Morais et al.—Neoproterozoic vase-shaped microfossils—Brazil
carbonate paleoenvironments (Maliva et al., 1989; Siever, 1992; Knoll, 2000). Thus, dissolved silica was evidently available throughout early Earth history for its eventual use by early protistans.
Indeed, members of the Arcellinida and Euglyphida are
among the most abundant silica-using protistans in present-day ecosystems (Cary et al., 2005; Wilkinson and Mitchell, 2010; Puppe et al., 2014; Lahr et al., 2015). Modern arcellinids exhibit three processes for the incorporation of silica into their tests: (1) by ingesting grains of quartz and/or phyllosilicates and agglutinating them into their tests, a mechanism characteristic of Diffugia Leclerc, 1815 (Châtelet et al., 2013); (2) via biomineralization of amorphous silica into test-encompassing scales, such as occurs in Quadrulella (Kosakyan et al., 2012); and (3) by kleptosquamy, in which siliceous scales previously produced by other testate amoebae are acquired by predation and reutilized (Lahr et al., 2015). Euglyphids are capable of depositing amorphous silica on their cellular membranes (Puppe et al., 2014). Although the oldest unambiguous fossils of this group date from the Paleogene, some 30–50 Ma ago (Barber et al., 2013), it is pertinent to note that Porter et al. (2003) interpreted the ~742 Ma old Chuar VSM Melicerion poikilon Porter, Meisterfeld, and Knoll, 2003, as possibly a siliceous-scaled euglyphid. Given these data, it seems plausible that silica biomineralization may have occurred in protistan lineages at least as early as the late Tonian of the Neoproterozoic when the Chuar Group VSMs were preserved. Secondary replacement is an alternative interpretation to
biomineralization as the process responsible for the presence of silica in the walls of the Urucum VSMs. Intrinsic controls on siliceous replacement of such walls include their original composition, the concentration of organic matter within them, and the skeletal ultrastructure of the fossil. Among the external controls of such silicification are the availability of silica, the chemistry of the depositional and/or early diagenetic setting and permeating pore-waters, and post-depositional diagenetic changes of the texture and composition of the fossil-hosting rocks (Butts, 2014). Secondary silicification of microfossils involves partial to complete replacement of the material making up the original structural components,which in the case ofVSMswould be their original test walls. For calcareous skeletal elements, this process consists of dissolution of calcium carbonate and subsequent precipitation of silica, promoted by the differing pH-related solubilities of CaCO3 and SiO2 and the propensity of dissolved silica to nucleate on degraded organic matter (Butts and Briggs, 2011; Butts, 2014). Primary features of walls are generally better preserved if they are permineralized in cryptocrystalline to microcrystalline quartz, as commonly occurs during early diagenetic silicification (cf., Calça and Fairchild, 2012) rather than by mold-filling mosaics of megaquartz, as commonly occurs during late-diagenetic replacement (Butts and Briggs, 2011). Did the Urucum VSMs exhibit original silica biominera-
lization orwere they secondarily replaced?Adetailed petrographic study of theVSMs is underway, but a fewpreliminary petrographic observations regarding the Urucum siliceous-walled specimens and the doublet (the “Unnamed form”) shown in Figure 6 are pertinent. First, silica in the doublet (Fig. 6.15, 6.16) appears more likely to have partially filled rather than to have replaced the test at
403
difficult to discern. Authigenic quartz is present within the same fields of view for both the holotype (Fig. 6.11) and the paratype (Fig. 6.13) of T. rata n. gen. n. sp., indicating that siliceous replacement of carbonate and other minerals locally affected the dolostone matrix. Based on these observations, secondary replacement by
the right of the specimen and to cut the test at its left, which is consistent with secondary silicification. Second, the entirely siliceous wall of Cycliocyrillium torquata Porter et al., 2003 (Fig. 6.2, 6.3) consists of a single quartz crystal (apparently exhibiting undulatory extinction) and lacks preserved relict wall substructure, which is an observation similarly consistent with secondary silicification. Third, the walls of the type specimen of Taruma rata n. gen. n. sp. (Fig. 6.10–6.13) also consist of quartz as in that of C. torquata, although the size of the quartz crystals is
