Urucum Formation

Morais et al. (Reference Morais, Fairchild, Lahr, Rudnitzki, Schopf, Garcia, Kudryavtsev and Romero2017) recently reviewed the geological context of this formation, which we briefly summarize here. The Brazilian VSMs occur in dolostone clasts within coarse-grained deposits near the base of the 400 m thick Urucum Formation, Jacadigo Group, in the Urucum District at Corumbá, Mato Grosso do Sul (MS) (Almeida, Reference Almeida1964; Boggiani and Alvarenga, Reference Boggiani, Alvarenga, Mantesso-Neto, Bartorelli, Carneiro and Brito-Neves2004; Freitas et al., Reference Freitas, Warren, Boggiani, De Almeida and Piacentini2011). This unit is overlain by a succession more than 300 m thick mostly of iron formation named the Santa Cruz Formation by Almeida (Reference Almeida1945) and Banda Alta Formation by Dorr (Reference Dorr1945) (see Morais et al., Reference Morais, Fairchild, Lahr, Rudnitzki, Schopf, Garcia, Kudryavtsev and Romero2017, p. 395). Current thinking is that deposition of the iron formation occurred under the influence of late Neoproterozoic glacial conditions (Barbosa, Reference Barbosa1949; Dorr, Reference Dorr1973; Urban et al., Reference Urban, Stribrny and Lippolt1992; Angerer et al., Reference Angerer, Hagemann, Walde, Halverson and Boyce2016; Frei et al., Reference Frei, Dossing, Gaucher, Boggiani, Frei, Árting, Crowe and Freitas2017).

The VSMs are at least as old as the basal deposits of the Jacadigo Group, whose minimum depositional age of 587 ± 7 Ma derives from the burial-heating age of cryptomelane in manganese formation near the base of the Santa Cruz Formation (⁴⁰Ar/³⁹Ar, Piacentini et al., Reference Piacentini, Vasconcelos and Farley2013). Geochronological evidence bearing upon the maximum age of the group includes a U-Pb sensitive high-resolution ion microprobe (SHRIMP) zircon age of 1,861 ± 5 Ma (Redes et al., Reference Redes, Sousa, Ruiz and Lafon2015) for the crystalline basement, an age of 1,300 Ma (Ar/Ar) for the last regional heating of the Rio Apa block, on which the Jacadigo Group lies (Cordani et al., Reference Cordani, Teixeira, Tassinari, Coutinho and Ruiz2010), and a K-Ar age of 889 ± 44 Ma by Hasui and Almeida (Reference Hasui and Almeida1970) for a basement granite feldspar. O'Connor and Walde (Reference O'Connor and Walde1986), however, alleged a much younger maximum depositional age of 623 ± 15 Ma for the Jacadigo Group based on K-Ar dating of acid volcanics of the Pimienta Formation, supposedly at the base of the Jacadigo equivalent (Boqui Group) in Bolivia, but unknown in Brazil; hence, the relevance of this date for Jacadigo sedimentation is conjectural (Angerer et al., Reference Angerer, Hagemann, Walde, Halverson and Boyce2016). The youngest detrital zircon in the Santa Cruz Formation suggests a maximum depositional age of 695 ± 17 Ma (U-Pb, Frei et al., Reference Frei, Dossing, Gaucher, Boggiani, Frei, Árting, Crowe and Freitas2017) for the upper part of the group. Consistent with this is a broadly permissive five-point Nd-Sm isochron age of 566 ± 110 Ma obtained by Viehmann et al. (Reference Viehmann, Bau, Bühn, Dantas, Andrade and Walde2016) for deposition of the iron formation. These data thus set a minimum age for the VSM-bearing clasts at about 587 Ma but do not precisely constrain their maximum age. The stratigraphic relationship between the rudites containing the VSM-bearing clasts in the Urucum Formation and the iron formation in the overlying Santa Cruz Formation means that the carbonate that furnished the clasts must have been deposited and reworked no later than the glacial episode that influenced ferruginous sedimentation (Angerer et al., Reference Angerer, Hagemann, Walde, Halverson and Boyce2016; Frei et al., Reference Frei, Dossing, Gaucher, Boggiani, Frei, Árting, Crowe and Freitas2017). In light of the 587 Ma burial heating age of the Santa Cruz Formation (Piacentini et al., Reference Piacentini, Vasconcelos and Farley2013), that episode was more likely related to the Cryogenian Sturtian or Marinoan events than to the 580-million-year-old Gaskiers glaciation.

The provenance of the VSM-bearing clasts is currently unknown, but all clasts present identical taphonomic and diagenetic signatures, different from those of intraformational Urucum carbonate beds investigated to date. This suggests a single source, possibly a pre-Jacadigo carbonate platform that has since been completely eroded. However, a source within the Urucum Formation cannot be ruled out and may yet be identified in poorly studied areas around Corumbá and along the Brazil–Bolivia border to the north and northwest.

Despite preservation of the carbonaceous test walls of the VSMs in these clasts, other primary sedimentary structures in general were adversely affected by diagenesis (especially dolomitization and recrystallization). Nevertheless, ghost-like textural features permit useful paleoenvironmental and diagenetic inferences. For example, the isopachous, fibrous to bladed habit of the carbonate encasing most of the VSMs (and less commonly coating inner test walls) (Figs. 2.1, 3.1, 3.13) is evidently a palimpsest of an originally calcitic and/or aragonitic cement suggestive of deposition and early diagenesis in a coastal marine setting (Tucker and Wright, Reference Tucker and Wright1990; Morais et al., Reference Morais, Fairchild, Lahr, Rudnitzki, Schopf, Garcia, Kudryavtsev and Romero2017). In one of the VSM-bearing clasts examined here, a finely laminated domed microbialite, lamination is clear to the unaided eye.

