area is about 70 m (Fig. 2).

Figure 2. Stratigraphic column and field photographs of the Sete Lagoas Formation at the Barreiro section, Santa Luzia quarry, Januária Municipality, Minas Gerais State, Brazil. (1) Thin-bedded limestone. (2) Intraclastic breccia. Sample horizons are marked with the sample number prefixes MP. Sample numbers in bold mark fossiliferous horizons. The CP- numbers refer to the palynological slides of the illustrated specimens.

The lower 15 m of the studied section consists mainly of dark gray, laminated, microcrystalline lime mudstones with a predominance of parallel bedding with microbial mats. However, there are cross-laminations in layers of fine calcareous grainstones. Microbial mats, silicified ooids, and dolomitic nodules are common at this level (Fig. 2). Intraclastic carbonate breccias, with flat pebbles ranging from <1 to 50 cm and light gray micritic matrix are present at 16 m of the section and above, intercalated with limestones. The top of the section, at around 66 m, is composed of light gray, oolitic, crystalline dolomitic grainstones, sometimes with intraclasts. Such carbonates are cross-stratified. This dolomitic interval presents incipient flat stratification, about 2 cm thick, defined by the changes in the amount of sand-size constituents.

A total of 79 stratigraphic levels were sampled. The curatorship of the rock samples, the remaining organic residues, and the palynological slides followed the protocol presented in Denezine et al. (2022). Each residual sample was coded with the MP prefix. All specimens recovered from the Sete Lagoas Formation herein illustrated are coded with the CP prefix. Each illustrated specimen is provided with a slide number followed by England Finder coordinates.

Organic-walled microfossils were extracted from thinly laminated lime mudstones and light- to dark-gray fine-grained limestone samples using acid maceration techniques. The samples were dissolved using hydrochloric and hydrofluoric acids. Residues were rinsed repeatedly in distilled water, and after the residues were settled following each rinse, the supernatant was decanted. No centrifugation was used, to minimize mechanical damage to organic-walled microfossils. No oxidative procedure was applied on organic residues. Transmitted-light photomicrographs were acquired using an Axio Imager.A2 microscope equipped with an AxioCam MRc digital camera (both Carl Zeiss). The organic-walled microfossils were also analyzed using epifluorescence microscopy; however, no fluorescence was observed in the specimens recovered.

Size analysis of Leiosphaeridia specimens is based on the measurement of their vesicle diameters. Vesicle diameter, along with vesicle wall thickness, was used to identify the four morphospecies of Leiosphaeridia present in the Sete Lagoas Formation: Leiosphaeridia crassa (Naumova, 1949), Leiosphaeridia jacutica (Timofeev, 1966), Leiosphaeridia minutissima (Naumova, 1949), and Leiosphaeridia tenuissima Eisenack, 1958.

Abundance data were collected in this study. All palynological slides were examined thoroughly, and complete specimens were counted. Due to their colonial nature or frequent preservation as fragments, the abundance of Siphonophycus robustum (Schopf, 1968) and Ghoshia januarensis n. sp. was not quantified.

Selected organic-walled microfossils from the Sete Lagoas Formation were analyzed using Raman spectroscopy. Specimens were placed on palynological slides and analyzed on a HORIBA JobinYvon LabRAM HR800 Raman microprobe equipped with a high-resolution 600 mm focal length spectrometer and a 514 nm argon laser source in the Department of Geosciences at Virginia Tech. The laser beam was focused to less than 10 μm in diameter with a 40× objective lens. Raman spectra were acquired using the software Labspec 5.0 with an acquisition time of less than one minute for each analysis and an excitation power of 600 mW.

Raman spectroscopy data were processed using Python modules. Baseline correction was applied to the raw data by adjusting a polynomial (third-order) curve using the Raman data from 800 and 2,100 cm^–1 that captures the Raman peaks of carbonaceous material. After baseline correction, the four Raman peaks of carbonaceous material (i.e., D₁, D₂, D₃, D₄) were decomposed using fitting G of Kouketsu et al. (2014).

The processed Raman data were subjected to principal component analysis (PCA) in Python. The peak position, peak height, and full width at half maximum of the four Raman peaks of carbonaceous material (D₁–D₄) were used in PCA. The Python package for PCA is publicly available (Mazoni, 2021), and PCA in this study used the Python modules Numpy (Harris et al., 2020), Scipy (Virtanen et al., 2020), and Rampy (Le Rosq, 2021).

Repository and institutional abbreviation

Types, figured specimens, and other specimens examined in this study are deposited in the Paleontological Collection under the prefix MP in the Museum of Geosciences (MGeo-UnB), University of Brasília, Brasília, Brazil.

Systematic paleontology

The suprageneric taxonomy follows the system of modern cyanobacteria and the informal classification of acritarchs (e.g., Butterfield et al., 1994; Sergeev and Schopf, 2010). Seven organic-walled microfossil species were recovered: Ghoshia januarensis n. sp., Germinosphaera bispinosa Mikhailova, 1986, Leiosphaeridia crassa (Naumova, 1949), Leiosphaeridia jacutica (Timofeev, 1966), Leiosphaeridia minutissima (Naumova, 1949), Leiosphaeridia tenuissima Eisenack, 1958, and Siphonophycus robustum (Schopf, 1968) (Figs. 3, 4). Two of them, Siphonophycus robustum and Ghoshia januarensis, are considered cyanobacteria. Four of them, Leiosphaeridia crassa, Leiosphaeridia jacutica, Leiosphaeridia minutissima, and Leiosphaeridia tenuissima, are sphaeromorph acritarchs traditionally regarded as protists. The phylogenetic affinity of Germinosphaera bispinosa is uncertain.

Figure 3. Organic-walled microfossils from the Sete Lagoas Formation at the Barreiro section. Slide number and England Finder coordinates (in parentheses) are given for each illustrated specimen. (1–3, 7, 10) Leiosphaeridia minutissima: (1) CP962 (S32); (2) CP962 (F48); (3) CP918 (K22); (7) CP964 (P29); (10) CP963 (F33). (4, 8, 11) Germinosphaera bispinosa, all in slide CP917 (EF coordinates: S26, I43, and O28, respectively). (5) Leiosphaeridia jacutica, CP913 (Y23). (6) Leiosphaeridia crassa, CP964 (H29). (9, 13) Leiosphaeridia tenuissima, all in slide CP914 (EF coordinates: Q30 and R23, respectively). (12) Siphonophycus robustum, CP960 (I50).

Figure 4. Ghoshia januarensis n. sp. from the Sete Lagoas Formation in the Barreiro section. (1) Holotype: CP916 (E46). Note dark spots inside cells indicated by white arrows. Yellow arrows indicate slightly deflated and deformed cells. (2–8) Paratypes: (2–4) CP919 (E18); (3) magnified view of the upper right part of (2), showing slightly deflated and deformed cells; (4) magnified view of the lower left part of (2), showing dark spot in terminal cell (arrow); (5, 6) CP919 (J16); (6) dark-field view of the central part of (5), showing a polyhedral cell (arrow in 5). (7) CP919 (J26); note polyhedral cell at branching point. (8) CP920 (N18/3), showing pointed terminal cell (arrow). (9) Specimen identified in a petrographic thin section of the Sete Lagoas Formation at the Barreiro section in the Januária area. Reproduced from Perrella Júnior et al. (2017) with permission.

Kingdom Eubacteria Woese and Fox, 1977
Phylum Cyanobacteria Stanier et al., 1978
Class Hormogoneae Thuret, 1875
Order Oscillatoriales Elenkin, 1949
Family Oscillatoriaceae Kirchner, 1900
Genus Siphonophycus Schopf, 1968

Type species

Siphonophycus kestron Schopf, 1968 (holotype: Paleobotany Collection Harvard University no. 58469, stage coordinates 33.6 × 101.4) from the black chert facies in the middle third of the late Precambrian Bitter Springs Formation, exposed on the south slope of the ridge about 1 mile north of Ross River Tourist Camp (Love's Creek Homestead), 40 miles northeast of Alice Springs, Northern Territory, Australia, by original designation.

Other species

Siphonophycus thulenema Butterfield in Butterfield et al., 1994; Siphonophycus septatum (Schopf, 1968); Siphonophycus robustum (Schopf, 1968); Siphonophycus typicum (Hermann, 1974); Siphonophycus kestron Schopf, 1968; Siphonophycus solidum (Golub, 1979); Siphonophycus punctatum Maithy, 1975; and Siphonophycus gigas Tang et al., 2013.

Original diagnosis by Schopf (1968)

“Thallus broad, tubular, nonseptate, unbranched, commonly quite long, finely rugose in surface texture. Thallus cylindrical, somewhat tapered toward apices, solitary, straight to slightly bent, up to 180 μm long (incomplete specimen), occasionally folded and distorted. Apices apparently capitate, more-or-less constricted adjacent to expanded, broadly conical, bluntly pointed terminus. Thallus quite broad, 8.3–15.00 μm wide, commonly about 12.5 μm wide (based on five specimens), ornamented and ringed by finely punctate surficial ridges regularly spaced out 2/3 μm apart. Reproductive structures unknown.”

Emended diagnosis by Knoll et al. (1991)

“Tubular, filamentous microfossils, nonseptate and unbranched, with little or no tapering toward filament termini; tubes truncated and open at ends or with closed, more or less hemispherical terminations; walls typically preserved as chagrenate to finely reticulate organic matter, but may be preserved as carbonate rinds.”

Remarks

The genus Siphonophycus is characterized by smooth and thin wall filaments without ornamentation. The taxon is traditionally interpreted as representing empty sheaths of filamentous cyanobacteria, but because of simple morphology, it could include a range of bacterial and eukaryotic organisms (Butterfield et al., 1994). Although it is here placed under cyanobacteria, we recognize that Siphonophycus is a form taxon, and several other genera of filamentous microfossils (e.g., Eomycetopsis, Tenuofilum, and Leiotrichoides) are regarded as synonyms of Siphonophycus (Knoll et al., 1991).

