Morais et al.—Neoproterozoic vase-shaped microfossils—Brazil
considerable diversification occurred among testate protists prior to the end of the Precambrian (Fiz-Palacios et al., 2014; Lahr et al., 2015). The present study analyzes VSMs first reported by
insufficient illustrations, and/or poor preservation have been reported fromAlaska (USA, Tindir Group, Allison and Awramik, 1989; Macdonald et al., 2010), Mongolia (Tsagaan Oloom Formation, Bosak et al., 2011), Namibia (Rasthof Formation, Bosak et al., 2011), Scotland (Bonahaven Formation, Anderson et al., 2013), Norway (Tanafjorden Formation, Vidal and Siedlecka, 1983; Vidal and Moczydłowska, 1995), India (Simla Slates, Nautiyal, 1978; Vindhyan Group, Maithy and Babu, 1988; Vaishnodevi Limestone, Venkatachala and Kumar, 1998), andRussia (UpperMin’yar Formation,Maslov et al., 1994; Maslov, 2004). Taken together, these numerous reports establish that
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of paired tests (Porter et al., 2003, p. 410, fig. 2.1), which we here refer to as “doublets,” that resemble products of asexual binary fission in modern testate amoebae. In addition to adding to the known diversity of Neoproterozoic protistans, the Urucum VSMs are exceptional in comparison with practically all other reported VSMs in their preservation of direct evidence of original wall composition, which in the great majority of the tests is carbonaceous (kerogenous). Some walls, however, are composed partially or entirely of silica, the origin and significance of which are discussed below.
Geological setting
Fairchild et al. (1978) from carbonate clasts in diamictite near the base of the Neoproterozoic Urucum Formation (Jacadigo Group, Brazil). To extend this work, we have here used, in addition to standard petrographic microscopy and scanning electron microscopy (SEM), two techniques recently introduced in studies of three-dimensionally well-preserved permineralized microfossils: confocal laser scanning microscopy (CLSM; Martí Mus and Moczydłowska, 2000; Schopf et al., 2006, 2016) and Raman spectroscopy (Schopf et al., 2002, 2016). We show that the Urucum VSMs include two species previously described from the Chuar Group (Porter et al., 2003) as well as others that are sufficiently distinct to warrant erection of three new monospecific genera, one of which is identifiable in figures of previously unnamed specimens from the Chuar Group. Additionally, we record an unnamed form ofVSMs that consists
The VSMs here reported occur in dolostone clasts within a diamictite near the base of the Urucum Formation in the Neoproterozoic Jacadigo Group of the Urucum massif in the southern Paraguay Fold Belt of west-central Brazil (Fig. 3; Fairchild et al., 1978; Freitas et al., 2011). The Jacadigo Group lies unconformably upon crystalline basement at the junction of the Rio Apa block with the Amazon craton and is composed of two units (Almeida, 1964) (Fig. 3): the ~ 280m thick Urucum Formation, consisting of coarse-grained arkoses (including diamictites), conglomerates and breccias, coarse- to medium- grained sandstones and subordinate siltstones and carbonates, having bedded manganese ore close to its top; and the conformably overlying Santa Cruz Formation, for a few tens of meters near its base composed of a granular iron-formation and varied siliciclastic deposits overlain by 300m of banded hematite-rich iron-formation intercalated with immature siliciclastic layers that include at least three beds of diamictite (Dorr, 1945; Almeida, 1964; Angerer et al., 2016).
Figure 3. Geologic setting of study area. (1)Simplified geological map of the southern Paraguay Fold Belt and (2) schematic stratigraphic column of Jacadigo Group (modified from Freitas et al., 2011); white star in part 1 indicates the approximate location of the fossiliferous locality at the northern end of Morraria do Rabicho.
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