Morais et al.—Neoproterozoic vase-shaped microfossils—Brazil
descriptions, below,mathematical averages are indicated by x and their standard deviation by σ. At the Instituto de Geociências, Universidade de São Paulo
(IGc-USP), the clast-hosted VSMs were studied in ~50µmthick petrographic thin sections using a Leica DM 750 P microscope equipped with a Leica MC 170 HD camera. Some images were obtained using a simplified version of the “white-card” technique of Folk (1987), by placing a small piece ofwhite paper beneath the thin section to diffuse the incoming light beam, thereby softening the visual effect of carbonate crystal boundaries and permitting a clearer view of individual rock components, especially of associated organic matter. Transmitted white light and plane-polarized light optical images of thin section-embedded specimens were also acquired at the University of California, Los Angeles (UCLA) using fluorescence-free microscopy immersion oil and a Leitz Orthoplan 2 microscope equipped with aNikon DS Microscope Digital Camera. Confocal laser scanning micrographs were obtained at
UCLA using an Olympus Fluoview 300 confocal laser scanning biological microscope system equipped with two Melles Griot lasers, a 488nm 20mW-output argon ion laser and a 633nm 10mW-output helium-neon laser. Images were acquired using a 100x oil-immersion objective, fluorescence-free microscopy immersion oil, and filters in the light-path to remove wavelengths <510nm (for 488nm laser excitation) and
<660nm (for 633 nm laser excitation) from the laser-induced fluorescence emitted by the specimens. Image-sets were subsequently processed using the VolView v3.4 3D-rendering computer program that permits image manipulation in three dimensions. Raman molecular-structural compositional analyses of the
fossils and associated minerals were carried out at UCLA using a T64000 triple-stage confocal laser-Raman system that permits acquisition both of point spectra and of Raman images that display the two-dimensional spatial distribution of the molecular- structural components of the specimens and their associated
minerals.ACoherent Innova argon ion laser provided excitation at 457.9 nmpermitting data to be obtained over a range from~300 to ~3000cm−1 using a single spectral windowcentered at 1800cm−1.
The laser power used was ~6–8mW over a ~1 µm spot, a con- figuration well below the threshold for radiation damage in kero-
genous fossils, and the thin sections were covered by a veneer of fluorescence-free microscopy immersion oil, the presence of which has been shown to have no discernible effect on the Raman spectra acquired (Schopf et al., 2005). Varying pixel intensities in the two-dimensional Raman images, acquired at the ~463 cm−1 band of quartz and the ~1600cm−1 “G” band of kerogen, correspond to the relative concentrations of the material analyzed.
Repositories and institutional abbreviations.—The petrographic thin sections analyzed here are designated by the prefixGP/5T, which is reserved for type and other published specimens belonging to the Micropaleontology Collection deposited in the Laboratório de Paleontologia Sistemática (LPS) of the Instituto de Geociências,Universidade de São Paulo, Brazil. Thin sectionGP/ L-3E-46, illustrated in Fairchild et al. (1978, p. 77, pl. 1, figs. 7–9), is deposited in the Paleobotany Collection (GP/3) in the LPS. The location of illustrated microfossils is indicated by the number of
Figure 4. Schematic representation of textural relationships of VSMs and matrix in dolostone clasts in diamictite of the Neoproterozoic Urucum Formation (Jacadigo Group, Corumbá, Brazil) showing tests of VSMs exhibiting external (1, 2, 4) and partial internal (2) fibrous to bladed palimpsest carbonate cement textures and matrices dominated by mosaic dolospar. (1) Transverse section of the organic test of an indeterminate VSM; (2) longitudinal section through siliceous test of Cycliocyrillium torquata Porter, Meisterfeld, and Knoll, 2003 (Fig. 6.2, 6.3); (3) longitudinal section through the siliceous test of the holotype of Taruma rata n. gen. n. sp. illustrated in Figure 6.10, 6.11; (4) longitudinal section through organic test of the holotype of Limeta lageniformis n. gen. n. sp. (GP/5T-2529 F) illustrated in Figure 6.9. Scale bars = 50 µm.
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the thin section in which it occurs and a location indicated by a letter (A, B, C, etc.) on maps of the thin sections deposited in the LPS. The designation HUPC for type specimens described by Porter et al. (2003) refers to theHarvardUniversity Paleobotanical Collections.
Preservational and paleoenvironmental considerations
The VSM-bearing clasts are predominantly dolomitic, containing small amounts of detrital quartz or other minerals and minor authigenic quartz (Fig. 6.11, 6.13). Dolomite having a fibrous to bladed habit forms a distinct rim around nearly all Urucum VSMs (Figs. 4, 5.1, 5.3, 5.6, 6.1). In such rims, the narrow(<10µmthick) layer closest to the wall represents early diagenetic isopachous calcite or aragonite cement that later recrystallized and later replaced by sparry dolomite while nevertheless conserving a palimpsest of the fibrous habit of the original carbonate (Figs. 4.1, 4.2, 4.4, 5.1, 5.3, 5.6, 6.1–6.9, 6.14–6.16). Such cement also coats the interiors of some tests or has completely filled them (Figs. 4.2, 6.2). More commonly, however, the tests are filled by mosaic sparry dolomite like that of their encompassing matrix. Asignificant difference between the Urucum VSMs and practically all such specimens previously described is that they preserve their original wall thicknesses and, importantly, evidence of their original wall composition. The great majority is carbonaceous (kerogenous) (e.g., Fig. 6.1, 6.4–6.9, 6.14), but some exhibit entirely siliceous or mixed kerogenous-siliceous walls, as documented by Raman imagery and optical petrography (Figs. 5.7, 5.8, 6.2, 6.3, 6.7, 6.10, 6.11). Not only have the VSMs retained their walls, but the original shapes of their tests are
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