Figure 2.

Photomicrographs of selected VSM species observed in petrographic thin sections of the stratigraphic units investigated here. Arrows point to base of collar or neck where present. EFS = England Finder Slide coordinates. Scale bars = 50 μm. (1–5) Cycliocyrillium simplex Porter et al., Reference Porter, Meisterfeld and Knoll2003. (1) Carbonaceous test enclosed by fibrous cement in dolostone clasts, Urucum Formation (GP/5T-2536E); (2) siliceous mold, Kwagunt Formation (AK-1053-13A); (3) siliceous mold in dolostone, Callison Lake Formation (F930-15-5); (4) organic test in siliceous ooid, Eleonore Bay Group (GR2880-8); (5) Siliceous mold in phosphatic nodule, Visingsö Group (V72-G14/Harvard University Herbaria Paleobotanical Collection 63100). (6–8) Cycliocyrillium torquata Porter et al., Reference Porter, Meisterfeld and Knoll2003. (6) Siliceous test enclosed by fibrous cement in dolostone clast, Urucum Formation (GP/5T-25532 G); (7) siliceous mold, Kwagunt Formation (AK-1053-13F); (8) siliceous mold in dolostone, Callison Lake Formation (F930-15.5). (9–13) Limeta lageniformis. (9) Carbonaceous test in dolostone clasts, Urucum Formation (GP/5T-2546/EFS = O23-1), photographed using white card technique (see text); (10) pyritized test, Kwagunt Formation (AK-1053-13F1/EFS = R19-3); (11) pyritized test, Kwagunt Formation (AK-1053-13F3/EFS = K28); (12) siliceous mold in dolostone, Callison Lake Formation (J1204-18.1/EFS = E60-3); (13) siliceous mold, in dolostone, Callison Lake Formation (J1204-18.1/EFS = F65-4). (14, 15) Obelix rootsii (Cohen et al., Reference Cohen, Irvine and Strauss2017) new combination. (14) Siliceous mold, Kwagunt Formation (AK-1053-13F1/EFS = J9-2); (15) siliceous mold in dolostone, Callison Lake Formation (F930-15.5).

Figure 3.

Photomicrographs of selected VSM species observed in petrographic thin sections of the stratigraphic units investigated here. Arrows point to base of collar or neck where present. EFS = England Finder Slide coordinates. Scale bars = 50 μm. (1, 2) Palaeoamphora urucumense Morais et al., Reference Morais, Fairchild, Lahr, Rudnitzki, Schopf, Garcia, Kudryavtsev and Romero2017. (1) Carbonaceous test in dolostone clast, Urucum Formation (GP/5T-2536B); (2) siliceous mold in dolostone, Callison Lake Formation (J1204-16.8/EFS = Y46-1). (3) Palaeoamphora sp.? Siliceous mold in dolostone, Callison Lake Formation. (4–9) Pakupaku kabin Riedman et al., Reference Riedman, Porter and Calver2017. (4) Siliceous mold, Kwagunt Formation (AK-1053-13FB); (5, 6) pyritized tests, Kwagunt Formation (8, AK-1053-13FC/EFS = J21-3; 9, AK-1053-13FB); (7–9) siliceous molds in dolostone, Callison Lake Formation (J1204-18.2). (10–12) Bonniea dacruchares Porter et al., Reference Porter, Meisterfeld and Knoll2003. (10) Lateral view, carbonaceous test in dolostone clast, Urucum Formation (GP/5T-2548 / EFS = V-25); (11) oblique-front view, carbonaceous test in dolostone clast, Urucum Formation (GP/5T-2547/ EFS = E36-2); (12) lateral view, siliceous mold, Kwagunt Formation (AK1053-13F). (13–15) Trigonocyrillium horodyskii (Bloeser, Reference Bloeser1985). (13, 14) Oral views, carbonaceous tests in dolostone clasts, Urucum Formation (13, GP/5T-2548/ EFS = U32-1; 14, GP/5T-2548/ EFS = G30-3); (15) SEM micrograph, siliceous mold extracted from Kwagunt Formation (courtesy S.M. Porter).

Other VSM-bearing deposits

The Chuar Group, exposed in the Grand Canyon, Arizona, consists mostly of variegated mudstones and interbedded meter-scale dolostones, including stromatolites, deposited in a shallow marine embayment within an intracratonic rift basin inboard of the rifted western margin of the Laurentia supercontinent (Karlstrom et al., Reference Karlstrom, Bowring, Dehler, Knoll and Porter2000; Dehler et al., Reference Dehler, Elrick, Karlstrom, Smith, Crossey and Timmons2001; Dehler et al., Reference Dehler, Gehrels, Porter, Heizler, Karlstrom, Cox, Crossey and Timmons2017). The initial 1,000 m corresponds to the Galeros Formation, made up of alternating shales, carbonates, and sandstones; this is followed by 590 m of stromatolites and shales of the Kwagunt Formation (Karlstrom et al., Reference Karlstrom, Bowring, Dehler, Knoll and Porter2000; Porter and Knoll, Reference Porter and Knoll2000, p. 362, fig. 1; Dehler et al., Reference Dehler, Elrick, Karlstrom, Smith, Crossey and Timmons2001). The VSMs occur as siliceous casts and molds in coastal shales of the Awatubi Member and in silicified microbialites within the overlying Walcott Member (Bloeser, Reference Bloeser1985; Horodyski, Reference Horodyski1993) but are most abundant in dolomitic nodules within carbonaceous shales at the top of the Walcott Member, where they occur as siliceous casts and molds, often coated by pyrite or iron oxides (Porter and Knoll, Reference Porter and Knoll2000; Porter et al., Reference Porter, Meisterfeld and Knoll2003). Organically preserved VSMs were reported by Bloeser (Reference Bloeser1985), but this has not been confirmed (Porter and Knoll, Reference Porter and Knoll2000).