Siphonophycus robustum (Schopf, 1968) emend. Knoll et al., 1991
Figure 3.12

1968

Eomycetopsis robusta Schopf, p. 685, pl. 82, figs. 2, 3, pl. 83, figs. 1–4.

1968

Eomycetopsis filiformis Schopf, p. 685, pl. 82, figs. 1, 4, pl. 83, figs. 5–8.

1979

Eomycetopsis robusta; Knoll and Golubic, p.149, fig. 4a, b.

1982

Eomycetopsis robusta; Mendelson and Schopf, p. 59, pl. 1, figs. 9, 10.

1984

Eomycetopsis robusta; Sergeev, p. 436, fig. 2a–ã.

1991

Eomycetopsis robusta; Hofmann and Jackson, p. 367, fig. 5.1–5.3, 5.8.

1991

Siphonophycus robustum (Schopf, 1968); Knoll et al., p. 565, fig. 10.3, 10.5.

1992

Eomycetopsis robusta; Zang and Walter, p. 314, pl. 17, figs. g–I, p. 308, pl. 18, fig. g.

1992

Eomycetopsis robusta; Sergeev, p. 93, pl. 7, figs. 9, 10, pl. 16, figs. 3, 6, 7, 10; pl. 19, figs. 1, 5–10, pl. 24, fig. 7.

1993

Eomycetopsis robusta; Golovenok and Belova, pl. 2, fig. å.

1994

Siphonophycus robustum; Butterfield et al., p. 64, fig. 26a, g.

1994

Siphonophycus robustum; Hofmann and Jackson, p. 10, fig. 11.5.

1994

Siphonophycus robustum; Sergeev, p. 250, fig. 8f.

1994

Siphonophycus robustum; Sergeev et al., pl. 3, fig. 6.

1995

Siphonophycus robustum; Kumar and Srivastava, p. 114, fig. 14c–e.

1995

Siphonophycus robustum; Zang, p. 172, figs. 26a, 32l, m.

1997

Siphonophycus robustum; Sergeev et al., p. 230, fig. 14a.

1998

Siphonophycus robustum; Kumar and Venkatachala, p. 63, fig. 6c.

2001

Siphonophycus robustum; Sergeev, p. 442, fig. 7.8, 7.9.

2001

Siphonophycus robustum; Sergeev and Lee, p. 6, pl. 1, figs. 1, 2, 7, 11, 12.

2001

Siphonophycus robustum; Samuelsson and Butterfield, p. 240, figs. 2b, 9h.

2003

Siphonophycus robustum; Gaucher et al., fig. 6c, d.

2003

Siphonophycus robustum; Gaucher and Germs, fig. 7.12.

2004

Siphonophycus robustum; Sharma and Sergeev, figs. 3c, 4a, 6b, e, 7c, f, 9e, 11f.

2004

Siphonophycus robustum; Sergeev and Lee, pl. 2, fig. 4.

2004

Siphonophycus robustum; Tiwari and Pant, fig. 3i, n.

2005

Siphonophycus robustum; Prasad et al., pl. 1, fig. 7, pl. 5, fig. 12.

2006

Siphonophycus robustum; Sergeev, p. 213, pl. 6, figs. 9, 10, pl. 17, fig. 1, pl. 19, figs. 8, 9, pl. 22, figs. 1, 2, 7, 8, 11, 12, pl. 25, figs. 1, 3, pl. 27, figs. 4, 5, pl. 28, fig. 2, pl. 36, figs. 1, 2, pl. 44, figs. 1–7, 13, pl. 46, figs. 7–10, pl. 48, fig. 4.

2008

Siphonophycus robustum; Kumar and Pandey, fig. 3a, b.

2008

Siphonophycus robustum; Sergeev et al., pl. 6, figs. 1, 5, 6, pl. 9, figs. 1–3, 5–7.

2009

Siphonophycus robustum; Tiwari and Pant, fig. 6a–c.

2009

Siphonophycus robustum; Dong et al., p. 30, fig. 6.12.

2010

Siphonophycus robustum; Sergeev and Schopf, p. 387, fig. 6.4.

2012

Siphonophycus robustum; Sergeev et al., p. 309, pl. 21, figs. 2, 4, 8–10.

2013

Siphonophycus robustum; Pandey and Kumar, p. 504, fig. 4e.

2013

Siphonophycus robustum; Knoll et al., fig. 4c.

2013

Siphonophycus robustum; Tang et al., fig. 13b, m.

2014

Siphonophycus robustum; Babu et al., fig. 3q.

2014

Siphonophycus robustum; Liu et al., fig. 110.1.

2015

Siphonophycus robustum; Vorob'eva et al., fig. 9.14.

2015

Siphonophycus robustum; Tang et al., fig. 18c.

2015

Siphonophycus robustum; Schopf et al., p. 716, fig. 11.11.

2016

Siphonophycus robustum; Porter and Riedman, p. 837, fig. 16.4.

2016

Siphonophycus robustum; Sergeev et al., fig. 8.4.

2016

Siphonophycus robustum; Baludikay et al., fig. 11n.

2017

Siphonophycus robustum; Tang et al., fig. 8a, c, d.

2017a

Siphonophycus robustum; Shi et al., fig. 6.3, 6.5.

2017b

Siphonophycus robustum; Shi et al., p. 721, fig. 3e, f.

2017

Siphonophycus robustum; Javaux and Knoll, p. 212, fig. 5.11.

2017

Siphonophycus robustum; Beghin et al., pl. 3, fig. i.

2017b

Siphonophycus robustum; Sergeev et al., p. 290, fig. 5.10, 5.11.

2019

Siphonophycus robustum; Li et al., fig. 15h.

2019

Siphonophycus robustum; Loron et al., fig. 3f.

2019

Siphonophycus robustum; Arrouy et al., fig. 6f.

2020

Siphonophycus robustum; Knoll et al., p. 6, fig. 3n, o.

2020

Siphonophycus robustum; Arvestål and Willman, p. 22, fig. 10f.

2020

Siphonophycus robustum; Shukla et al., p. 496, fig. 5e.

2021

Siphonophycus robustum; Miao et al., p. 17, fig. 9e.

2022

Siphonophycus robustum; Denezine et al., fig. 11.6.

For additional synonyms, see Butterfield et al. (1994).

Holotype

Paleobotanical collections, Harvard University (thin section Bit. Spr. 10-1, number 58491), from Neoproterozoic Bitter Springs Formation, Amadeus Basin, Australia (Schopf, 1968, pl. 83, fig. 1).

Original diagnosis by Schopf (1968)

“Filaments commonly solitary, occasionally in groups of a few entangled filaments, rarely showing plectenchymatous organization. Lateral walls approximately1/3–3/4 μ thick, markedly coriaceous, coarsely and irregularly granular in surface texture. Filaments up to 135 μ long (incomplete filament), more-or-less regularly cylindrical with a variance in diameter of less than 0.8 μ from the widest to the most narrow portion of the filament; 2.8–4.2 μ in diameter with an average width (20 filaments measured of 3.5 μ). Septate portions of filament vary in length, commonly less than 25 μ long, with filaments commonly constricted or overlapping at the septa; overlapping portions commonly with rounded ends. Reproductive structures unknown.”

Emended diagnosis by Knoll and Golubic (1979)

“Filaments cylindrical; unbranched; tubular (nonseptate); bent, sinuous and tortuous; partially flattened, circular to elliptical in cross section; intertwined to form more or less dense meshworks; long. Surface coarsely to irregularly granular in texture. Occasional cylindrical and evenly spaced inclusions, homogeneously filled with fine-grained carbonaceous matter and centrally located in the ‘bore’ of the tube. Filaments tubular with average diameters expressed as mean ± standard deviation 2.95 ±μm (range 2.0–4.4, n = 60). Occasional long cylindrical inclusions, 1.09 ± 0.36 μm (n = 8) in diameter, 3–4 μm long located centrally within tubular filaments.”

Emended diagnosis by Knoll et al. (2020)

“A species of Siphonophycus with tubes 2–4 μm in cross-sectional diameter.”

Occurrence in the studied section

MP2985, MP2995, MP3040, MP3708, MP3709, and MP3710.

Illustrated specimen

CP960 (3 μm in diameter).

Remarks

Filamentous microfossils from the Sete Lagoas Formation are scarce and restricted to Siphonophycus robustum (Schopf, 1968).

Order Stigonematales Geitler, 1925
Family Capsosiracea Geitler, 1925
Genus Ghoshia Mandal and Maithy in Mandal et al., 1984

Type species

Ghoshia bifurcata Mandal and Maithy in Mandal et al., 1984.

Original diagnosis presented by Mandal and Maithy in Mandal et al. (1984)

“Thallus heterotrichous, erect filaments arising from basal horizontally creeping thallus, densely packed, truly laterally branched, with cells in one or two series; sheath absent; reproduction not observed.”

Ghoshia januarensis new species
Figure 4

2017

Fossil filaments consisting of aligned rounded cells, Perrella Júnior et al., p. 138. fig. 7h.

2022

Goshia sp.; Denezine et al., fig. 11.5.

Type specimens

Holotype: CP916. Paratypes: CP919 and CP920. Specimens are housed in the Research Collection, Museum of Geosciences, Institute of Geosciences, University of Brasília, Federal District, Brazil.

Type locality

Sete Lagoas Formation, Bambuí Group, Santa Luzia quarry, Municipality of Januária, Minas Gerais State, Brazil.

Type horizon

Intraclastic breccia from the Sete Lagoas Formation, Bambuí Group. Stratigraphic level: between 31.5 and 36.4 m.

Diagnosis

A species of Ghoshia characterized by spherical to doliform cells that are 3–10 μm in diameter. Cells are organized to form uniserial chains that branch irregularly.

Occurrence in the studied section

MP2980, MP3013, MP3015, MP3040, MP3710, MP3714, MP3718, MP3723, and MP3724.

