GEOLOGY | Volume 45 | Number 5 | www.gsapubs.org 387
A Laurentian record of the earliest fossil eukaryotes
Zachary R. Adam1,2, Mark L. Skidmore1, David W. Mogk1, and Nicholas J. Butterfield3
1Department of Earth Sciences, Montana State University, Bozeman, Montana 59717, USA
2Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts 02138, USA
3Department of Earth Sciences, University of Cambridge, Cambridge CB2 3EQ, UK
ABSTRACT
The oldest evidence of eukaryotes in the fossil record comes from 
a recurrent assemblage of morphologically differentiated late Paleo-
proterozoic to early Mesoproterozoic microfossils. Although widely 
distributed, the principal constituents of this Tappania-Dictyosphaera-
Valeria assemblage have not hitherto been recognized on Lauren-
tia. We have recovered all three taxa from a shallow-water shale 
succession in the early Mesoproterozoic Greyson Formation (Belt 
Supergroup, Montana, USA). An exceptionally preserved population 
of Tappania substantially expands the morphological range of this 
developmentally complex organism, suggesting phylogenetic place-
ment within, or immediately adjacent to, crown-group eukaryotes. 
Correspondence with Tappania-bearing biotas from China, India, 
Australia, and Siberia demonstrates an open-ocean connection to 
the intracratonic Belt Basin and, along with broadly co-occurring 
macrofossils Grypania and Horodyskia, supports the recognition of 
a globally expressed biozone. The Greyson Formation, along with 
contiguous strata in Glacier National Park, is unique in preserving all 
currently confirmed taxa of early eukaryotic and macroscopic fossils.
INTRODUCTION
The fossil record of eukaryotes extends back to at least the late Paleo-
proterozoic (Butterfield, 2015), and a recurrent assemblage of distinctive 
microfossils from China (Ruyang Group; Yin, 1997; Yin et al., 2005), Aus-
tralia (Roper Group; Javaux et al., 2001; Javaux and Knoll, 2016), India 
(Semri Group; Prasad et al., 2005), and Siberia (Kamo Group; Nagovitsin, 
2009) points to the presence of an ecologically coherent biota extending 
from ca. 1650 Ma to ca. 1400 Ma. Typified by process-bearing Tappania, 
reticulate-walled Dictyosphaera/Shuiyoushaeridium, and concentrically 
striated Valeria, these earliest fossil eukaryotes offer key insights into the 
early evolution of the clade, and enable practical applications to paleo-
biogeography and biostratigraphic correlation.
Proterozoic eukaryotes are also richly represented on Laurentia; how-
ever, their oldest confirmed records on this craton are of late Mesopro-
terozoic age (Butterfield and Chandler, 1992; Butterfield, 2000; Hofmann 
and Jackson, 1994). Older fossils are known from North America, notably 
Grypania and Horodyskia from the early Mesoproterozoic Belt Super-
group (Walter et al., 1976; Fedonkin and Yochelson, 2002), but neither 
of these macroscopic forms is unambiguously eukaryotic (Butterfield, 
2009). Apart from our report of long-ranging Valeria in the Chamberlain 
Formation (Adam et al., 2016), the same is true for spheroidal and filamen-
tous microfossils that have been recovered from Belt Supergroup strata 
(Horodyski, 1980). Here we report a full Tappania-Dictyosphaera-Valeria 
assemblage from the Greyson Formation of the lower Belt Supergroup, 
filling a major gap in the early eukaryotic record.
GEOLOGICAL SETTING
The Belt Supergroup contains as much as 15 km of strata, extend-
ing from British Columbia to central Montana, including an eastward-
extending limb termed the Helena embayment (Winston and Link, 1993) 
(Fig. 1A). This intracratonic sedimentary succession has been interpreted 
as a restricted marine (Horodyski, 1993; Lyons et al., 2000), possibly even 
a lacustrine (Winston, 1993), deposit. The paleogeographic relationships 
of the Belt Basin are poorly constrained, with different reconstructions 
identifying Siberia, Australia, or Antarctica as the conjugate rift margin 
(Sears and Price, 2003).