The minimum age of the Chuar VSMs is now constrained by a chemical abrasion-thermal ionization mass spectrometry (CA-IDTIMS) U–Pb age of 729.0 ± 0.9 Ma for a tuff ~15 m above the level of VSM-bearing dolomite nodules near the top of the group; a 751 ± 7.6 Ma Re-Os date on early diagenetic sulfides underlies the oldest Grand Canyon VSMs, in the Awatubi Member (Rooney et al., Reference Rooney, Austermann, Smith, Li, Selby, Dehler, Schmitz, Karlstrom and Macdonald2017).

In northwestern Canada, the Callison Lake Formation (Mount Harper Group) was deposited in marginal marine embayments close to the western margin of the Laurentia supercontinent (Strauss et al., Reference Strauss, Macdonald, Halverson, Tosca, Schrag and Knoll2015), much like the Kwagunt Formation, so that the two formations are considered closely correlative. The 530 m thick Callison Lake Formation consists of mixed siliciclastics, carbonates, and evaporites, subdivided into four members, with VSMs present in both the lowest (Heterolithic) and highest (Transitional) members (Strauss et al., Reference Strauss, Rooney, Macdonald, Brandon and Knoll2014, Reference Strauss, Macdonald, Halverson, Tosca, Schrag and Knoll2015; Cohen et al., Reference Cohen, Irvine and Strauss2017). VSMs are crudely preserved in stromatolitic dolostone in the former member but are silicified and thus better preserved in the organic-rich silty shale of the latter (Strauss et al., Reference Strauss, Rooney, Macdonald, Brandon and Knoll2014, p. 659, fig. 1; Cohen et al., Reference Cohen, Irvine and Strauss2017). In both members, VSMs are most abundant within organic-rich intervals. Re-Os geochronology of the fossiliferous strata indicates that the VSMs were deposited between 752.7 ± 5.5 Ma (Strauss et al., Reference Strauss, Rooney, Macdonald, Brandon and Knoll2014) and 739.9 ± 6.1 Ma (Rooney et al., Reference Rooney, Strauss, Brandon and Macdonald2015; Strauss et al., Reference Strauss, Macdonald, Halverson, Tosca, Schrag and Knoll2015).

Comparative taphonomy

Preservational pathways and quality vary among known VSM assemblages. All assemblages described to date were deposited in marine environments, ranging from peritidal to basinal, and in all cases, tests are preserved three-dimensionally. Porter and Knoll (Reference Porter and Knoll2000) and Porter et al. (Reference Porter, Meisterfeld and Knoll2003) provided detailed descriptions of preservational modes for Kwagunt VSMs. While fossils occur in a range of environmental settings, the best-preserved assemblage was found in early diagenetic dolostone nodules within basinal carbonaceous shales near the top of the Walcott Member, deposited near maximum flooding (Porter and Knoll, Reference Porter and Knoll2000; Porter et al., Reference Porter, Meisterfeld and Knoll2003). VSMs are preserved as silica-filled casts of test interiors and, sometimes, test walls; penecontemporaneous coatings of pyrite (now commonly iron oxides) that formed during decay by sulfate-reducing bacteria replicate test walls, but preservation of organic walls is, at best, rare (Bloeser, Reference Bloeser1985). Strauss et al. (Reference Strauss, Rooney, Macdonald, Brandon and Knoll2014) and Cohen et al. (Reference Cohen, Irvine and Strauss2017) also found that the predominant preservational mode in Callison Lake VSMs was infilling of test interiors by silica, although Cohen et al. (Reference Cohen, Irvine and Strauss2017) also observed organic, argillaceous, and carbonate (siderite or diagenetic dolomite) infillings. Only rarely were cell walls preserved. The abundant VSMs in shales and nodular cherts of the Togari Formation, Tasmania, are preserved much like those in the Kwagunt and Callison Lake formations (Riedman et al., Reference Riedman, Porter and Calver2017).

Set against this, the taphonomy of Urucum VSM populations is different, reflecting the distinct depositional and diagenetic setting (Morais et al., Reference Morais, Fairchild, Lahr, Rudnitzki, Schopf, Garcia, Kudryavtsev and Romero2017). Despite dolomitization and partial recrystallization of the original carbonate of the dolostone clasts, exceptional preservation of original test shapes and, importantly, the organic (now kerogenous) walls of Urucum VSMs was imparted by a thin isopachous rim of drusy cements formed penecontemporaneously with burial (Morais et al., Reference Morais, Fairchild, Lahr, Rudnitzki, Schopf, Garcia, Kudryavtsev and Romero2017, figs. 4.1, 4.2, 6.2). Preservation by isopachous cements is known for Neoproterozoic cyanobacteria (Knoll et al., Reference Knoll, Fairchild and Swett1993), but to the best of our knowledge has not previously been implicated in Precambrian protistan preservation. The empty interior of most tests was eventually filled by (now) sparry mosaic dolomite. In several tests, silica has clearly replaced the wall, but most test walls are kerogenous. Confocal laser scanning microscopy has revealed silica clearly associated with carbon in the wall of a specimen of Cyclocyrillium simplex Porter et al., Reference Porter, Meisterfeld and Knoll2003. Indeed, Morais et al. (Reference Morais, Fairchild, Lahr, Rudnitzki, Schopf, Garcia, Kudryavtsev and Romero2017) have argued, judging from the thickness and unique shape of the wall and characteristics of the encasing cement, that the test wall silica in the holotype of Taruma rata Morais et al., Reference Morais, Fairchild, Lahr, Rudnitzki, Schopf, Garcia, Kudryavtsev and Romero2017 may reflect biomineralization rather than diagenesis.

