Journal of Paleontology, 91(3), 2017, p. 393–406. Copyright © 2017, The Paleontological Society. This is an Open Access article, distributed under the terms of the Creative Commons Attribution licence (
http://creativecommons.org/licenses/by/4.0/), which permits unrestricted re-use, distribution, and reproduction in any medium, provided the original work is properly cited. 0022-3360/15/0088-0906 doi: 10.1017/jpa.2017.16
Carbonaceous and siliceous Neoproterozoic vase-shaped microfossils (Urucum Formation, Brazil) and the question of early protistan biomineralization
Luana Morais,1 Thomas Rich Fairchild,2 Daniel J.G. Lahr,3 Isaac D. Rudnitzki,4,5 J. William Schopf,6,7,8,9 Amanda K. Garcia,6,7 Anatoliy B. Kudryavtsev,6,7 and Guilherme R. Romero1
1Graduate program in Geochemistry and Geotectonics, Institute of Geosciences, University of São Paulo. Rua do Lago, 562, Cidade Universitaria,
CEP: 05508-080, São Paulo, Brazil ⟨
lumorasoa@yahoo.com.br⟩, ⟨
graffaeli@gmail.com⟩ 2Department of Sedimentary and Environmental Geology, Institute of Geosciences, University of São Paulo, Rua do Lago, 562, Cidade
Universitária, CEP: 05508-080, São Paulo, Brazil ⟨
trfairch@hotmail.com⟩ 3Department of Zoology, Institute of Biosciences, University of São Paulo, Rua do Matão, travessa 14, 101, Cidade Universitária,
CEP: 05508-090, São Paulo, Brazil ⟨
dlahr@ib.usp.br⟩ 4Departament of Geophysics, Institute of Astronomy, Geophysics and Atmospheric Sciences, University of São Paulo, Rua do Matão, 1226, CEP:
05508-900 São Paulo, Brazil ⟨
idrgeo@gmail.com⟩ 5Federal University of Ouro Preto, Department of Geology, Ouro Preto, Rua Diogo de Vasconcelos, 122, Minas Gerais, CEP: 35400-000 6Department of Earth, Planetary, and Space Sciences, 595 Charles E. Young Drive East, University of California, Los Angeles, CA 90095, United
States ⟨
schopf@ess.ucla.edu⟩, ⟨
agarcia9@ucla.edu⟩, ⟨
kudryavtsev@ess.ucla.edu⟩ 7Center for the Study of Evolution and the Origin of Life, 595 Charles E. Young Drive East, University of California, Los Angeles, CA 90095,
United States 8Molecular Biology Institute, 495 Hilgard Avenue, University of California, Los Angeles, CA 90095, United States 9University of Wisconsin Astrobiology Research Consortium, Department of Geosciences, 1215 W. Dayton St., University of Wisconsin, Madison, WI 53706, United States
Abstract.—Vase-shaped microfossils (VSMs) occur in dolomitic extraclasts of indeterminate provenance within the basal diamictite of the Neoproterozoic Urucum Formation (Jacadigo Group) of west-central Brazil, having an age constrained between 889±44 Ma (K-Ar; basement rocks) and 587±7Ma(40Ar/39Ar age of early metamorphic cryptomelane in overlying manganese ore). Early isopachous carbonate cement entombed these VSMs, preserving rare direct evidence of original wall composition that is carbonaceous (now kerogenous) in practically all specimens. Some tests are siliceous or composed of a quartz-kerogen mixture; secondary replacement explains some features of these tests, but original biomineralization seems more likely for others. This interpretation, coupled with test morphology, suggests affinity to arcellinid testate amoebae. Five VSM taxa are recognized in the deposit: Cycliocyrillium simplex Porter, Meisterfeld, and Knoll, 2003, and C. torquata Porter, Meisterfeld, and Knoll, 2003, originally described in the Chuar Group (USA), and three new monospecific genera—Palaeoamphora urucumense n. gen. n. sp., Limeta lageniformis n. gen. n. sp., and Taruma rata n. gen. n. sp. Most of the taxonomically important characteristics of these VSMs occur also in extant testate amoebae, but the combinations of some characters, such as organic-walled tests having exceptionally long necks that exhibit terminal apertures (L. lageniformis n. gen. n. sp.), are evidently novel additions to the known diversity of Neoproterozoic VSMs. Evidence of glacially influenced deposition in the conformably overlying Santa Cruz Formation may indicate that the Urucum Formation slightly preceded or was penecontemporaneous with a major Neoproterozoic glaciation, although the VSM-hosting extraclasts must be older, possibly rivaling the age of the testate amoebae of the Chichkan Formation (766±7Ma) that are currently regarded as the oldest record of protists in the geological record.
Introduction
Molecular phylogenies and the fossil record indicate that eukaryotic diversification appreciably preceded the earlyCambrian appearance of metazoanswith hard parts (Knoll, 2014; Butterfield, 2015). Vase-shaped microfossils (VSMs), commonly interpreted to be the oldest evidence of heterotrophic protists in the pre-metazoan fossil record, occur widely in Neoproterozoic rocks younger than ~750Ma. As illustrated in Figure 1, themorphology
of these microfossils is similar to such unicellular eukaryotes as chitinozoans, tintinnids, foraminifera, testate amoebae, and acritarchs (Bloeser et al., 1977; Fairchild et al., 1978; Bloeser, 1985; Schopf, 1992; Porter et al., 2003; Bosak et al., 2012). Affinities of VSMs to testate amoebae were first postulated by Schopf (1992, p. 592) and later acknowledged and corroborated by Porter and Knoll (2000, p. 360), an interpretation now generally accepted for most VSMs (e.g., Bosak et al., 2011; Dalton et al., 2013; Strauss et al., 2014). Extant testate amoebae have been
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