The Helena embayment preserves the lowermost strata of the Belt 
Supergroup, representing a broadly transgressive-regressive sedimentary 
succession. The fossils described here come from unoxidized shales of 
the Greyson Formation (Montana, USA) that overlie subtidal carbonate-
shale deposits of the Newland Formation, and are overlain in turn by mud-
cracked redbeds of the Spokane Formation (Fig. 1B). The Greyson Forma-
tion crops out extensively in the Little and Big Belt Mountains, including 
the type section of Grypania at Deep Creek (Walter et al., 1976), and is 
broadly correlative with the Appekunny Formation in Glacier National 
Park (Fig. 1A; Horodyski, 1993; Slotznick et al., 2016), including the 
type section of Horodyskia (Fedonkin and Yochelson, 2002). Our study is 
based on a roadcut section of the Greyson Formation ~50 km northeast of 
Deep Creek, along Newlan Creek Road near White Sulphur Springs, Mon-
tana (46.668°N, 110.884°W) (Fig. 1C), where it comprises ~1100 m of 
GEOLOGY, May 2017; v. 45; no. 5; p. 387–390 | Data Repository item 2017114 | doi:10.1130/G38749.1 | Published online 15 March 2017
© 2017 The Authors. Gold Open Access: This paper is published under the terms of the CC-BY license.
Figure 1. Geology of the fossiliferous Newlan Creek Road section of the 
Greyson Formation (Montana, USA). A: Extent of the Belt Supergroup, 
with the Helena embayment to the east. B: Stratigraphy of the Belt 
Supergroup within the Helena embayment; fossil horizons are marked 
by black stars. M.—member. C: Local bedrock geology, including the 
Grypania locality at Deep Creek and the Tappania-Dictyosphaera-
Valeria–bearing section along Newlan Creek Road.
388 www.gsapubs.org | Volume 45 | Number 5 | GEOLOGY
finely laminated mudstones with isolated sandstone layers. We recognize 
three informal constituent units: (1) a lower siltstone member (~350 m); 
(2) a middle shale member (~435 m); and (3) an upper siltstone-sandstone 
member (~315 m). Multiple small-scale shallowing-upward sequences 
in the two upper members are interpreted to reflect sedimentation in 
shallow-shelf to subshoreface environments.
The age of the Greyson Formation is constrained by a 1454 ± 9 Ma 
U-Pb date from a bentonite layer in the overlying Helena Formation of 
the Middle Belt Carbonate Group (Evans et al., 2000), and is corroborated 
by 1470–1460 Ma sills intruding correlative Lower Belt Group strata of 
the Prichard Formation (Sears et al., 1998). The maximum age is set as 
1710 Ma by the youngest age of a suite of zircons recovered from the 
underlying Neihart Quartzite (Mueller et al., 2016), although basement 
rocks in northern Idaho dated at 1576 ± 13 Ma underlie a unit thought 
to be correlative to the Neihart Quartzite (Evans and Fischer, 1986; see 
the GSA Data Repository1).
PALEONTOLOGY
Nine unoxidized mudstone horizons of the Greyson Formation at 
Newlan Creek were collected for microfossil processing. Samples were 
broken into 1–3 cm fragments, allowed to disaggregate with minimal 
agitation in 48% hydrofluoric acid, and fossils were individually picked 
from rinsed slurries.
Microfossils were recovered from five horizons, all within or imme-
diately below the shale member (Fig. 1B). The assemblages are domi-
nated by simple spheroidal microfossils up to 300 µm in diameter (Leio-
sphaeridia spp.) regularly accompanied by a variety of paired, colonial, 
and budding forms, including Satka favosa and Gemmuloides doncookii; 
medial-split release structures are also common (Figs. 2C–2F, 2H). Such 
morphologies are common among extant protistan-grade eukaryotes, but 
the absence of any demonstrably crown-group features leaves these fos-
sils taxonomically unresolved. Filamentous microfossils (Siphonophycus, 
Oscillatoriopsis; Figs. 2A, 2B) are most likely benthic photosynthesizers 
(Butterfield and Chandler, 1992), providing an independent measure of 
shallow-water deposition in the Greyson Formation. The longitudinally 
striated filament Lineaforma elongata (Figs. 2G, 2I) may prove to be 
biostratigraphically useful with all currently reported occurrences limited 
to the early Mesoproterozoic (Vorob’eva et al., 2015; Adam et al., 2016; 
Javaux and Knoll, 2016).