Type species

Cycliocyrillium simplex Porter et al., Reference Porter, Meisterfeld and Knoll2003

Other species

Cycliocyrillium torquata Porter et al., Reference Porter, Meisterfeld and Knoll2003

Diagnosis

Bulbous to pyriform VSMs having a circular aperture either flush with the test or associated with an uncurved terminal oral extension (collar or short neck); angle between apertural plane and oral axis 90°.

Remarks

We here modify the generic and specific diagnoses for Cycliocyrillium and Cycliocyrillium torquata by using ‘terminal oral extension’ as a general term to include the ‘collars’ and ‘necks’ used in previous descriptions of VSMs. In Morais et al. (Reference Morais, Fairchild, Lahr, Rudnitzki, Schopf, Garcia, Kudryavtsev and Romero2017), for the genus Cycliocyrillium, we formalized an upper limit of 10% total test length for such extensions, but subsequent published systematic descriptions and empirical observations during this petrographic survey have shown that this is not a practical limit to differentiate between C. torquata and Limeta lageniformis Morais, Fairchild, and Lahr in Morais et al., Reference Morais, Fairchild, Lahr, Rudnitzki, Schopf, Garcia, Kudryavtsev and Romero2017, as explained in the following. By the same token, we have transferred Cycliocyrillium rootsii (Cohen et al., Reference Cohen, Irvine and Strauss2017) to a new genus of long-necked VSMs, Obelix (discussion follows).

Holotype

HUPC# 64455, upper Tonian Kwagunt Formation, Chuar Group, Arizona, USA (Porter et al., Reference Porter, Meisterfeld and Knoll2003).

Diagnosis

Cycliocyrillium with a smooth apertural margin, flush with the rest of the test.

Occurrence

See Figure 4.

Figure 4.

Species of vase-shaped microfossils (VSMs) and their occurrence in 14 stratigraphic units, six of which (shaded at left) were investigated in this study. Stars highlight principal discoveries of this survey. Light shading shows the distribution of the biostratigraphically significant ‘Cycliocyrillium simplex assemblage’ of Riedman et al. (Reference Riedman, Porter and Calver2017). References per unit: Kwagunt Formation, USA (Porter et al., Reference Porter, Meisterfeld and Knoll2003; Morais et al., Reference Morais, Fairchild, Lahr, Rudnitzki, Schopf, Garcia, Kudryavtsev and Romero2017); Callison Lake Formation, Canada (Strauss et al., Reference Strauss, Rooney, Macdonald, Brandon and Knoll2014; Cohen et al., Reference Cohen, Irvine and Strauss2017; Morais et al., Reference Morais, Fairchild, Lahr, Rudnitzki, Schopf, Garcia, Kudryavtsev and Romero2017; Riedman et al., Reference Riedman, Porter and Calver2017); Visingsö Group, Sweden (Knoll and Vidal, Reference Knoll and Vidal1980; Martí Mus and Moczydłowska, Reference Martí Mus and Moczydłowska2000; Porter et al., Reference Porter, Meisterfeld and Knoll2003; Moczydłowska et al., Reference Moczydłowska, Pease, Willman, Wickström and Agić2017; Riedman et al., Reference Riedman, Porter and Calver2017); Ryssö Formation, Svalbard (Knoll and Calder, Reference Knoll and Calder1983; Porter et al., Reference Porter, Meisterfeld and Knoll2003; Riedman et al., Reference Riedman, Porter and Calver2017); Eleonore Bay Group, Greenland (Vidal, Reference Vidal1979; Green et al., Reference Green, Knoll and Swett1988; Riedman et al., Reference Riedman, Porter and Calver2017); Urucum Formation, Brazil (Fairchild et al., Reference Fairchild, Barbour and Haralyi1978; Morais et al., Reference Morais, Fairchild, Lahr, Rudnitzki, Schopf, Garcia, Kudryavtsev and Romero2017); Togari Group, Tasmania (Porter et al., Reference Porter, Meisterfeld and Knoll2003; Riedman et al., Reference Riedman, Porter and Calver2017); Draken Formation, Svalbard (Knoll et al., Reference Knoll, Swett and Mark1991; Porter et al., Reference Porter, Meisterfeld and Knoll2003; Riedman et al., Reference Riedman, Porter and Calver2017); Elbobreen Formation, Svalbard (Porter et al., Reference Porter, Meisterfeld and Knoll2003; Martí Mus, Reference Martí Mus2001); Backlundtoppen Formation, Svalbard (Knoll et al., Reference Knoll, Swett and Burkhardt1989; Porter et al., Reference Porter, Meisterfeld and Knoll2003; Riedman et al., Reference Riedman, Porter and Calver2017); Pahrump Group, USA (Horodyski, Reference Horodyski1993; Porter et al., Reference Porter, Meisterfeld and Knoll2003; Dehler, Reference Dehler2014); Uinta Mountain Group, Utah (Porter et al., Reference Porter, Meisterfeld and Knoll2003); Chichkan Formation, Kazakhstan (Sergeev and Schopf, Reference Sergeev and Schopf2010); Jabal Rockham Dolostone, Saudi Arabia (Binda and Bokhari, Reference Binda and Bokhari1980; Porter et al., Reference Porter, Meisterfeld and Knoll2003).