Description

Uniserial cell chains that branch irregularly. Cells are spherical (Fig. 4.1), doliform (Fig. 4.2–4.6, 4.9), or polyhedral (Fig. 4.7), with smooth cell walls. Side branches arise more or less perpendicularly to the main branches. Cells at the branching points are often polyhedral (Fig. 4.7). Cells 3–10 μm in diameter. Deformation folds, likely resulting from compression, are present in some cells (Fig. 4.2, 4.7, 4.8).

Etymology

In reference to the Municipality of Januária, Minas Gerais State, Brazil.

Illustrated specimens

CP916, CP919, and CP920.

Remarks

The Sete Lagoas specimens are somewhat similar to Arctacellularia German in Timofeev et al., 1976 in their uniserial filaments consisting of spherical, doliform, and polyhedral cells. However, unlike the Sete Lagoas specimens, Arctacellularia does not branch. The Sete Lagoas specimens are also similar to the Devonian cyanobacteria Langiella Croft and George, 1959, Kidstoniella Croft and George, 1959, and Rhyniella Croft and George, 1959 in having branching filaments. However, these Devonian genera can be distinguished by the presence of morphologically differentiated heterocysts and akinetes or by the presence of a sheath (Croft and George, 1959). The Sete Lagoas specimens are best placed in the genus Ghoshia, which is characterized by branching uniserial filaments consisting of largely undifferentiated cells. The new species proposed here, Ghoshia januarensis, resembles Ghoshia bifurcata Mandal and Maithy in Mandal et al., 1984 in cell size but differs in its more variable cell shape; Ghoshia januarensis has spherical, doliform, and polyhedral cells, whereas Ghoshia bifurcata is said to have “drum-shaped to rectangular” cells (Mandal et al., 1984). In addition, some specimens of Ghoshia bifurcata (including the holotype; Mandal et al., 1984, pl. 4, fig. 30) seem to have cell aggregates that are not uniserially organized.

A specimen from the Sete Lagoas Formation in the Januária area illustrated as “fossil filaments consisting of aligned rounded cells” (Perrella Júnior et al., 2017, fig. 7H) shares the same characteristics of Ghoshia januarensis, including uniserial and branching filaments consisting of spherical cells. Thus, this specimen is here identified as Ghoshia januarensis. It is important to point out that the specimen illustrated in Perrella Júnior et al. (2017) was observed in a petrographic thin section, ruling out the possibility of modern contamination.

Raman spectra of the analyzed microfossils (Fig. 5) display well-developed D₁ and D₂ bands positioned at 1,350 cm^–1 and 1,620 cm^–1, respectively. These characteristics are typical of organic matter spectra (Kouketsu et al., 2014). The Raman data show that the four analyzed specimens of Ghoshia januarensis, including the holotype, are distinct from other organic-walled microfossils from the Sete Lagoas Formation (Fig. 5). Relative to other organic-walled microfossils from the Sete Lagoas Formation, Ghoshia januarensis specimens exhibit broader peaks (Fig. 5.1). PCA analysis of Raman parameters also shows that Ghoshia januarensis specimens are separate from other organic-walled microfossils from the Sete Lagoas Formation (Fig. 5.2). Therefore, it is possible that specimens of Ghoshia januarensis have different thermal history from other organic-walled microfossils in the Sete Lagoas Formation, indicating that the former could be contaminations. However, the three specimens of Ghoshia januarensis that were analyzed for Raman spectroscopy in Fig. 5.2 overlap with the other organic-walled microfossils largely along the second PCA axis; the difference is mainly along the primary PCA axis. A similar situation is found in the four specimens of Leiosphaeridia minutissima that were analyzed for Raman spectroscopy (labeled as A, B, G, and H in Fig. 5.2): they exhibit a limited range and overlap with other organic-walled microfossils from the Sete Lagoas Formation along the primary PCA axis, but two specimens (labeled A and B in Fig. 5.2) are distinct from all other specimens along the secondary PCA axis. Although subtle differences in carbonaceous material Raman characteristics could be taken as evidence for different degrees of thermal maturation (Kouketsu et al., 2014), recent studies show that such differences can result from differences in organic precursors (Qu et al., 2015; Pang et al., 2020). Considering that Ghoshia januarensis specimens showed no fluorescence as would modern organic contaminations, and that Ghoshia januarensis has been found in a petrographic thin section of the Sete Lagoas Formation (Perrella Júnior et al., 2017), we conclude that Ghoshia januarensis is indigenous to the Sete Lagoas Formation.

Figure 5. Raman spectroscopic data of organic-walled microfossils and amorphous organic matter from the Sete Lagoas Formation at the Barreiro section. (1) Baseline-corrected and fitted Raman spectra. Legends are shown in Figure 5.2. Note that Raman spectra of Ghoshia januarensis (J from holotype and I, K from paratypes) have broader peaks of carbonaceous matter around 1,350 cm^–1 and 1,600 cm^–1 relative to other Sete Lagoas organic-walled microfossils. (2) Principal component analysis of deconvolved Raman data. Samples: A–B and J, CP916; C–D, CP917; E, I, K, CP920; F, H, MP3728; G, MP3723.

Group Acritarcha Evitt, 1963
Subgroup Sphaeromorphitae Downie et al., 1963
Genus Leiosphaeridia Eisenack, 1958

Type species

Leiosphaeridia baltica Eisenack, 1958 (in Eisenack, 1958b).

Other species

Fensome et al. (1990) revised all Leiosphaeridia species and listed 167 valid species.

Original diagnosis presented by Eisenack (1958b) in German

“Hohlkugelförmige, dünnwandige und aus einer sehr widerstandsfähigen, hellgelb bis dunkelrotbraun durchscheinenden organischen Substanz bestehende Organismenreste, die oft in scheibenförmig zusammengepreßtem Zustande oder auch unregelmäßig verfaltet überliefert sein können. Wand, auch in erwachsenem Zustande, stets ohne Wandporen (Unterschied zu Tasmanites). Pylome vorhanden.”

Translation of original diagnosis presented by Eisenack (1958b)

Hollow spherical, thin-walled organism residues consisting of a very resistant, light yellow to dark red-brown translucent, organic substance, which can often be preserved as a disc-shaped compressed state or an irregularly folded structure. Wall, even when fully grown, always without wall pores (in contrast to Tasmanites). Pylome present.

Emended diagnosis by Downie and Sarjeant (1963)

“Spherical to ellipsoidal bodies without processes, often collapsed or folded, with or without pylomes. Walls granular, punctate or unornamented, thin. Without divisions into fields and without transverse or longitudinal furrows or girdles.”

Emended diagnosis by Jankauskas et al. (1989) in Russian

“Сфероидальные оболочки с гладкой, точеечной или зеррнистой поверхиостью размером от 2–3 до 750 мкм. Толщина стенки от долей микрометра до 3–10 мкм. В ископаемом состоянии сплющены и осложнены складками смятия различной формы и размеров.”

Translation of emended diagnosis by Jankauskas et al. (1989)

Spheroidal vesicle with a smooth, punctate, or granular surface ranging in size from 2–3 to 750 μm. The wall thickness varies from fractions of a micrometer to 3–10 μm. The specimens are flattened and can have compressional folds of various shapes and sizes.

Remarks

A great number of species of the genus Leiosphaeridia have been reported from the Proterozoic, and many of them have very long stratigraphic ranges, e.g., from the Paleoproterozoic to the Mesozoic (Lamb et al., 2009). There are even reports of Leiosphaeridia species from the Miocene (Hannah et al., 2000). Because of its simple morphologies, the genus Leiosphaeridia is regarded as a form taxon with diverse phylogenetic affinities, and it is classified in the Acritarcha (Jankauskas et al., 1989; Grey, 2005; Sergeev and Schopf, 2010), although Sergeev and Schopf (2010) consider this taxon as belonging to the Kingdom Protista, a proposition followed here. It is important to point out that some authors relate Leiosphaeridia species to chlorophyceaens (Moczydlowska et al., 2011; Moczydłowska, 2016). Downie and Sarjeant (1963) emended the diagnosis of the genus Leiosphaeridia to exclude the reference of the vesicle color since it could reflect diagenetic features. Moreover, the maceration protocol could affect the color of organic vesicles due to the use of oxidizing solutions. Jankauskas et al. (1989) specified that the diameter of the vesicle of Leisophaeridia species ranges from 2–3 to 750 μm. Furthermore, Jankauskas et al. (1989) divided the smooth-walled Leiosphaeridia species into four species according to vesicle diameter and wall thickness, a form-taxonomical scheme followed in the present work. Butterfield et al. (1994) suggested that Leiosphaeridia should be restricted to spheroidal fossils with vesicle walls less than 2 μm thick so that it can be differentiated from Chuaria circularis Walcott, 1899, which has thicker vesicle walls (2–3 μm single-wall thickness).

Leiosphaeridia crassa (Naumova, 1949) Jankauskas in Jankauskas et al., 1989
Figure 3.6

1949

Leiotriletes crassus Naumova, p. 54, pl. 1, figs. 5, 6, pl. 2, figs. 5, 6.

1973

Leiopsophosphaera crassa; Pykhova, p. 99, pl. 2, fig. 3.

1989

Leiosphaeridia crassa (Naumova, 1949) Jankauskas in Jankauskas et al., p. 75, pl. 9, figs. 5–10.

1992

Leiosphaeridia crassa; Zang and Walter, p. 289, pl. 9, figs. a–k, pl. 12, fig. k, pl. 14, figs. e, h.

1994

Leiosphaeridia crassa; Butterfield et al., p. 40, figs. 16f, 23k.

1994

Leiosphaeridia crassa; Hofmann and Jackson, p. 22, fig. 1.19–1.29.

1994

Leiosphaeridia crassa; Knoll, fig. 4b.

1995

Leiosphaeridia crassa; Zang, p. 166, figs. 21d, 28c, d.

