398
Journal of Paleontology 91(3):393–406
an unnamed form of paired tests here referred to as “doublets.” Two of the five taxa, Cycliocyrillium simplex and C. torquata, were originally described by Porter et al. (2003), and the other three are here assigned to new genera and species: Paleoam- phora urucumense n. gen. n. sp., Limeta lageniformis n. gen. n. sp., and Taruma rata n. gen. n. sp.
Vase-shaped Microfossils Genus Cycliocyrillium Porter, Meisterfeld, and Knoll, 2003
Type species.—Cycliocyrillium simplex Porter et al., 2003. Other species.—Cycliocyrillium torquata Porter et al., 2003.
Diagnosis.—Bulbous to pyriform VSMs having a circular aperture either flush with the test or associated with a narrow collar or short uncurved neck, not exceeding one-tenth the total length of the specimen; angle between the apertural plane and the aboral axis ~90°.
Figure 5. Optical, CLSM, Raman images, and a Raman spectrum of thin section-embedded VSMs from dolostone clasts in diamictite of the Neoproterozoic Urucum Formation (Jacadigo Group, Corumbá, Brazil). (1) Optical image of organic test of Cycliocyrillium simplex Porter, Meisterfeld, and Knoll, 2003 (GP/5T-2537 A); (2) Raman spectrum of test shown in (1), indicating the principal bands of the embedding dolomite and quartz and the “D,”“G,” and second-order carbonaceous kerogen bands of its test; (3–5) optical (3) and CLSM images (4, 5)of Palaeoamphora urucumense n. gen. n. sp. that exhibits a constricted short neck subtending its aperture (GP/5T-2537 B); (6–8) optical image (6) and corresponding Raman images (7, 8) of a test attributed to C. simplex (GP/5T-2537 C) showing a close association between kerogen (7) and silica (8).
preserved as well, despite the post-depositional recrystallization and dolomitization that has alteredmuch of the primary rock fabric. We conclude that the very early isopachous carbonate cement was fundamental in preserving the walls of the Urucum VSMs (Figs. 4, 5, 6). Attempts were unsuccessful to identify the source of the
VSM-bearing clasts among carbonate intraclasts present in the continental debris flow-dominated alluvial fan facies of the Urucum Formation (Freitas et al., 2011), all such intraclasts proving to be nonfossiliferous and calcitic rather than dolomitic. Indeed, the isopachous cement encasing the VSMs documented here is much more characteristically marine (Tucker and Wright, 1990) than continental. Such a marine origin for the Urucum VSMs is also consistent with the paleoenvironment inferred for practically all other occurrences of VSMs (e.g., Porter and Knoll, 2000). Available evidence thus indicates that the VSM-bearing clasts should be regarded as extraclasts of unknown provenance apparently derived from pre-Urucum marine rocks.
Systematic paleontology
Of the approximately 3,000 specimens of VSMs detected in clasts from the Urucum Formation, 55 specimens having long- itudinal sections oriented parallel to the thin section surface were examined in detail and differentiated into five species and
Description.—Specimens from the Urucum Formation have pyriform tests (L = 50–139 µm, x = 87 µm, σ = 20 µm; W = 42–96 µm, x = 62 µm, σ = 14 µm), and exhibit a circular aperture (aperture diameter, AD = 11–22 µm, x = 22 µm, σ = 6 µm) without a collar or neck. Aspect ratios (L/W) range
Remarks.—As originally described by Porter et al. (2003, p. 415), the presence of an uncurved neck is a diagnostic feature for the genus Cycliocyrillium, but the neck is absent in the type species C. simplex. Therefore, we have emended the generic and specific diagnoses to allow for the lack of a neck or collar in the genus and to differentiate clearly between C. simplex, which has no collar or neck, and C. torquata, which does. The dimensions of the specimens of Cycliocyrillium in the
Urucum Formation are similar to those of the type material presented by Porter et al. (2003) from the Neoproterozoic Chuar Group. Unlike the type material, which is preserved as molds, siliceous casts, and carbonate substitutions, the specimens described here exhibit walls of varied composition. Nearly all are carbonaceous (kerogenous) (Figs. 4.1, 6.1), which we interpret as indicating an originally organic wall. A few are siliceous (Fig. 6.2, 6.3) or a carbonaceous-siliceous mixture (Fig. 4.6–4.8). Their possible origins are discussed in the next section.
Cycliocyrillium simplex Porter, Meisterfeld, and Knoll, 2003 Figures 5.1, 5.6, 6.1
2003 Cycliocyrillium simplex Porter et al., p. 415, fig. 6.1–6.9.
Holotype.—HUPC# 64455, upper Tonian (~742±6 Ma), Kwagunt Formation, Chuar Group, Arizona.
Materials.—N = 24; GP/5T: 2529 B, C, D, E, I; 2531 A, B, D, E; 2532 B, H; 2534 A, C, E; 2535 A; 2536 F, I, L; GP/5T: 2539 B, C, E, F; 2541 A, C.
Diagnosis.—Specimens of Cycliocyrillium with smooth aper- tural margin flush with the rest of the test.
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