Description

Morphometric data for C. simplex in the Kwagunt Formation, Urucum Formation, Callison Lake Formation, Upper Eleonore Bay Group, Visingsö Group, and Ryssö Formation gathered from the literature and obtained in the present study petrographic survey are summarized in Table 1 (supplementary material). These data allow the following morphometric characterization of this species: length (L): 34–203 μm, x̅ = 97 μm, N = 188; width (W): 18–102 μm, x̅ = 59 μm, N = 188; aperture diameter (AD): 7–48 μm, x̅ = 22 μm, N = 178; aspect ratio (L/W): 1.0–3.2, x̅ = 1.7, N = 170.

Compared to previous studies (Porter et al., Reference Porter, Meisterfeld and Knoll2003; Cohen et al., Reference Cohen, Irvine and Strauss2017; Morais et al., Reference Morais, Fairchild, Lahr, Rudnitzki, Schopf, Garcia, Kudryavtsev and Romero2017), the new data extend the ranges for these features, as can be seen in Table 1 (supplementary data). This is not surprising, given that more specimens (N = 188) were considered here than in Porter and colleagues’ (Reference Porter, Meisterfeld and Knoll2003) original description (N = 10). C. simplex in the Upper Eleonore Bay Group tends to be slightly more elongate (L/W up to 2.3) than at other localities, as is also true for specimens in the Visingsö Group (L/W up to 2.7), which exhibit, moreover, the narrowest widths for this species (W = 18–34 μm).

Remarks

Nearly all specimens of this species consist of siliceous or, more rarely, phosphatic molds and casts (Visingsö Group). Specimens in the Urucum Formation are an important exception, for they possess carbonaceous walls, possibly reinforced by silica (Morais et al., Reference Morais, Fairchild, Lahr, Rudnitzki, Schopf, Garcia, Kudryavtsev and Romero2017). C. simplex is the most common, though not necessarily the most abundant, VSM taxon in the six stratigraphic units investigated. It is present in 13 of the 15 units listed in Figure 4 and is suspected to be present in the other two.

Holotype

HUPC# 64453, upper Tonian Kwagunt Formation, Chuar Group, Arizona, USA (Porter et al., Reference Porter, Meisterfeld and Knoll2003).

Emended diagnosis

Cycliocyrillium with a short terminal oral extension, sometimes thickened relative to test wall, that begins at a slight to moderate inflection in the test wall and then proceeds in a parallel fashion (or tapers slightly) for a short distance not exceeding 20% total test length before terminating at the apertural edge.

Occurrence

See Figure 4.

Description

Data from both previously published systematic descriptions and new data obtained in the present study (see Table 2, supplementary material) allow the following morphometric characterization of this species: length (L): 45–145 μm, x̅ = 77 μm, N = 111; width (W): 35–91 μm, x̅ = 59 μm, N = 111; aperture diameter (AD): 12–53 μm, x̅ = 30 μm, N = 111; collar length (CL): 3–19 μm, x̅ = 11 μm, N = 99; body length (BL): 50–123 μm, x̅ = 84 μm, N = 28; ratio of collar length to total length (CL/L), as a percentage: 4%–19%, x̅ = 10%, N = 28; aspect ratio (L/W): 1.15–2.3, x̅ = 1.5, N = 40; aspect ratio for test body excluding collar (BL/W): 1.1–1.9, x̅ = 1.4, N = 28.

Remarks

In their original description, Porter et al. (Reference Porter, Meisterfeld and Knoll2003) included the presence of a collar or short neck as a diagnostic feature of C. torquata. Consistent with a comment by Porter et al. (Reference Porter, Meisterfeld and Knoll2003, p. 415), Morais et al. (Reference Morais, Fairchild, Lahr, Rudnitzki, Schopf, Garcia, Kudryavtsev and Romero2017) limited relative neck length in C. torquata and in the generic description of Cycliocyrillium itself to 10% or less of total test length as a diagnostic character to differentiate this species (and genus) from the long-necked VSM Limeta lageniformis. However, in the present study, we found collar lengths in C. torquata of up to 13% total test length in thin sections of the Kwagunt Formation (N = 14) and 4% to 19% in the Urucum Formation (N = 10) (Morais et al., Reference Morais, Fairchild, Lahr, Rudnitzki, Schopf, Garcia, Kudryavtsev and Romero2017). Furthermore, all data points in Cohen and colleagues’ (Reference Cohen, Irvine and Strauss2017, fig. 16) scatter plot for 71 specimens of C. torquata in the Callison Lake Formation indicate relative collar lengths from about 10% to 20% total test length. Our results for an additional four specimens from this formation—8% to 17%—are consistent with this observation. For these reasons, we have redefined the limit for relative neck length in C. torquata as 20% total test length. Specimens with relative neck lengths greater than 20% should be classified as Limeta lageniformis or Obelix rootsii (Cohen, Irvine, and Strauss, Reference Cohen, Irvine and Strauss2017) new combination, as discussed in the following.

Type species

Limeta lageniformis Morais, Fairchild, and Lahr in Morais et al., Reference Morais, Fairchild, Lahr, Rudnitzki, Schopf, Garcia, Kudryavtsev and Romero2017.

Diagnosis

Test body subglobose to pyriform with a simple aperture at the terminus of a long tapering to cylindrical neck 20% or more total length of test.

Remarks

Neck length comprises 20% or more of the total test length in Limeta. The arbitrary upper limit of 50% for this value proposed in the original diagnosis is meaningless and thus abandoned (Morais et al., Reference Morais, Fairchild, Lahr, Rudnitzki, Schopf, Garcia, Kudryavtsev and Romero2017).

Holotype

GP/5T:2529 F, Neoproterozoic Urucum Formation, Jacadigo Group, Corumbá, Mato Grosso do Sul, southwest Brazil.