1999

Leiosphaeridia crassa; Yin and Guan, p. 131, figs. 3.8, 4.5, 5.3, 5.5, 5.7, 5.11, 6.2–6.6, 6.9, 6.12.

2004

Leiosphaeridia crassa; Javaux et al., fig. 4e–i.

2004

Leiosphaeridia crassa; Sergeev and Lee, p. 21, pl. 3, figs. 4, 5.

2004

Leiosphaeridia crassa; Tiwari and Pant, p. 1736, fig. 3v.

2005

Leiosphaeridia crassa; Grey, p. 179, figs. 63a–c, 64a–d.

2005

Leiosphaeridia crassa; Marshall et al., fig. 1e.

2005

Leiosphaeridia crassa; Prasad et al., pl. 1, figs. 1, 2, pl. 4, fig. 16, pl. 5, fig. 18, pl. 9, figs. 10, 11.

2006

Leiosphaeridia crassa; Javaux and Marshal, fig. 3.4–3.6.

2006

Leiosphaeridia crassa; Sergeev and Seong-Joo, p. 15, pl. 2, figs. 2a–c, 5.

2008a

Leiosphaeridia crassa; Moczydłowska, p. 84, figs. 7a, 8g.

2008b

Leiosphaeridia crassa; Moczydłowska, fig. 2g.

2008

Leiosphaeridia crassa; Sergeev et al., pl. 7, figs. 5, 6.

2009

Leiosphaeridia crassa; Yin et al., figs. 3a, 3h, 3l, 4d, 4f, 4h, 5a, 5c.

2009

Leiosphaeridia crassa; Tiwari and Pant, figs. 7d, e, 8h, 8o, p.

2009

Leiosphaeridia crassa; Stanevich et al., p. 32, pl. 3, figs. 3, 4.

2010

Leiosphaeridia crassa; Sergeev and Schopf, p. 395, fig. 15.3–15.6.

2011

Leiosphaeridia crassa; Strother et al., fig. 1a, e.

2011

Leiosphaeridia crassa; Couëffé and Vecolii, figs. 6.2, 7.1, 7.7.

2013

Leiosphaeridia crassa; Tang et al., fig. 4b.

2014

Leiosphaeridia crassa; Lottaroli et al., fig. 10.2.

2014

Leiosphaeridia crassa; Babu et al., fig. 3f.

2015

Leiosphaeridia crassa; Tang et al., fig. 4d.

2015

Leiosphaeridia crassa; Nagovitsin and Kochnev, fig. 1.55, 1.56.

2016

Leiosphaeridia crassa; Baludikay et al., fig. 8a–c.

2016

Leiosphaeridia crassa; Porter and Riedman, p. 833, fig. 13.2, 13.6.

2016

Leiosphaeridia crassa; Sergeev et al., fig. 4.2.

2017

Leiosphaeridia crassa; Javaux and Knoll, p. 209, fig. 4.6.

2017

Leiosphaeridia crassa; Agic et al., p. 110, fig. 8a–c.

2017a

Leiosphaeridia crassa; Sergeev et al., fig. 3.14.

2017b

Leiosphaeridia crassa; Sergeev et al., pl. I, fig. 6.

2017

Leiosphaeridia crassa; Beghin et al., pl. 2, figs. c, d.

2017

Leiosphaeridia crassa; Suslova et al., fig. 3.1–3.4.

2019

Leiosphaeridia crassa; Anderson et al., p. 510, fig. 8a–e.

2018

Leiosphaeridia crassa; Riedman et al., fig. 5.15.

2019

Leiosphaeridia crassa; Arrouy et al., fig. 6d, e.

2019

Leiosphaeridia crassa; Li et al., fig. 4f.

2020

Leiosphaeridia crassa; Arvestål and Willman, p. 11, fig. 6j, k, m.

2020

Leiosphaeridia crassa; Knoll et al., p. 6, fig. 3g.

2020

Leiosphaeridia crassa; Shukla et al., p. 502, fig. 6g.

2020

Leiosphaeridia crassa; Pang et al., fig. 2m.

For additional synonyms, also see Jankauskas et al. (1989) and Zang and Walter (1992).

Type material

Naumova (1949) did not designate a holotype for Leiotriletes crassus. Subsequently, Jankauskas in Jankauskas et al. (1989) designated one specimen of Leiotriletes crassus published by Naumova (1949) as a “holotype” (Naumova, 1949, pl. 1, fig. 3). In addition, he designated another specimen from a different locality and a different stratigraphic unit as a “lectotype” (Jankauskas et al., 1989, LitNIGRI, N 16-800-2942/9, specimen 2, table 9, fig. 5). By so doing, the selection of a holotype by Jankauskas can, according to the International Code of Nomenclature for Algae, Fungi, and Plants, be taken as the designation of a lectotype (Turland et al., 2018). In addition, the specimen designated by Jankauskas as a “lectotype” should be regarded as a neotype. According to the same code, a lectotype always takes precedence over a neotype. However, the lectotype designated by Jankauskas was a specimen of Leiotriletes simplicissimus (Naumova, 1949), a species he synonymized with a different species of Leiosphaeridia, Leiosphaeridia minutissima. Thus, the lectotype designated by Jankauskas is not valid, and the neotype designated by Jankauskas in Jankauskas et al. (1989) is here considered the valid type specimen of Leiosphaeridia crassa.

Original diagnosis presented by Naumova (1949) in Russian

“В очертании спора округлой или округло-овальной формы. Поверхность экзины гладкая, экзина очень толстая и плотная. Форма имеет складки смятия, щель разверзания, простая. Широко распространена в нижнем кембрии Прибалтики.”

Translation of original diagnosis presented by Naumova (1949)

In outline, the spore is round or round–oval. The surface of the exine is smooth, very thick, and dense. The form has compressional folds, an opening gap, and is simple. Widespread in the lower Cambrian of the Baltic.

Emended diagnosis by Javaux and Knoll (2017) and Knoll et al. (2020)

“A species of Leiosphaeridia with smooth, pliant walls with lanceolate folds and a modal diameter of less than 70 μm.”

Occurrence in the studied section

Fourteen specimens were recovered. They range from ~18 to ~62 μm in diameter: MP3719 and MP3720.

Illustrated specimen

CP964 (diameter ~27 μm).

Remarks

Leiotriletes crassus Naumova, 1949 was originally published with only a description, without a diagnosis. Although the International Code of Nomenclature for Algae, Fungi, and Plants states that either a description or a diagnosis is sufficient for the valid publication of a name (Turland et al., 2018, Art. 38.1), it is strongly recommended that both the diagnosis and description be presented when describing a new species (Hassemer et al., 2020). Later, Jankauskas in Jankauskas et al. (1989) reviewed some species of Leiosphaeridia and transferred Leiotriletes crassus Naumova, 1949 to the genus Leiosphaeridia. When Leiotriletes crassus was transferred to the genus Leiosphaerida, the species epithet was changed to crassa, so the gender of the epithet agrees with the gender of the genus name. Thus, this species became Leiosphaeridia crassa (Naumova, 1949) Jankauskas in Jankauskas et al., 1989. In addition, Jankauskas et al. (1989) did not include in their synonym list the species Leiopsophosphaera crassa Pykhova, 1973. Finally, Fensome et al. (1990), also in a work of taxonomic revision, transferred Leiopsophosphaera crassa Pykhova, 1973 to Leiosphaeridia crassa (Pykhova, 1973). However, Fensome et al. (1990) did not consider the study of Jankauskas et al. (1989) and regarded Leiotriletes crassus Naumova, 1949 as taxonomically uncertain (Grey, 2005). Thus, Leiosphaeridia crassa (Pykhova, 1973) is a junior homonym of Leiosphaeridia crassa (Naumova, 1949) Jankauskas in Jankauskas et al., 1989. Nonetheless, Leiopsophosphaera crassa Pykhova, 1973 is considered by several authors (Yin and Guan, 1999; Grey, 2005) as a synonym of Leiosphaeridia crassa (Naumova, 1949), a synonymy followed in this study. Leiosphaeridia crassa differs from Leiosphaeridia minutissima in its thicker vesicle wall, and it differs from Leiosphaeridia tenuissima and Leiosphaeridia jacutica in vesicle size (Jankauskas et al., 1989).

Leiosphaeridia jacutica (Timofeev, 1966) Mikhailova and Jankauskas in Jankauskas et al., 1989
Figure 3.5

1966

Kildinella jacutica Timofeev, p. 30, pl. 7, fig. 2, pl.19, fig 9, pl. 61, fig. 5, pl. 67, fig. 8, pl. 72, fig. 1.

1989

Leiosphaeridia jacutica (Timofeev, 1966) Mikhailova and Jankauskas in Jankauskas et al., p. 77, pl. 12, figs. 3, 7, 9.

1992

Leiosphaeridia jacutica; Butterfield and Chandler, fig. 5e.

1994

Leiosphaeridia jacutica; Butterfield et al., p. 42, fig. 16h.

1994

Leiosphaeridia jacutica; Hofmann and Jackson, p. 22, fig. 17.1–17.4.

1995

Leiosphaeridia jacutica; Kumar and Srivastava, p. 106, fig. 11k.

2001

Leiosphaeridia jacutica; Sergeev, p. 444, fig. 8.7–8.10.

2004

Leiosphaeridia jacutica; Javaux et al., fig. 4a–d, 4m.

2005

Leiosphaeridia jacutica; Grey, p. 183, fig. 63g.

2005

Leiosphaeridia jacutica; Marshall et al., fig. 1c.

2005

Leiosphaeridia jacutica; Prasad et al., pl. 3, figs. 13, 14, pl. 4, fig. 12, pl. 9, fig. 25, pl. 10, fig. 6.

2006

Leiosphaeridia jacutica; Sergeev and Seong-Joo, p. 14, pl. 2, fig. 6.

2006

Leiosphaeridia jacutica; Javaux and Marshal, fig. 3.1–3.3.