The most significant fossils in the Greyson microbiota are morphologi-
cally differentiated vesicles assignable to Tappania plana, Dictyosphaera 
macroreticulata, and Valeria lophostriata. Only Valeria (Figs. 3D, 3E) has 
been previously documented on Laurentia: in the middle Neoproterozoic 
Chuar Group of Arizona (Porter and Riedman, 2016), the late Mesopro-
terozoic Bylot basin system of Canada (Butterfield and Chandler, 1992; 
Hofmann and Jackson, 1994), and the Lower Belt Group Chamberlain 
Formation  (Adam et al., 2016). With a total age range exceeding 700 m.y., 
it offers no useful stratigraphic resolution (Hofmann, 1999). There are, 
however, good grounds for recognizing its fine concentric microstructure 
as diagnostically eukaryotic (Javaux et al., 2001); modern analogs are 
found in the microfibrillar cell-wall construction of glaucophyte algae 
(cf. Willison and Brown, 1978).
Dictyosphaera is distinguished by its relatively large vesicles (50–300 
µm) and a complex polygonal wall structure well beyond the limits of 
prokaryotic morphogenesis (Yin et al., 2005; Agić et al., 2015). Although 
the inner surface of the wall is not seen in the Greyson population (Figs. 
3A–3C), the specimens correspond closely with the Ruyang Group type 
material in terms of outer wall ornamentation, size, medial split structures, 
and the presence of a large intracellular inclusion in some unsplit specimens 
1 GSA Data Repository item 2017114, geochronological constraints on the 
Greyson Formation, is available online at http://www.geosociety.org /datarepository 
/2017/ or on request from editing@geosociety.org.
Figure 2. Filamentous and spheroidal microfossils from the Greyson 
Formation (Montana, USA). A: Oscillatoriopsis longa. B: Siphonophycus 
sp. C: Colonial Satka favosa. D: Leiosphaeridia sp. with a medial split. 
E: Colonial spheroidal form with single larger cell developing a lateral 
extension. F: Colonial Coneosphaera sp. G, I: Lineaforma elongata. H: 
Gemmuloides doncookii. Scale bar applies to all images.
Figure 3. Eukaryotic microfossils from the Greyson Formation (Mon-
tana, USA). A–C: Dictyosphaera macroreticulata, with reticulate wall 
structure, intracellular inclusion (A) and medial split (C). D, E: Vale-
ria lophostriata, with concentric wall microstructure. Dark scale bar 
applies to A–D.
GEOLOGY | Volume 45 | Number 5 | www.gsapubs.org 389
(Fig. 3A; cf. Pang et al., 2013). The absence of an acanthomorphic (Shuiy-
ousphaeridum) phase seen in the late Paleoproterozoic Ruyang Group and 
Semri Group biotas (Pang et al., 2013; Singh and Sharma, 2014; Agić et 
al., 2015) is most likely due to small sample size and local taphonomy.
The most conspicuous and common eukaryote in the Greyson Forma-
tion biota is Tappania plana (Fig. 4), a microfossil distinguished by tubular 
processes and broader neck-like extensions (Yin, 1997). Most Greyson 
Formation Tappania specimens correspond to previously described popu-
lations, but the large sample size reveals a substantially new account of its 
anatomy. Vesicles range from 30 to 150 µm (x = 89 µm, standard deviation 
= 23 µm, N = 53) with shapes varying from equidimensional to elongate 
(Fig. 4C). Both the processes (N ≤ 20) and neck-like extensions (N ≤ 4) 
vary in number and are randomly distributed on vesicles, although many 
specimens show clear evidence of hemispherical polarization (Figs. 4B, 4D, 
4E, 4H–4J). Process branching shows a broad range of expression, includ-
ing equidimensional (Figs. 4B,4E, 4H, 4I) and size-reductive bifurcations 
(Figs. 4B, 4I, 4J), second-order branching (Figs. 4B, 4H), and vesicles 
bearing multiple branched process (Figs. 4B, 4I). As in the Kamo Group 
and Roper Group populations, there are rare instances of processes with 
internal partitions (Fig. 4F). Some specimens also bear outgrowths directly 
comparable to T. tubata (Fig. 4J), a putative second species.