Diagnosis

As for genus.

Occurrence

See Figure 4.

Description

The data in Table 3 (supplementary material) allow the following morphometric characterization of this species (N = 25, unless indicated otherwise); length (L): 67–198 μm, x̅ = 111 μm; width (W): 29–85 μm, x̅ = 53 μm; aperture diameter (AD): 9–32 μm, x̅ = 18 μm; neck length (NL): 17–81 μm, x̅ = 35 μm; body length (BL): 28–136 μm, x̅ = 76 μm; ratio of neck length to total length (NL/L), as a percentage: 20%–48%, x̅ = 31%, N = 15; aspect ratio (L/W): 1.4–3.4, x̅ = 2.1; aspect ratio for test body, excluding the neck (BL/W): 0.9–2.5, x̅ = 1.5.

Remarks

Specimens of Limeta lageniformis identified here in the Kwagunt Formation (N = 8) and Callison Lake Formation (N = 7) are similar to one another in shape, with average aspect ratios (L/W) of 2.3 and 2.4, respectively, and average body length-to-width ratios (BL/W) of 1.7. Compared to the type material from the Urucum Formation (Morais et al., Reference Morais, Fairchild, Lahr, Rudnitzki, Schopf, Garcia, Kudryavtsev and Romero2017), in which L/W = 1.8 and BL/W = 1.2 (see Table 3, supplementary material), they display slightly more elongate test-body shapes and more obtuse flexural angles (Morais et al., Reference Morais, Fairchild, Lahr, Rudnitzki, Schopf, Garcia, Kudryavtsev and Romero2017).

Other Proterozoic microfossils exhibiting long oral extensions include single specimens in the upper Tonian Ryssö Formation of Sweden illustrated by Knoll and Calder (Reference Knoll and Calder1983, p. 491, pl. 61.13) and in the early Tonian Jiayuan Formation of China illustrated by Xiao et al. (Reference Xiao, Shen, Tang, Kaufman, Yuan, Li and Qian2014, p. 216, fig. 5B). More work is needed to demonstrate the affinity of these fossils with VSMs.

Type species

Cycliocyrillium rootsi Cohen, Irvine, and Strauss, Reference Cohen, Irvine and Strauss2017 (p. 16, figs. 4G–J).

Diagnosis

Bipartite subglobose test, consisting of an approximately hemispheroidal aboral portion and a long, broad-based neck with concave to straight walls tapering markedly for much of the fossil before reaching the aperture.

Etymology

In reference to the ovoid body shape of the character ‘Obélix’ in the French comic strip Astérix, the Gaul, created by Albert Uderzo and René Goscinny.

Remarks

Cohen et al. (Reference Cohen, Irvine and Strauss2017) erected the species Cycliocyrillium rootsi for a set of VSMs with a rounded basal portion and a long, tapering neck comprising 40% to 50% total test length. This bipartite organization is incompatible with its inclusion not only in Cycliocyrillium Porter et al., Reference Porter, Meisterfeld and Knoll2003 but in Limeta as well, as diagnosed here. Hence, we created the new genus Obelix to accommodate this species.

Holotype

Sample YG644.1 (slide J1204-17.9 [4]), England Finder coordinates L19-0 (Cohen et al., Reference Cohen, Irvine and Strauss2017, fig. 4G).

Occurrence

See Figure 4.

Description

Table 4 (supplementary material) presents morphometric data for the type material of Obelix rootsii n. comb. in the Callison Lake Formation and for three specimens identified as such in the present survey. Two of these are new examples from the same formation and differ from the four specimens illustrated in the original study (Cohen et al., Reference Cohen, Irvine and Strauss2017, figure 4G–J); the other is from the Walcott Member of the upper Kwagunt Formation. The data in Table 4 (supplementary material) allow the following morphometric characterization of this species: length (L): 45–85 μm, x̅ = 62 μm, N = 9; width (W): 31–75 μm, x̅ = 53 μm, N = 9; aperture diameter (AD): 12–29 μm, x̅ = 21 μm, N = 9; neck length (NL): 24–50 μm, x̅ = 31 μm, N = 9; body length (BL): range not provided in original paper, but 19–35 μm for specimens CLK 1, CLK 2, and KWG 1 in Table 4 (supplementary material); average of 32 μm obtained by subtracting average NL from average L of type material. Range of relative neck lengths given by Cohen et al. (Reference Cohen, Irvine and Strauss2017) as 40%–50% is here extended to 58%: average for four illustrated specimens in Cohen et al. (Reference Cohen, Irvine and Strauss2017, figure 4G–J) plus new specimens is 51% (N = 7). Aspect ratios (L/W) indicated in Cohen et al. (Reference Cohen, Irvine and Strauss2017, fig. 16) range from 1.0 to 1.25, but new material extends this range to 1.4, with an average of 1.2 (N = 7). Similarly, aspect ratios for test body (BL/W) of illustrated material plus new specimens range from 0.47 to 0.65, averaging 0.60 (N = 7).

Remarks

We here corrected the suffix for the specific epithet to rootsii in conformance with the International Code of Nomenclature (ICN) 60.8b. The ratios of neck to total length greater than 40% reported by Cohen et al. (Reference Cohen, Irvine and Strauss2017) for this species do not fit the diagnosis for the genus Cycliocyrillium Porter et al., Reference Porter, Meisterfeld and Knoll2003, as emended by Morais et al. (Reference Morais, Fairchild, Lahr, Rudnitzki, Schopf, Garcia, Kudryavtsev and Romero2017). Although the genus Limeta is available for VSMs having relative neck lengths 20% or greater total test length, the bipartite subglobose shape of the test (L/W = 1.2) and the long, tapering, broad-based neck in C. rootsi clearly differentiate it from L. lageniformis and justify the erection of the new genus Obelix to accommodate this species.