2009

Leiosphaeridia jacutica; Stanevich et al., p. 32, pl. 3, fig. 2.

2009

Leiosphaeridia jacutica; Vorob'eva et al., p. 185, fig. 14.13.

2010

Leiosphaeridia jacutica; Nemerov et al., fig. 6.8, 6.9.

2010

Leiosphaeridia jacutica; Prasad et al., pl. 1, fig. 3.

2013

Leiosphaeridia jacutica; Tang et al., fig. 4d.

2014

Leiosphaeridia jacutica; Babu et al., fig. 3l.

2015

Leiosphaeridia jacutica; Chiglino et al., p. 643, fig. 5b.

2015

Leiosphaeridia jacutica; Tang et al., figs. 4f, g, 5a.

2015

Leiosphaeridia jacutica; Nagovitsin and Kochnev, fig. 4.43.

2015

Leiosphaeridia jacutica; Vorob'eva et al., fig. 7.6.

2016

Leiosphaeridia jacutica; Baludikay et al., fig. 8d.

2016

Leiosphaeridia jacutica; Porter and Riedman, p. 833, fig. 13.3.

2016

Leiosphaeridia jacutica; Sergeev et al., fig. 4.1, 4.6, 4.7.

2016

Leiosphaeridia jacutica; Singh and Sharma, p. 80, pl. 1, figs. 9, 10.

2017

Leiosphaeridia jacutica; Javaux and Knoll, p. 209, fig. 4.4, 4.5.

2017a

Leiosphaeridia jacutica; Sergeev et al., fig. 3.1, 3.9–3.11.

2017b

Leiosphaeridia jacutica; Sergeev et al., pl. I, fig. 5.

2017

Leiosphaeridia jacutica; Beghin et al., pl. 2, fig. e.

2017

Leiosphaeridia crassa; Tang et al., fig. 3c.

2017

Leiosphaeridia jacutica; Tang et al., fig. 3d.

2019

Leiosphaeridia jacutica; Anderson et al., p. 12, fig. 8f–k.

2019

Leiosphaeridia jacutica; Arrouy et al., fig. 6b, c.

2019

Leiosphaeridia jacutica; Li et al., fig. 4h.

2020

Leiosphaeridia jacutica; Arvestål and Willman, p. 11, fig. 6i, 6l.

2020

Leiosphaeridia jacutica; Knoll et al., p. 6, fig. 2g.

2020

Leiosphaeridia jacutica; Shukla et al., p. 502, fig. 6l.

2020

Leiosphaeridia jacutica; Pang et al., fig. 2f.

2021

Leiosphaeridia jacutica; Han et al., fig. 3a–d.

Holotype

IGD Russian Academy of Sciences no. 451/1, from upper Riphean, Lakhanda Group, Neryuen Formation, Siberia (Timofeev, 1966, pl. 7, fig. 2).

Diagnosis by Javaux and Knoll (2017) and Knoll et al. (2020)

“A species of Leiosphaeridia characterized by smooth, pliant walls with lanceolate folds and a modal diameter greater than 70 μm.”

Occurrence in studied section

Four specimens were recovered. They range from ~74 to ~98 μm in diameter: MP2990, MP3719, and MP3714.

Original description by Timofeev (1966) in Russian

“Оболочки диаметром 150–250 мк, сферические, толстые, однослойные, с поверхностью от гладкой до грубошагреневой, с резко очерченными, крупными, серповидными, иногда угловатымц складками. Цвет темно-желтый, желто-коричневый.”

Translation of original description by Timofeev (1966)

The vesicles are 150–250 microns in diameter, spherical, thick, single-layered, with a smooth to coarse shagreen surface that bears sharply defined, large, crescent-shaped, sometimes angular folds. Color dark yellow, yellow-brown.

Illustrated specimen

CP913 (diameter ~81 μm).

Remarks

Timofeev (1966) described the new species Kildinella jacutica Timofeev, 1966 and designated a holotype with the description of this species, but no diagnosis was provided. Later, Mikhailova and Jankauskas in Jankauskas et al. (1989) proposed that Kildinella jacutica should be transferred to Leiosphaeridia jacutica (Timofeev, 1966). They also designated a neotype for Leiosphaeridia jacutica. This neotype is not valid since Timofeev (1966) had designated a holotype in its publication, and there is no report of the loss of this holotype. Leiosphaeridia jacutica differs only by the larger size compared with Leiosphaeridia crassa (Jankauskas et al., 1989). The specimen illustrated by Tang et al. (2017, fig. 3) may not be Leiosphaeridia crassa but Leiosphaeridia jacutica because its diameter is around 90 μm. Leiosphaeridia jacutica differs from Bambuites erichsenii in its sphaeromorphic vesicle without processes.

Leiosphaeridia minutissima (Naumova, 1949) Jankauskas in Jankauskas et al., 1989
Figure 3.1–3.3, 3.7, 3.10

1949

Leiotriletes minutissimus Naumova, p. 52, pl. 1, figs. 1, 2, pl. 2, figs. 1, 2.

1989

Leiosphaeridia minutissima (Naumova, 1949) Jankauskas in Jankauskas et al., p. 79, pl. 9, figs. 1–4, 11.

1992

Leiosphaeridia minutissima; Butterfield and Chandler, fig. 3a, i.

1994

Leiosphaeridia minutissima; Hofmann and Jackson, p. 21, fig. 23.9–23.15.

2003

Leiosphaeridia minutissima; Gaucher and Germs, fig. 6.10–6.12.

2005

Leiosphaeridia minutissima; Grey, p. 184, fig. 63d.

2005

Leiosphaeridia minutissima; Blanco and Gaucher, fig. 11b.

2005b

Leiosphaeridia minutissima; Gaucher et al., fig. 6d.

2005

Leiosphaeridia minutissima; Prasad et al., pl. 9, figs. 1, 3.

2008

Leiosphaeridia minutissima; Gaucher et al., p. 491, fig. 3a.

2008a

Leiosphaeridia minutissima; Moczydłowska, p. 84, fig. 8h.

2008b

Leiosphaeridia minutissima; Moczydłowska, figs. 2f, 6d.

2010

Leiosphaeridia minutissima; Nemerov et al., fig. 6.7.

2011

Leiosphaeridia minutissima; Couëffé and Vecolii, fig. 7.3.

2013

Leiosphaeridia minutissima; Tang et al., fig. 4a.

2015

Leiosphaeridia minutissima; Chiglino et al., p. 642, fig. 5a.

2015

Leiosphaeridia minutissima; Tang et al., fig. 4c.

2015

Leiosphaeridia minutissima; Nagovitsin and Kochnev, fig. 4.57, 4.58.

2015

Leiosphaeridia minutissima; Schopf et al., p. 724, fig. 13.10.

2016

Leiosphaeridia minutissima; Baludikay et al., fig. 8e.

2016

Leiosphaeridia minutissima; Porter and Riedman, p. 834, fig. 13.1, 13.5.

2017

Leiosphaeridia minutissima; Javaux and Knoll, p. 210, fig. 4.7, 4.8.

2017a

Leiosphaeridia minutissima; Shi et al., fig. 11.6, 11.7.

2017

Leiosphaeridia minutissima; Beghin et al., pl. 2, figs. g, h.

2017

Leiosphaeridia minutissima; Tang et al., fig. 3a.

2017

Leiosphaeridia minutissima; Suslova et al., fig. 3.6–3.11.

2017

Leiosphaeridia minutissima; Agic et al., p. 110, fig. 8g, h.

2018

Leiosphaeridia minutissima; Yin et al., fig. 4h, 4j, 4l.

2018

Leiosphaeridia minutissima; Javaux and Lepot, fig. 2e.

2019

Leiosphaeridia minutissima; Lei et al., fig. 3.13, 3.14.

2019

Leiosphaeridia minutissima; Arrouy et al., fig. 5a–g, 5j.

2019

Leiosphaeridia minutissima; Li et al., fig. 4e.

2019

Leiosphaeridia minutissima; Shang et al., p. 24, fig. 21a.

2020

Leiosphaeridia minutissima; Arvestål and Willman, p. 11, fig. 6c–g.

2020

Leiosphaeridia minutissima; Knoll et al., p. 6, fig. 2a, 2c.

2020

Leiosphaeridia minutissima; Shukla et al., p. 502, fig. 6e, k, m.

2020

Leiosphaeridia minutissima; Pang et al., fig. 2n.

2021

Leiosphaeridia minutissima; Loron et al., fig. 6.2.

2022

Leiosphaeridia minutissima; Denezine et al., fig. 11.1, 11.2.

Type material

Naumova (1949) did not designate a holotype for Leiotriletes minutissimus. Afterward, Jankauskas in Jankauskas et al. (1989) designated one of the specimens published by Naumova (1949, pl. 1, fig. 1) as the “holotype”. In addition, he designated another specimen from a different locality and a different stratigraphic unit as a “lectotype” (Jankauskas et al., 1989, LitNIGRI, N 16-800-2942/9, table 9, fig. 1). According to the International Code of Nomenclature for Algae, Fungi, and Plants (Turland et al., 2018), the “holotype” selected by Jankauskas should be regarded as a lectotype and the “lectotype” regarded as a neotype. According to the same code, a lectotype takes precedence over a neotype. Thus, the lectotype designated by Jankauskas in Jankauskas et al. (1989) (Naumova, 1949, pl. 1, fig. 1) is the valid type specimen of Leiosphaeridia minutissima.

Diagnosis presented by Javaux and Knoll (2017)

“A species of Leiosphaeridia characterized by smooth walls with sinuous folds and a modal diameter less than 70 μm.”

Emended diagnosis presented by Knoll et al. (2020)

“A species of Leiosphaeridia characterized by thin, smooth walls with sinuous folds and a modal diameter less than 70 μm.”