Greyson Formation Tappania specimens exhibit a number of features not 
seen in other populations, most notably the differentiation of an outer wall 
(Figs. 4A, 4B, 4G). Although often expressed as a simple enveloping sheath, 
this relatively translucent layer can also be seen forming processes that 
are not present, or only partially developed, on the more robust inner wall; 
such independent morphogenesis demonstrates a particularly sophisticated 
level of cytoskeletal control. A further subset of specimens has terminally 
flared processes (Figs. 4A, 4B, 4D, 4G, 4H), suggesting suspension within 
a third (unpreserved) outer envelope or adventitious substrate attachment.
In a conventional form-taxonomic context, the novel features and 
variation exhibited by Greyson Formation Tappania specimens would 
warrant the establishment of multiple new form genera and form spe-
cies. Even within this relatively large population, however, there are no 
obvious modes in size or form. Such morphological continuity, anchored 
to the presence of diagnostic neck-like extensions, points to pronounced 
ontogenetic and/or ecophenotypic variation (cf. Yin et al., 2005; Javaux 
and Knoll, 2017). On the basis of current evidence, Tappania represents 
a single biological species, T. plana.
DISCUSSION
The complex wall morphologies and microstructures exhibited by 
Greyson Formation Tappania, Dictyosphaera, and Valeria specimens are 
diagnostically eukaryotic. Such features are entirely outside the capacity 
of any organisms lacking a fully motorized cytoskeleton and endomem-
brane system (Javaux et al., 2001; Cavalier-Smith, 2002; Pang et al., 2013; 
Agić et al., 2015; Butterfield, 2015). The sophistication of this cytologi-
cal machinery is particularly apparent in Tappania, where the inducible 
development of at least three separate components (neck-like extensions, 
processes, and independently deployed outer wall) demonstrates a level of 
morphogenetic control comparable to that of all but the most derived living 
protists. In this light, there is an increasingly secure case for placing Tap-
pania within, or immediately adjacent to, crown-group Eukarya, despite 
its unresolved affiliations (Javaux and Knoll, 2016). This same conclusion 
may be drawn for Dictyosphaera and Shuiyousphaeridium (Pang et al., 
2013; Agić et al., 2015), although not for fundamentally simpler Valeria.
All previous records of Tappania and Dictyosphaera came from a 
relatively narrow range of late Paleoproterozoic to early Mesoproterozoic 
strata, pointing to their utility as biostratigraphic markers (Yin et al., 2005; 
Agić et al., 2015; Javaux and Knoll, 2017). This potential is bolstered by the 
presence of two distinct forms exhibiting the same temporal distribution, 
along with a third (Valeria) that is conspicuously different. Extension of 
a Tappania-Dictyosphaera-Valeria assemblage zone into Laurentia now 
confers a corroborating global signal. Notably, the ca. 1450 Ma minimum 
age for the Greyson Formation biota overlaps the age of Roper Group fos-
sils (cf. Javaux et al., 2001), which are bracketed in turn by the marginally 
younger Kamo Group and older Ruyang Group and Semri Group biotas.
A Tappania-Dictyosphaera-Valeria biozone is further supported by 
the accompanying occurrence of Grypania and Horodyskia. Despite the 
fundamentally unresolved affiliations of these oldest-known macrofossils, 
both contribute a possible biostratigraphic signal. In addition to its type 
occurrence in the Greyson Formation, Grypania spiralis is known from 
the early Mesoproterozoic Changchengian System of north China (Du et 
al., 1986) and the late Paleoproterozoic Semri Group of India (Sharma 
and Shukla, 2009), along with a less resolved population in the 1.87 Ga 
Negaunee Iron-Formation of Michigan, USA (Han and Runnegar, 1992). 