Obelix rootsii n. comb. differs distinctly in shape from L. lageniformis, as expressed by average values of both L/W (1.2 and 2.1, respectively) and BL/W (0.6 and 1.5, respectively). Necks in both initiate at flexures or inflections in the test walls, but in O. rootsii this occurs near maximum test width, whereas in L. lageniformis it occurs closer to the oral terminus of the body.

Obelix rootsii can be distinguished from longitudinal sections passing through the indented portions at the oral end of Melanocyrillium hexodiadema by the marked inflection at the base of the neck and absence of the invagination and/or longitudinal ridges in the apertural region typical of M. hexodiadema.

Type species

Palaeoamphora urucumense Morais, Fairchild, and Lahr in Morais et al., Reference Morais, Fairchild, Lahr, Rudnitzki, Schopf, Garcia, Kudryavtsev and Romero2017.

Diagnosis

VSMs having a pyriform test with a funnel-like collar or neck and simple aperture.

Remarks

Two siliceous molds of VSMs with flaring terminal oral extensions were discovered in thin sections of the Callison Lake Formation during the present survey, qualifying them for inclusion in Palaeoamphora. In one (Fig. 3.2), the relative length of the funnel-like neck falls within the range of previously described specimens from the Urucum and Kwagunt formations (less than one-sixth total test length), while in the other (Fig. 3.3), it is much larger (42%). For this reason, we have chosen to remove any limit to relative length of the oral extension from the generic diagnosis.

Holotype

GP/5T-2534 F, Neoproterozoic Urucum Formation, Jacadigo Group, Corumbá, Mato Grosso do Sul, southwest Brazil.

Diagnosis

As for genus.

Occurrence

See Figure 4.

Description

Published data on the Urucum and Kwagunt formations plus new data on specimen CLK 3 from the Callison Lake Formation (see Table 5, supplementary material) allow the following morphometric characterization of this species: length (L): 57–135 μm, x̅ = 111 μm, N = 10; width (W): 37–109 μm, x̅ = 48 μm, N = 10; aperture diameter (AD): 24–66 μm, x̅ = 48 μm, N = 7; neck length (NL): 5–19 μm, x̅ = 14 μm, N = 10; body length (BL): 48–124 μm, x̅ = 92 μm, N = 7; ratio of neck length to length (NL/L), as a percentage: 8%–17%, x̅ = 13%, N = 7; aspect ratio (L/W): 1.2–2.2, x̅ = 1.,; N = 10; aspect ratio for test body in VSMs with necks (BL/W): 1.1–1.4, x̅ = 1.2, N = 7.

New specimens CLK 3 (Fig. 3.2) and CLK 4 (Fig. 3.3) from the Callison Lake Formation are smaller than previously studied specimens in the Urucum and Kwagunt formations (see Table 6, supplementary material). CLK 3 (Fig. 3.2) furnishes the lower size limits for L, W, AD, and BL in this species. The divergent collar walls of its oral extension allow it to be confidently included in P. urucumense. It, like other known specimens, is also relatively thin walled. The clearly divergent walls in CLK 4 (Fig. 3.3) are characteristic of Palaeoamphora, but its long neck and thick walls differentiate it from all currently known specimens of P. urucumense, as discussed in the following.

Remarks

Although Porter et al. (Reference Porter, Meisterfeld and Knoll2003, fig. 17.1–17.3) have previously distinguished specimens in the Kwagunt Formation that fit in the Palaeoamphora urucumense diagnosis, no specimen was identified in thin section on this survey.

Occurrence

Callison Lake Formation, Northwest Canada (see Fig. 1.2).

Description

Morphometric data for the specimen in Figure 3.3 are as follows (N = 1): length (L): 67.1 μm; width (W): 50.3 μm; aperture diameter (AD): 21.8 μm; neck length (NL): 28.1 μm; body length (BL): 39.0 μm; ratio of neck length to length (NL/L), as a percentage: 42%; aspect ratio (L/W): 1.3; aspect ratio for test body (BL/W): 0.8; wall thickness (?) up to 14 μm in the oral extension.

Material

Petrographic thin section J1204-18.1 (G56).

Remarks

Specimen in Figure 3.3 differs from specimens of P. urucumense in its greater relative neck length (42% of total test length) and, consequently, in its lower body aspect ratio of 0.8 (as compared to the average of 1.2 for P. urucumense). The great apparent wall thickness in the oral extension requires corroboration. Specimen CLK 4 is placed in open nomenclature as Palaeoamphora sp.?, pending discovery of additional examples.

Type species

Pakupaku kabin Riedman, Porter, and Calver, Reference Riedman, Porter and Calver2017.

Holotype

CPC43685, slide Tas 10-41(par), England Finder coordinates F69-0, from Bertha Road locality, lower Black River Dolomite, Togari Group, Tasmania (Riedman et al., Reference Riedman, Porter and Calver2017, fig. 6.1, 6.2).

Diagnosis

Vase-shaped microfossil having a globose to pyriform test with biconvex (lanceolate) or elliptical aperture, when viewed along the oral-aboral axis. In longitudinal section, aperture appears as a wedge-shaped gap.

Occurrence

See Figure 4.

Description

The data in Table 6 (supplementary material) allow the following morphometric characterization of this species: length (L): 70.2–143.4 μm, x̅ = 98.4 μm, N = 11; width (W): 42.5–83.6 μm, x̅ = 63.3, μm N = 11; aperture diameter (AD): 16.4–32.9 μm, x̅ = 24.2 μm, N = 10; aspect ratio (L/W): 1.1–1.9, x̅ = 1.6, N = 11.