Occurrence in studied section

A total of 359 specimens were recovered. They range from ~4.4 to ~68.2 μm in diameter: MP3719, MP2977, MP2979, MP2980, MP2983, MP2985, MP2986, MP2987, MP2988, MP2992, MP2993, MP2994, MP2995, MP2998, MP2999, MP 3002, MP 3004, MP 3005, MP 3006, MP 3007, MP 3011, MP 3012, MP 3013, MP 3015, MP 3016, MP3028, MP 3030, MP3031, MP3033, MP3034, MP3035, MP3036, MP3705, MP3707, MP3708, MP3709, MP3710, MP3712, MP3713, MP3714, MP3715, MP3716, MP3719, and MP3720.

Description presented by Naumova (1949) in Russian

“Очертание споры округлое. Экзина очень тонкая, прозрачная, наблюдаются многочисленные складки смятия. Поверхность экзины гладкая. Щель разверзания трехлучевая, простая, плохо различимая нз-за складок смятия.”

Translation of description presented by Naumova (1949)

The outline of the vesicle is round. The exine is very thin, transparent, with numerous compressional folds. The surface of the exine is smooth. The opening slit is three-beam, simple, poorly distinguishable due to the compressional folds.

Illustrated specimens

CP918 (diameter ~32 μm), CP962 (diameters 32~ μm and ~39 μm), CP963 (diameter ~38 μm), and CP964 (diameter ~45 μm).

Remarks

The basionym of Leiosphaeridia minutissima (Naumova, 1949) is Leiotriletes minutissimus Naumova, 1949. As for Leiotriletes crassus, Naumova (1949) did not present a diagnosis for this species but provided a detailed description. Subsequently, Jankauskas in Jankauskas et al. (1989) transferred this species to Leiosphaeridia minutissima (Naumova, 1949) without presenting a diagnosis. When Leiotriletes minutissimus was transferred to the genus Leiosphaerida, the epithet was changed to minutissima, so the gender of the epithet agrees with the gender of the genus name. The first formal diagnosis for Leiosphaeridia minutissima was presented by Javaux and Knoll (2017), emended later by Knoll et al. (2020).

Leiosphaeridia tenuissima Eisenack, 1958
Figure 3.9, 3.13

1958a

Leiosphaeridia tenuissima Eisenack, p. 391, pl. 1, figs. 2, 3.

1958b

Leiosphaeridia tenuissima; Eisenack, pl. 2, figs. 1, 2.

1989

Leiosphaeridia tenuissima; Jankauskas et al., p. 81, pl. 9, figs. 12, 13.

1994

Leiosphaeridia tenuissima; Butterfield et al., p. 42, fig. 16i.

1994

Leiosphaeridia tenuissima; Hofmann and Jackson, p. 22, fig. 15.16–15.18.

1998

Leiosphaeridia tenuissima; Zhang et al., p. 32, fig. 9.7.

1998

Leiosphaeridia spp. div.; Zhang et al., p. 32, fig. 9.8, 9.9

1999

Leiosphaeridia tenuissima; Turnau and Racki, p. 267, pl. 5, fig. 1.

2000

Leiosphaeridia tenuissima; Gaucher, p. 68, pl. 11, fig. 5.

2003

Leiosphaeridia tenuissima; Gaucher and Germs, fig. 6.6.

2004

Leiosphaeridia tenuissima; Javaux et al., fig. 4j–l.

2004

Leiosphaeridia tenuissima; Gaucher et al., fig. 4d.

2005a

Leiosphaeridia tenuissima; Gaucher et al., p. 549, fig. 8g–h.

2005b

Leiosphaeridia tenuissima; Gaucher et al., fig. 6a–b, 6e–h.

2005

Leiosphaeridia tenuissima; Blanco and Gaucher, fig. 11a.

2005

Leiosphaeridia tenuissima; Grey, p. 184, fig. 63h.

2005

Leiosphaeridia tenuissima; Marshall et al., fig. 1d.

2005

Leiosphaeridia tenuissima; Prasad et al., pl. 1, fig. 3, pl. 2, fig. 10, pl. 3, fig. 15, pl. 4, fig. 17, pl. 8, figs. 16, 17.

2006

Leiosphaeridia tenuissima; Gaucher and Germs, p. 207, figs. 7d, f, g, 8b–f.

2007

Leiosphaeridia tenuissima; Javaux, fig. 1.18, 1.19.

2008

Leiosphaeridia tenuissima; Gaucher et al., p. 491, fig. 3b–i.

2009

Leiosphaeridia tenuissima; Stanevich et al., p. 32, pl. 3, fig. 5.

2010

Leiosphaeridia tenuissima; Prasad et al., pl. 1, fig. 1.

2010

Leiosphaeridia tenuissima; Buick, fig. 1e.

2013

Leiosphaeridia tenuissima; Tang et al., fig. 4c.

2014

Leiosphaeridia tenuissima; Liu et al., fig. 101.

2014

Leiosphaeridia tenuissima; Vorob'eva and Petrov, fig. 6b.

2015

Leiosphaeridia tenuissima; Schopf et al., p. 724, fig. 13.9.

2015

Leiosphaeridia tenuissima; Nagovitsin and Kochnev, fig. 4.59.

2015

Leiosphaeridia tenuissima; Chiglino et al., p. 640, fig. 4a–c.

2015

Leiosphaeridia tenuissima; Tang et al., fig. 4e.

2015

Leiosphaeridia tenuissima; Vorob'eva et al., fig. 7.8.

2016

Leiosphaeridia tenuissima; Baludikay et al., fig. 8f.

2016

Leiosphaeridia tenuissima; Porter and Riedman, p. 834, fig. 13.4.

2016

Leiosphaeridia tenuissima; Sergeev et al., fig. 4.2.

2016

Leiosphaeridia tenuissima; Singh and Sharma, p. 81, pl. 1, figs. 12, 15.

2017

Leiosphaeridia tenuissima; Beghin et al., pl. 2, fig. j.

2017

Leiosphaeridia tenuissima; Tang et al., fig. 3b.

2017

Leiosphaeridia tenuissima; Agic et al., p. 112, fig. 8d, f.

2017

Leiosphaeridia tenuissima; Suslova et al., fig. 3.13, 3.14.

2017a

Leiosphaeridia tenuissima; Sergeev et al., fig. 3.12.

2017a

Leiosphaeridia minutissima; Sergeev et al., fig. 3.13.

2017b

Leiosphaeridia tenuissima; Sergeev et al., pl. 1, figs. 7, 9.

2019

Leiosphaeridia tenuissima; Anderson et al., p. 512, figs. 8l, m, 15k.

2019

Leiosphaeridia tenuissima; Arrouy et al., figs. 6a, 7a–d.

2019

Leiosphaeridia tenuissima; Li et al., fig. 4g.

2019

Leiosphaeridia tenuissima; Tang et al., fig. 1.2–1.5.

2019

Leiosphaeridia tenuissima; Wan et al., fig. 4f.

2020

Leiosphaeridia tenuissima; Arvestål and Willman, p. 12, fig. 6a, b.

2020

Leiosphaeridia tenuissima; Shukla et al., p. 502, fig. 6a–d, 6f.

2020

Leiosphaeridia tenuissima; Pang et al., fig. 2c.

2021

Leiosphaeridia tenuissima; Han et al., fig. 3e.

2021

Leiosphaeridia tenuissima; Tang et al., fig. 9a.

2021

Leiosphaeridia tenuissima; Loron et al., fig. 6.1, 6.3.

2022

Leiosphaeridia tenuissima; Denezine et al., fig. 11.3.

Holotype

Preparation A3, 3 number 4, from the Dictyonema shales of the Ordovician Baltic, Nikolskaya on the Tossna, southeast Leningrad (Eisenack, 1958a, pl. 1, fig. 2).

Original diagnosis presented by Eisenack (1958a) in Germany

“Wand äußerst dünn und zart, glasklar durchscheinend, ohne Wandporen; nur in flachgedrücktem Zustand in Form von fast kreisrunden Scheibchen überliefert. Pylome nicht beobachtet. Ø um rd 100 μm schwankend.”

Translation of original diagnosis presented by Eisenack (1958a)

Wall extremely thin and delicate, crystalline translucent, without wall pores; only preserved in the flattened state in the form of almost circular disks. Pyloma not observed. Size around 100 μ.

Emended diagnosis by Javaux and Knoll (2017)

“A species of Leiosphaeridia characterized by smooth walls with sinuous folds and a modal diameter (rather than maximum diameter) greater than 70 μm; the wall color is not a diagnostic criteria.”

Occurrence in the studied section

Fourteen specimens were recovered. They range from ~72 to ~126 μm in diameter: MP3002, MP3007, MP2994, MP3707, MP3709, MP3013, MP3714, MP3719, and MP3720.

Illustrated materials

CP914 (diameters: ~81 μm in Fig. 3.9 and ~86 μm in Fig. 3.13).

Remarks

Both Leiosphaeridia tenuissima Eisenack, 1958 and Leiosphaeridia minutissima (Naumova, 1949,) are simple sphaeromorphs and have a thin and translucent wall less than 0.5 μm thick. However, Jankauskas et al. (1989) differentiated them on the basis of vesicle size, defining specimens smaller than 70 μm in diameter as Leiosphaeridia minutissima and specimens larger than 70 μm as Leiosphaeridia tenuissima. The specimen illustrated by Sergeev et al. (2017a, fig. 3.13) as Leiosphaeridia minutissima is better identified as Leiosphaeridia tenuissima due to its greatest diameter of about 105 μm.

Subgroup Acanthomorphitae Downie et al., 1963
Genus Germinosphaera Mikhailova, 1986

Type species

Germinosphaera bispinosa Mikhailova, 1986.

Other species

Germinosphaera guttaformis Mikhailova in Jankauskas et al., 1989; Germinosphaera alveolata Miao et al. (2019).

Original diagnosis presented by Mikhailova (1986) in Russian

“Оболочки округлые, округло-овальные, плотные, толстые, гладкие или шагреневые, проросшие. Отростки, которые могут ветвиться, наблюдаются на одном или двух поолюсах.”

