404
Journal of Paleontology 91(3):393–406
Porter, Meisterfeld, and Knoll, 2003. Were this the case, secondary siliceous replacement might have been favored both by the quantity of degraded organic matter in the thick walls, as suggested above, and by the additional sites for silica nucleation afforded by the fine-grained silica present within the walls. This possibility would require that the silica in the mixed carbonaceous-siliceous wall be primary in origin rather than a product of secondary replacement. Although it seems unlikely, it is also possible that the thick
wall of Taruma rata n. gen. n. sp.was originally inorganic, perhaps being calcareous or composed of cryptocrystalline chalcedony or amorphous opaline silica. If originally calcareous, the wall as preserved would have to have been subsequently replaced by quartz.However, if originally composed of very fine-grained silica, the preserved wall would more likely have been recrystallized rather than silica-replaced, a mode of preservation that might explain how it attained its quartzose compositionwithout alteration of its shape and thickness or dissolution of its encompassing dolomitized rind of early diagenetic fibrous cement. Interestingly, both these possibilities would require the occurrence of biominer- alization, for which at present there is no definitive evidence. It nevertheless remains conceivable that further detailed study of the siliceous walls of Taruma rata (e.g., Fig. 6.11, 6.13) might reveal textural evidence, such as the occurrence of carbonate inclusions or relict ultrastructure, indicative of biomineralization that,were it to be substantiated,would be unlikely to have resulted from either silica-agglutination or kleptosquamy, processes that both give rise to irregular test surfaces (Fig. 7.1). A final question to be considered is “why are organic walls
preserved in almost all Urucum VSMs when they are so rarely known in other occurrences of VSMs?” On the basis of the evidence available, we suggest that the answer lies in their encasement during very early diagenesis by a rind of isopachous carbonate cement, which we interpret to have been fundamental to impeding significant subsequent diagenetic degradation (cf., Butts and Briggs, 2011).
Conclusions
Vase-shaped microfossils of the Neoproterozoic Urucum Forma- tion (Jacadigo Group, west-central Brazil) are diverse and exhibit well-preserved walls, providing insight into the biological affinity and evolutionary status of this evidently early originating group of protozoans. Fibrous carbonate cement entombed the tests during early diagenesis, conserving their shape and preserving evidence of their original organic and organo-siliceous compositions. Secondary silica evidently replaced some tests, the occurrence of which does not exclude the possibility of some Urucum VSMs being originally biomineralized. In conjunction with previously described VSMs, the Urucum specimens substantiate the global distribution, early diversification, and abundance of arcellinid testate amoebae in Neoproterozoic oceans.
Acknowledgments
Adoctoral scholarship provided by FAPESP (Proc. 2013/12852-1) supported L.M.; A.K.G. was supported by a UCLA-Eugene V. Cota-Robles Fellowship and CSEOL (the UCLA Center for the Study of Evolution and the Origin of Life); D.J.G.L. is supported
by FAPESP (Proc. 2013/04585-3); and A.B.K., by CSEOL and WARC (the University of Wisconsin Astrobiology Research Consortium). The authors also thank M.F. Zaine, C. Simonetti, and P.C. Boggiani who participated in earlier research on these fossils, the Journal’s reviewers, S. Porter (University of California, Santa Barbara [UCSB]) and M. Marti Mus (Universidad de Extremadura, Spain), and editor M. Moczydłowska-Vidal, for useful suggestions.
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