Remarks

Pakupaku kabin was first described from the Togari Group by Riedman, Porter, and Calver (Reference Riedman, Porter and Calver2017). Its principal diagnostic feature is its wedge-shaped aperture, reminiscent of the mouth of the video game character ‘Pac-Man’ or the open gap common in pistachio nuts (Riedman et al., Reference Riedman, Porter and Calver2017, fig. 6.1–6.6). In the present survey, specimens of Pakupaku kabin were observed in dolomite nodules from the Kwagunt Formation (N = 4) and in silicified shales from the Callison Lake Formation (N = 3). In both cases, however, the VSMs are globose; hence, we have emended the original diagnosis to include this term and reworded the diagnosis for greater clarity as well.

Type species

Bonniea dacruchares Porter, Meisterfeld, and Knoll, Reference Porter, Meisterfeld and Knoll2003.

Other species

Bonniea pytinaia Porter, Meisterfeld, and Knoll, Reference Porter, Meisterfeld and Knoll2003; Bonniea makrocurtos Cohen, Irvine, and Straus, Reference Cohen, Irvine and Strauss2017.

Diagnosis

VSMs with a curved ‘neck’ and circular aperture.

Holotype

HUPC# 64409, upper Tonian Kwagunt Formation, Chuar Group, Arizona, USA (Porter et al., Reference Porter, Meisterfeld and Knoll2003).

Diagnosis

Specimens of the genus Bonniea with tests having length-to-width ratios less than 2 and concavity of the neck greater than 30°. Aperture wall may be slightly thickened.

Occurrence

See Figure 4.

Description

The two specimens of B. dacruchares identified in the Urucum Formation measure 65 and 88 μm in length and 46 and 65 μm in width with aspect ratios (L/W) of 1.4 and 1.3 and aperture diameters of 15 and 23 μm. The specimen in Figure 3.10 exhibits a concavity of 66° and a curvature of 20°. Test walls are carbonaceous.

Remarks

The length, width, aperture diameter, and concavity of the Urucum specimens lie within the intervals given by Porter et al. (Reference Porter, Meisterfeld and Knoll2003) for the type material. These morphometric parameters are consistent as well with ranges observed in the Callison Lake Formation (Cohen et al., Reference Cohen, Irvine and Strauss2017) and Togari Group (Riedman et al., Reference Riedman, Porter and Calver2017). Forms identified as B. dacruchares in the Ryssö Formation by Porter et al. (Reference Porter, Meisterfeld and Knoll2003) have not been formally described. Several other examples of B. dacruchares have been identified in the Urucum Formation, but as they present less than ideal orientations in thin sections, they could not be properly measured. The specimen in lateral view in Figure 3.10 and that in oblique frontal view in Figure 3.11 clearly exhibit divergent oral and aboral axes (i.e., curvature; Porter et al., Reference Porter, Meisterfeld and Knoll2003, fig. 7) that distinguish them from all other Urucum VSMs.

Type species

Trigonocyrillium horodyskii (Bloeser, Reference Bloeser1985) Porter, Meisterfeld, and Knoll Reference Porter, Meisterfeld and Knoll2003.

Other species

Trigonocyrillium fimbriatum (Bloeser, Reference Bloeser1985) Porter, Meisterfeld, and Knoll Reference Porter, Meisterfeld and Knoll2003.

Diagnosis

Pyriform to bulbous VSMs with an equilateral triangular aperture (Bloeser, Reference Bloeser1985).

Holotype

SEM plug CS-30; log 197, UCLA Catalogue Number 58975, upper Tonian Kwagunt Formation, Chuar Group, Arizona, USA (Bloeser, Reference Bloeser1985).

Diagnosis

Specimens of the genus Trigonocyrillium with unadorned apertural margins.

Occurrence

See Figure 4.

Description

The three tests of Trigonocyrillium horodyskii identified in the Urucum Formation range from 72 to 84 μm in length (x̅ = 79 μm; σ = 6.2 μm; N = 3) and 68 to 80 μm in width (x̅ = 72 μm; σ = 6.9 μm; N = 3), values consistent with the ranges given for the type material by Porter et al. (Reference Porter, Meisterfeld and Knoll2003): L = 75 to 125 μm (x̅ = 96 μm; σ = 19 μm; N = 22); W = 45 to 90 μm (x̅ = 65 μm; σ = 15 μm; N = 22). All three are bulbous, with aspect ratios (L/W) between 1.0 to 1.2, as compared to values of 1.2 to 1.8 for the type material. The triangular apertures present rounded vertices and approximately equal sides, whose averages fall between 19 and 30 μm in the three Urucum specimens. Similar values of 22 to 49 μm were measured for four appropriately oriented specimens of the type material of Porter et al. (Reference Porter, Meisterfeld and Knoll2003, fig. 5.2, 5.8, 5.18, 5.19). The walls of the Urucum specimens are carbonaceous.

Remarks

The unadorned triangular aperture allows identification of these VSMs as T. horodyskii; their size and globose shape are also consistent with the type material. In contrast to the molds and casts of this species in the Kwagunt, Visingsö, and Draken assemblages (Knoll and Vidal, Reference Knoll and Vidal1980; Knoll and Calder, Reference Knoll and Calder1983; Porter et al., Reference Porter, Meisterfeld and Knoll2003), the Urucum specimens have carbonaceous walls as recognized by Raman spectroscopy (Morais et al., Reference Morais, Fairchild, Lahr, Rudnitzki, Schopf, Garcia, Kudryavtsev and Romero2017).