quartz of the originally organic walls of the Urucum VSMs seems plausible. However, because only a small minority of the Urucum VSMs has siliceous walls and because such specimens are limited to three of the five taxa here described (two species of Cycliocyrillium Porter, Meisterfeld, and Knoll, 2003 and Taruma rata n. gen. n. sp.) and the unnamed doublets, it would appear secondary silicification was not a selective process. Nevertheless, secondary silicification is not an entirely
satisfactory explanation for all Urucum specimens. For example, were this to have occurred for Cycliocyrillium torquata Porter et al., 2003 (Fig. 6.2, 6.3), silica-substitution would have to have been a substrate-specific process that affected only the originalwall of the tests without distorting or disrupting them or altering the adjacent carbonate, including the delicate early diagenetic cement that coats the original wall both inside and out. In this regard, it is significant that the holotype of Taruma rata n. gen. n. sp, (Fig. 6.10) is encased by an external coating of mosaic dolomite
substituting an early diagenetic cement. The thin carbonaceous film that coats the inner side of its wall, evidently delimiting the ellipsoidal internal chamber of the test, quite plausibly represents a relict of the originally carbonaceous wall internal to an originally inorganic thick outerwall thatwas preferentially replaced by quartz (Figs. 5.7, 6.11). The paratype of this taxon is surrounded by
mosaic dolomite, but exhibits a clearly defined internal rind of fibrous carbonate (Fig. 6.12, 6.13) that, as inC. torquata (Fig. 6.2), was not disrupted when its wall was presumably replaced by quartz.
Although much evidence points to secondary siliceous
replacement of the original organic walls of Urucum VSMs, the foregoing considerations raise twofundamental questions: (1) what was the original composition of the replaced walls, and (2) how were they replaced without altering delicate features in their immediate vicinity? As an example, what might have been the original
composition of the robust walls of Taruma rata n. gen. n. sp.? The simplest explanation would be that like nearly all other Urucum VSMs, they too were originally carbonaceous. If so, their voluminous walls would have offered many more sites for silica nucleation (Maliva and Siever, 1988) than the thinner- walled VSMs, resulting in their preferential silicification. However, it is also possible that the thick walls of T. rata n. gen. n. sp. were originally a mixture of organic matter and silica, as is shown in Figure 5.6–5.8 for a specimen of C. simplex
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88 |
Page 89 |
Page 90 |
Page 91 |
Page 92 |
Page 93 |
Page 94 |
Page 95 |
Page 96 |
Page 97 |
Page 98 |
Page 99 |
Page 100 |
Page 101 |
Page 102 |
Page 103 |
Page 104 |
Page 105 |
Page 106 |
Page 107 |
Page 108 |
Page 109 |
Page 110 |
Page 111 |
Page 112 |
Page 113 |
Page 114 |
Page 115 |
Page 116 |
Page 117 |
Page 118 |
Page 119 |
Page 120 |
Page 121 |
Page 122 |
Page 123 |
Page 124 |
Page 125 |
Page 126 |
Page 127 |
Page 128 |
Page 129 |
Page 130 |
Page 131 |
Page 132 |
Page 133 |
Page 134 |
Page 135 |
Page 136 |
Page 137 |
Page 138 |
Page 139 |
Page 140 |
Page 141 |
Page 142 |
Page 143 |
Page 144 |
Page 145 |
Page 146 |
Page 147 |
Page 148 |
Page 149 |
Page 150 |
Page 151 |
Page 152 |
Page 153 |
Page 154 |
Page 155 |
Page 156 |
Page 157 |
Page 158 |
Page 159 |
Page 160 |
Page 161 |
Page 162 |
Page 163 |
Page 164 |
Page 165 |
Page 166 |
Page 167 |
Page 168 |
Page 169 |
Page 170 |
Page 171 |
Page 172 |
Page 173 |
Page 174 |
Page 175 |
Page 176 |
Page 177 |
Page 178 |
Page 179 |
Page 180 |
Page 181 |
Page 182 |
Page 183 |
Page 184 |
Page 185 |
Page 186 |
Page 187 |
Page 188 |
Page 189 |
Page 190 |
Page 191 |
Page 192 |
Page 193 |
Page 194 |
Page 195 |
Page 196 |
Page 197 |
Page 198 |
Page 199 |
Page 200 |
Page 201 |
Page 202 |
Page 203 |
Page 204 |
Page 205 |
Page 206 |
Page 207 |
Page 208 |
Page 209 |
Page 210 |
Page 211 |
Page 212 |
Page 213 |
Page 214 |
Page 215 |
Page 216