Translation of original diagnosis presented by Mikhailova (1986)

The shells are round or round-oval, dense, thick, smooth or shagreen, sprouted. Processes are observed at one or two poles.

Emended diagnosis by Butterfield et al. (1994)

“Spheroidal vesicles with 1–6 open-ended, tubular, and occasionally branched processes that communicate freely with the vesicle. Multiple processes usually restricted to a single ‘equatorial’ plane, but otherwise non-uniformly distributed on the vesicle.”

Emended diagnosis by Miao et al. (2019)

“Vesicle spheroidal, teardrop-shaped to slightly irregular outline, having psilate or low relief sculptured alveolar wall surface and bearing a single to multiple processes. Processes are simple tubular or occasionally branching, and open-ended. Processes are distributed [irregularly] on the vesicle wall, if multiple, and may be predominantly, but not exclusively, distributed in the equatorial plane of the vesicle.”

Germinosphaera bispinosa Mikhailova, 1986
Figure 3.4, 3.8, 3.11

1986

Germinosphaera bispinosa Mikhailova, p. 33, fig. 6.

1986

Germinosphaera unispinosa Mikhailova, p. 33, fig. 5.

1989

Germinosphaera bispinosa; Jankauskas et al., p. 142, pl. 47, fig. 2.

1989

Germinosphaera tadasii Weis in Jankauskas et al., p. 143, pl. 47, figs. 3–5.

1989

Germinosphaera unispinosa Jankauskas et al., p. 143, pl. 47, fig. 1.

1991

Germinosphaera sp.; Knoll et al., p. 557, fig. 19.6.

1993

Gemmispora rudis Yan in Yan and Liu, pl. I, figs. 6, 7.

1994

Germinosphaera fibrilla (Ouyang et al., 1974); Butterfield et al., p. 38, fig. 17a–h.

1994

Germinosphaera bispinosa; Butterfield et al., p. 38, fig. 16d, e.

1994

Germinosphaera jankauskasii Butterfield in Butterfield et al., p. 38, fig. 16a–c.

1995

Germinosphaera sp. cf. G. unispinosa; Zang, p. 164, fig. 26k, l.

1999

Germinosphaera unispinosa; Yin and Guan, p. 128, fig. 5.2, 5.4, 5.6, 5.9.

2005

Germinosphaera bispinosa; Prasad et al., p. 44, pl. 11, fig. 3.

2005

Germinosphaera unispinosa; Prasad et al., p. 44, pl. 11, figs. 1, 2.

2007

Germinosphaera unispinosa; Yin and Yuan, fig. 2.11.

2009

Germinosphaera sp.; Vorob'eva et al., p. 191, fig. 13.13–13.15, 13.17.

2016

Germinosphaera bispinosa; Baludikay et al., fig. 6a–c.

2017

Germinosphaera bispinosa; Loron and Moczydłowska, p. 24, pl. 1, fig. 3.

2019

Germinosphaera bispinosa; Li et al., fig. 10c–g.

2019

Germinosphaera bispinosa; Loron et al., fig. 8e–f.

2019

Germinosphaera bispinosa; Miao et al., p. 187, fig. 5d, f.

2021

Germinosphaera bispinosa; Miao et al., p. 14, fig. 5d, e.

2022

Germinosphaera bispinosa; Denezine et al., fig. 11.4.

Holotype

Number 882/2 from the Krasnoyarsk region, River Uderei; Upper Riphean, Dashkin Formation (Mikhailova, 1986, fig. 6).

Diagnosis by Butterfield in Butterfield et al. (1994)

“A species of Germinosphaera with psilate vesicles 13–35 μm in diameter. Processes 2.5–3.5 μm wide and, when multiple, arranged equatorially on the vesicle.”

Emended diagnosis by Miao et al. (2019)

“Spheroidal to slightly elongate or irregular vesicle with one to multiple tubular processes. Vesicle wall psilate. Processes may [be] arranged irregularly or equatorially on the vesicle wall when multiple.”

Occurrence in the studied section

Twenty-three specimens were recovered: MP3036 and MP3714.

Description

Vesicles are 23.4–34.8 μm in diameter, bearing one to two processes. When two processes are present, they are inserted at two opposing ends of the vesicle (Fig. 3.4). Processes typically taper slightly toward their distal end (Fig. 3.4, lower process, 3.8) or are more or less cylindrical (Fig. 3.11). One of the processes in the specimen illustrated in Fig. 3.4 is apparently constricted at the base. However, it is uncertain whether this constriction is a taphonomic feature related to the twisting of the process. Processes are 1–3 μm in maximum diameter and 22.6–123.0 μm in preserved length.

Illustrated material

CP917.

Remarks

Mikhailova (1986) established two species of Germinosphaera, Germinosphaera unispinosa and Germinosphaera bispinosa. Two additional species were published by Jankauskas et al. (1989), Germinosphaera guttaformis Mikhailova in Jankauskas et al., 1989 and Germinosphaera tadasii Weiss in Jankauskas et al., 1989. These species were distinguished by the number of processes and the psilate versus shagrinate nature of vesicle walls. However, Butterfield et al. (1994) considered the possibility that the processes in Germinosphaera represent growth structures in vegetative stages, analogous to the modern xanthophyte Vaucheria. As such, they emended the diagnosis of Germinosphaera and the diagnosis of G. bispinosa, and they synonymized G. unispinosa with G. bispinosa. Miao et al. (2019) further emended the diagnosis of Germinosphaera and considered shagrinate vesicle walls to represent taphonomic alteration. Furthermore, they noted that the vesicle diameters of different species could overlap each other. Thus, they proposed that G. tadasii and G. jankauskasii, which are characterized by shagrinate vesicle walls, were junior synonyms of G. bispinosa. Following Miao et al. (2019), Germinosphaera currently has three species: Germinosphaera bispinosa Mikhailova, 1986, Germinosphaera guttaformis Mikhailova in Jankauskas et al., 1989, and Germinosphaera alveolata Miao et al., 2019.

Species diversity and abundance

The analysis of 80 samples, 53 of which contained microfossils, yielded a modest diversity of organic-walled microfossils, including seven species of four genera: Siphonophycus robustum (Schopf, 1968), Leiosphaeridia crassa (Naumova, 1949), Leiosphaeridia jacutica (Timofeev, 1966), Leiosphaeridia minutissima (Naumova, 1949), Leiosphaeridia tenuissima Eisenack, 1958, Germinosphaera bispinosa Mikhailova, 1986, and Ghoshia januarensis new species. Following Butterfield et al. (1994), the four morphospecies of Leiosphaeridia are differentiated on the basis of their vesicle diameter and wall thickness: Leiosphaeridia minutissima has thin-walled vesicles less than 70 μm in diameter, Leiosphaeridia tenuissima has thin-walled vesicles 70–200 μm in diameter, Leiosphaeridia crassa has thicker-walled vesicles less than 70 μm in diameter, and Leiosphaeridia jacutica has thicker-walled vesicles 70–800 μm in diameter (Fig. 6). Only one species of acanthomorphs is reported, Germinosphaera bispinosa, a smooth vesicle with one or two unbranched processes that are either cylindrical or slightly tapered toward the distal end.

Figure 6. Abundance and size distribution of Leiosphaeridia species from the Sete Lagoas Formation at the Barreiro section.

Filamentous microfossils are common in the Sete Lagoas Formation. Tubular filamentous microfossils recovered in this work are represented by the morphospecies Siphonophycus robustum, which is interpreted as remains of cyanobacterial sheaths. This work follows Knoll et al. (1991), Butterfield et al. (1994), and Tang et al. (2013) in distinguishing Siphonophycus species according to their filament diameter: Siphonophycus thulenema, 0.5 μm in diameter; Siphonophycus septatum, 1–2 μm; Siphonophycus robustum, 2–4 μm; Siphonophycus typicum, 4–8 μm; Siphonophycus kestron, 8–16 μm; Siphonophycus solidum, 16–32 μm; Siphonophycus punctatum, 32–64 μm; and Siphonophycus gigas, 64–128 μm. In addition to tubular filaments, branching filaments of uniserially chained cells from the Sete Lagoas Formation are identified as Ghoshia januarensis new species.

The Sete Lagoas assemblage is numerically dominated by sphaeromorphs. Nearly all fossiliferous samples contain the sphaeromorph genus Leiosphaeridia, and Leioshpaeridia minutissima is the most abundant species (Fig. 6), with 359 specimens (~93% of all Leiosphaeridia specimens) and 1–64 occurrences per horizon in 45 horizons (Fig. 7). By contrast, Leiosphaeridia crassa, Leiosphaeridia jacutica, Leiosphaeridia tenussima, and Germinosphaera bispinosa are rare, represented by 14, 4, 9, and 19 specimens, respectively. About 73% of acritarch specimens recovered from the Sete Lagoas Formation are <40 μm in diameter, highlighting the predominance of small organic-walled microfossils in this unit.

Figure 7. Stratigraphic distribution and relative abundance of organic-walled microfossils from the Sete Lagoas Formation at the Barreiro section.

The organic-walled microfossil assemblage recovered in this study is taxonomically different from those of previous micropaleontological studies of the Sete Lagoas Formation (e.g., Fairchild et al., 1996; Table 1). This difference is likely related to variations in paleoenvironment, paleoecology, taphonomy, and fossil preparation techniques. Previous micropaleontological studies of the Sete Lagoas Formation were focused exclusively on cherts, particularly silicified stromatolites and microbialites. Microfossils recovered in those studies were dominated by benthic microorganisms that constructed or dwelled in microbial mats. For example, Fairchild et al. (1996) documented abundant filamentous and coccoidal microfossils (e.g., Siphonophycus, Myxococcoides, Gloeodiniopsis) from silicified microbialites of the Sete Lagoas Formation in the State of Goiás (their localities 20–22), more than 350 km to the west of the Barreiro section investigated in this study; only rare acritarchs identified as cf. Leioshpaeridia sp. were reported (see Fairchild et al., 1996, fig, 5h, table 2). By contrast, our investigation was focused on the lime mudstone and fine-grained limestone facies dominated by planktonic microfossils such as Leioshpaeridia minutissima. It is well known that Precambrian chert and fine-grained siliciclastic facies tend to host taxonomically distinct microfossils, with benthic microbial mat communities dominating the former facies and planktons prevailing in the latter (Butterfield and Chandler, 1992). Thus, the taxonomic difference between this and previous studies of the Sete Lagoas Formation is likely a result of paleoenvironmental and paleoecological differences.

