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Journal of Paleontology 91(3):393–406
all previously described VSM species in these characteristics, thereby justifying the erection of the species. The walls in both are siliceous, which may represent secondary replacement, although, as discussed in the next section, biomineralization might better explain some petrographic features of the walls. Also, the walls thicken (holotype) or bend abruptly inward (paratype) at the oral pole greatly restricting the aperture. The shape and slight thickening of the wall at the oral pole of a third specimen (Fig. 6.14) are reminiscent of the holotype. However, this specimen differs from the type material in several ways. It is smaller (L = 70.0 µm; W = 56.1 µm), the oral surface is less well defined, the aperture much larger (27.7 µm), and the wall is kerogenous and much thinner (WT = 3.6 µm), especially as measured in the apertural plane (6.5 µm). Thus, the specimen is attributed to Taruma rata n. gen. n. sp. with reservations in the expectation that future finds will clarify its situation.
Unnamed Form Figure 6.15, 6.16
Remarks.—Among the Urucum VSMs studied in detail are six pairs of contiguous or nearly contiguous globose tests of unequal size, each between 50 and 100 µm in diameter. All six pairs of tests (or “doublets”) have carbonaceous walls, but silica is associated with two pairs, including the pair shown in Figure 6.15 and 6.16. This and four other pairs exhibit vestiges of specimen-encompassing isopachous fringing cement. Doublets are formed in many extant testate amoebae and
(Gassowsky, 1936), the long necks (Fig. 7.1, 7.2) of which comprise about a third of the total body length (Ogden and Ellison, 1988). However, modern Difflugia has an agglutinated rather than an organic test like that of L. lageniformis n. gen. n. sp. Other possible analogues among the arcellinids occur within the hyalosphenid genus Padaungiella Kosakyan et al., 2012, but the necks of such taxa are comparatively short and laterally compressed (Fig. 7.3, 7.4) rather than being long and cylindrical as in L. lageniformis n. gen. n. sp. (Fig. 7.5, 7.6). Urucum VSMs are also morphologically similar to
foraminifera, rhizarian protistans (Fig. 1). In particular, the shapes of the two species of Cycliocyrillium and of L. lageniformis n. gen. n. sp. resemble, respectively, the Paleozoic calcareous foraminifers Eolagena minuta Lipina, 1959 (cf., Loeblich and Tappan, 1964, p. C322, fig. 232, parts 5–7), and the internal cavity of the globular proloculus and undivided tubular chamber of Syzrania bella Reytlinger, 1950 (cf., Loeblich and Tappan, 1988, p. 109, pl. 430.1; Fig. 6.8). Such similarities, however, may merely represent evolutionary convergence of body form among morphologically simple protistans, given that the possible affinity of organic-walled VSMs and thick-walled calcitic foraminifera is as yet unsubstantiated. A strong case for the affinity of the Chuar Goup VSM
other protists (e.g., tintinnids) during asexual binary fission. Construction of the daughter tests initiates at the aperture of the parent cell and the two cells/tests remain attached until cytokinesis is complete (Tappan, 1993). From the ~770–740 Ma old Uinta Mountain Group of Utah, Porter et al. (2003) illustrated a VSM doublet of claviform tests joined at their apertures (p. 410, fig. 2.1) that appears to represent just this process. Preliminary three-dimensional CLSM imaging suggests that the Urucum doublets may be similarly joined, but the evidence is inconclusive.
Affinities of Urucum VSMs and the question of silica biomineralization
Affinities.—It is now consensus that most VSMs represent testate amoebae (Schopf, 1992; Porter and Knoll, 2000; Porter et al., 2003) having affinities to amoebozoan arcellinids and, less commonly, to euglyphids of the Rhizaria. For example, Porter et al. (2003, fig. 17.1–17.3) illustrated unnamed VSMs having a funnel-like neck from the Chuar Group that they compared to the modern arcellinid Microamphora pontica Valkanov, 1970, although the possibility that these specimens might represent fragmented doublets of a Cycliocyrillium species was also considered. The discovery of similar Urucum specimens that exhibit complete necks supports the comparison with arcellinids such as M. pontica and the inclusion both of damaged and complete forms within a distinct taxon, here designated Palaeoamphora urucumense n. gen. n. sp. Another Urucum VSM, Limeta lageniformis n. gen. n. sp.,
similarly invites comparison with arcellinids, specifically with Quaternary testate amoebae such as Difflugia gassowski
Melicerion poikilon Porter et al., 2003 to alveolate euglyphids has been put forward by Porter et al. (2003) who compared the regular pattern of holes present in numerous fossil tests to the points of insertion of siliceous scales in the organic test of modern euglyphids. No isolated siliceous scales were reported.
Wall composition.—A particularly notable feature of the Urucum VSMs that sets them apart from most other occurrences of VSMs is that the walls are preserved in practically all of the tests. Although the great majority of the Urucum specimens have thin carbonaceous (kerogenous) walls, silica is an impor- tant wall component in others. Raman spectroscopy has demonstrated that the wall of the specimen of Cycliocyrillium simplex Porter et al., 2003, shown in Figure 5.6–5.8 is a mixture of kerogen and silica. Petrographic microscopy shows that the walls of the specimen of Cycliocyrillium torquata Porter et al., 2003, shown in Figure 6.2 and 6.3, as well as the holotype and paratype of Taruma rata n. gen. n. sp. (Fig. 6.10–6.13) are siliceous. Silica is also a wall component of the unnamed “doublet” of tests shown in Figure 6.15 and 6.16, an occurrence that raises the interesting question of whether such silica repre- sents secondary replacement or might instead evince primary siliceous biomineralization in early protistans. Biological use of silica is a major factor that controls
the present-day global silica cycle and occurs in diverse extant groups, including protists (e.g., diatoms, ciliates, and testate amoebae),metazoans (viz., sponges), and plants, such as horsetails (Equisetum) and grasses (cf., Hodson et al., 2005; Cornelis et al., 2013; Lahr et al., 2015). Throughout most of the Precambrian, prior to the emergence of silica utilization by eukaryotes (Knoll, 2014), continental erosion provided large amounts of silica to the marine environment (Sarmiento, 2013), as evidenced by the abundance of such abiotically precipitated siliceous rocks as banded iron formations and, of particular paleontologic importance, by the abundance of chert-permineralized microbiotas in Precambrian
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