Taphonomic differences may also have played a role in the taxonomic difference between this and previous studies. Silicification of microfossils is fundamentally a three-dimensional cast-and-mold process at the cellular level (Xiao and Tang, 2021), whereas organic-walled microfossils in fine-grained siliciclastic facies are preserved through two-dimensional compression of recalcitrant organic structures (Butterfield, 1990), aided by clay mineral coating (e.g., Anderson et al., 2011). Thus, the taxonomic difference between chert and lime mudstone and fine-grained limestone facies of the Sete Lagoas Formation is related at least partially to taphonomic variations. However, because both paleoecological and taphonomic processes are intertwined with paleoenvironmental conditions, it is impossible to disentangle the paleoecological and taphonomic factors that may have contributed to the observed taxonomic differences.

Finally, methodological differences may also have played a part in the taxonomic difference between this and previous micropaleontological studies of the Sete Lagoas Formation. Organic-walled microfossils preserved in cherts are typically observed in petrographic thin sections, whereas those in fine-grained siliciclastic rocks can be extracted for microscopic analysis. Such methodological difference can lead to biases in microfossil recovery and taxonomic identification, as shown in Ediacaran acritarchs (Xiao et al., 2023), which have been processed using petrographic thin sections (for those preserved three-dimensionally in cherts), hydrofluoric acid extraction (for those preserved two-dimensionally in shales), and both thin sections and acetic acid extraction (for those preserved three-dimensionally in phosphatic carbonate rocks). Therefore, the taxonomic differences between this and previous studies of the Sete Lagoas Formation likely result from a combination of paleoenvironmental, paleoecological, taphonomic, and methodological factors.

Stratigraphic distribution and biostratigraphy

Organic-walled microfossils in the Sete Lagoas Formation at the Barreiro section range from the base of the measured section to 57.5 m stratigraphic height (Fig. 7). Ghoshia januarensis has the greatest range of all species recovered in this work, occurring at nine stratigraphic levels from 0.4 m to 57.5 m. Leiosphaeridia minutissima is the longest-ranging sphaeromorph, occurring at 45 stratigraphic levels from the base of the section to 36.4 m. Leiosphaeridia tenuissima ranges from 1.3 to 36.4 m and is present in nine horizons, showing almost the same stratigraphic range as Leiosphaeridia minutissima. As a minor component of the assemblage, Leiosphaeridia jacutica was recovered from three levels in the interval of 2.8–31.5 m, Leiosphaeridia crassa from two levels in 31.5–36.4 m, Germinosphaera bispinosa from two horizons in 23.5–26.5 m, and Siphonophycus robustum from four levels in 1.9–28.0 m.

Except for Leiosphaeridia crassa and Germinosphaera bispinosa, all recovered species have their first appearance within 2 m above the base of the studied section, where there is a predominance of lime mudstone. Leiosphaeridia crassa and Germinosphaera bispinosa first emerge in the middle part of the section below the intraclastic breccia beds. The disappearance of organic-walled microfossils in the Sete Lagoas Formation is gradual, although three species (Leiosphaeridia crassa, Leiosphaeridia minutissima, and Leiosphaeridia tenuissima) disappear at approximately 37 m. No organic-walled microfossils other than Ghoshia januarensis were recovered above 37 m, right before a great abundance of intraclastic breccias, which were interpreted as seismic deposits by Okubo et al. (2020).

With the exception of Ghoshia januarensis, organic-walled microfossils from the Sete Lagoas Formation described in this paper have very long stratigraphic ranges when global data are considered. For example, the four Leiosphaeridia species recovered in this work range from the Mesoproterozoic to the Cambrian (Grey, 2005). Both Germinosphaera bispinosa and Siphonopycus robustum are known from the late Paleoproterozoic to the Paleozoic (Butterfield et al., 1994; Sergeev et al., 2012; Miao et al., 2019).

Because leiospheric sphaeromorphs have rather long stratigraphic ranges globally, they have limited utility for global biostratigraphic correlation, which casts doubt on the biostratigraphic significance of leiosphere-based biozones. Nonetheless, Grey (2005) established the Ediacaran Leiosphere Palynoflora, and Gaucher and Sprechmann (2009) proposed the Early Ediacaran Leiosphere Palynoflora. Both were regarded as early Ediacaran (ca. 635–580 Ma) acritarch biozones. More recent studies from South China, however, recovered abundant and diverse acanthomorphs from early Ediacaran strata (Zhou et al., 2007; Liu and Moczydłowska, 2019; Ouyang et al., 2021), indicating that the leiosphere-based biozones of Grey (2005) and Gaucher and Sprechmann (2009) are controlled by local environmental, regional biogeographical, or taphonomic factors.

It is perceived that the terminal Ediacaran (ca. 550–539 Ma) is characterized by a leiosphere assemblage (Knoll and Walter, 1992; Gaucher and Sprechmann, 2009) (Fig. 17). Gaucher and Sprechmann (2009) presented the Late Ediacaran Leiosphere Palynoflora, which is a low-diversity assemblage characterized by small sphaeromorphs (<150 μm) such as Leiosphaeridia minutissima and Leiosphaeridia tenuissima, among others (Fig. 17). In addition, there are occurrences of Chuaria circularis, as well as Bavlinela faveolata Shepeleva, 1962, Soldadophycus bossii Gaucher et al., 1996, and small acanthomorphs, such as Asteridium spp. The Late Ediacaran Leiosphere Palynoflora, sensu Gaucher and Sprechmann (2009), has been documented in the Nama Group in Namibia (Germs et al., 1986), the Holgat Formation of the Port Nolloth Group in Namibia (Gaucher et al., 2005a), the Mulden Group in Namibia (Gaucher and Germs, 2007), the Tent Hill Formation in Australia (Damassa and Knoll, 1986), Cijara Formation in Spain (Palacios, 1989), the Cango Caves and Gamtoos groups in South Africa (Gaucher and Germs, 2006), the Dengying Formation in South China (Yin and Yuan, 2007), the Arroyo del Soldado Group in Uruguay (Gaucher, 2000; Gaucher et al., 2003), the Sierras Bayas Group in Argentina (Cingolani et al., 1991; Gaucher et al., 2005b), the La Providencia Group in Argentina (Arrouy et al., 2019), and the Corumbá Group in Brazil (Zaine, 1991; Gaucher et al., 2003; Tobias, 2014). In Namibia, Argentina, Uruguay, and Brazil (Germs et al., 1986; Gaucher et al., 2003, 2005b; Tobias, 2014), the Late Ediacaran Leiosphere Palynoflora occurs in association with biomineralized tubular fossils such as Cloudina lucianoi (Beurlen and Sommer, 1957), Cloudina riemkeae Germs, 1972, and Corumbella werneri Hahn et al., 1982, which are potential index fossils for the terminal Ediacaran.

Given the predominance of a depauperate leiosphere assemblage in the Sete Lagoas Formation (this study) and the previous report of Cloudina and Corumbella from this unit (Warren et al., 2014), it is tempting to consider that the organic-walled microfossil assemblage reported in this study is correlated with the Late Ediacaran Leiosphere Palynoflora. There are, however, two caveats. First, as discussed by Xiao and Narbonne (2020), several recent studies have shown acanthomorphs as a group may extend from the lowermost Ediacaran to the terminal Ediacaran stage in Mongolia (Anderson et al., 2017, 2019) and in Siberia (Golubkova et al., 2015; but see Vorob'eva et al., 2009), to rocks considered younger than the Shuram Excursion (Ouyang et al., 2017) or even to the early Cambrian (Grazhdankin et al., 2020). Second, a systematic description of the purported Cloudina and Corumbella fossils from the Sete Lagoas Formation (Warren et al., 2014; Perrella Júnior et al., 2017) is needed to assess their species-level identification and to support their biostratigraphic significance. Nonetheless, considering the maximum age constraint of ~557 Ma provided by detrital zircons from the upper Sete Lagoas Formation (Paula-Santos et al., 2015), the organic-walled microfossil assemblage reported in this paper is consistent with a terminal Ediacaran age interpretation.

Conclusions

A modest diversity of organic-walled microfossils is reported from the Sete Lagoas Formation of Bambuí Group at the Barreiro section in the Januária area of the São Francisco basin, central Brazil. Seven species are described: Siphonophycus robustum (Schopf, 1968), Ghoshia januarensis new species, Leiosphaeridia crassa (Naumova, 1949), Leiosphaeridia jacutica (Timofeev, 1966), Leiosphaeridia minutissima (Naumova, 1949), Leiosphaeridia tenuissima Eisenack, 1958, and Germinosphaera bispinosa Mikhailova, 1986. The first two species are considered cyanobacteria, the four Leiosphaeridia species are considered possible protists, and the phylogenetic affinity of Germinosphaera bispinosa is uncertain. All species occur in the lower part of the studied section, but only Ghoshia januarensis extends to the upper portion of the studied section. The assemblage is numerically dominated by Leiosphaeridia, with Leiosphaeridia minutissima being the most abundant species. The predominance of Leiosphaeridia indicates that the Sete Lagoas organic-walled microfossil assemblage may be correlated with the Late Ediacaran Leiosphere Palynoflora, consistent with a terminal Ediacaran age interpretation inferred from detrital zircon data, Cloudina, and Corumbella from the Sete Lagoas Formation. However, we emphasize that further investigation is needed to test this age interpretation.