Horodyskia is biologically more problematic and extends from the early 
Mesoproterozoic Appekunny Formation in Glacier National Park (Mon-
tana) through Mesoproterozoic–Neoproterozoic strata in Australia (Grey 
et al., 2010; Calver et al., 2010) and the Ediacaran of south China (Dong et 
al., 2008). Even so, its first appearance notably is within the narrow time 
frame delineated by the cooccurrence of Tappania plana, Dictyosphaera 
macroreticulata, Valeria lophostriata, and Grypania spiralis.
Insofar as the Greyson Formation in the Helena embayment is strati-
graphically contiguous with the Appekunny Formation in Glacier National 
Park, this shallow shelf succession is unique in preserving a complete 
roster of all known early eukaryotic and macroscopic fossil taxa, along 
with a comprehensive record of simple spheroidal and filamentous form 
taxa. The succession demonstrates a clear connection between the Helena 
embayment and the global oceans (cf. Winston, 1993), with important 
implications for early Proterozoic paleogeography and the global distri-
bution of the planet’s oldest known eukaryotes.
ACKNOWLEDGMENTS
Z. Adam was supported by the National Science Foundation Graduate Research 
Fellowship Program, the Lewis and Clark Fund for Exploration Research (NASA 
Astrobiology Institute and American Philosophical Society), the Tobacco Root 
Geological Society, the Belt Association, the D.L. Smith Fund, and a Geobiology 
Postdoctoral Fellowship from the Agouron Institute. We thank D. Winston, G. Zieg, 
J. Schmitt, and D. Lageson for useful discussions. We thank S. Xiao, E. Javaux, 
and S. Porter for insightful manuscript review comments.
Figure 4. Eukaryotic Tappania plana from the Greyson Formation (Mon-
tana, USA), showing variably branched processes (B, E, H–J), terminally 
flared processes (A, B, D, G, H), hemispherically polarized processes 
(B, D, E, H–J), septate processes (F), tubular outgrowth comparable to T. 
tubata (J), and an outer wall capable of independent process formation 
(blue arrows in A, B, G). Scale bar applies to all images.
390 www.gsapubs.org | Volume 45 | Number 5 | GEOLOGY
REFERENCES CITED
Adam, Z.R., Skidmore, M.L., and Mogk, D.W., 2016, Paleoenvironmental implica-
tions of an expanded microfossil assemblage from the Chamberlain Formation, 
Belt Supergroup, Montana, in MacLean, J.S., and Sears, J.W., eds., Belt Basin: 
Window to Mesoproterozoic Earth: Geological Society of America Special 
Paper 522, p. 101–119, doi: 10 .1130 /2016 .2522 (04).
Agić, H., Moczydłowska, M., and Yin, L.-M., 2015, Affinity, life cycle, and intra-
cellular complexity of organic-walled microfossils from the Mesoproterozoic 
of Shanxi, China: Journal of Paleontology, v. 89, p. 28–50, doi: 10 .1017 /jpa 
.2014 .4.
Butterfield, N.J., 2000, Bangiomorpha pubescens n. gen., n. sp.: Implications for 
the evolution of sex, multicellularity, and the Mesoproterozoic/Neoproterozoic 
radiation of eukaryotes: Paleobiology, v. 26, p. 386–404, doi: 10 .1666 /0094 
-8373 (2000)026 <0386: BPNGNS>2 .0 .CO;2.
 Butterfield, N.J., 2009, Modes of pre-Ediacaran multicellularity: Precambrian 
Research, v. 173, p. 201–211, doi: 10 .1016 /j .precamres .2009 .01 .008.
 Butterfield, N.J., 2015, Early evolution of the Eukaryota: Palaeontology, v. 58, 
p. 5–17, doi: 10 .1111 /pala .12139.
Butterfield, N.J., and Chandler, F.W., 1992, Paleoenvironmental distribution of 
Proterozoic microfossils, with an example from the Agu Bay Formation, Baf-
fin Island: Palaeontology, v. 35, p. 943–957.
Calver, C.R., Grey, K., and Laan, M., 2010, The ‘string of beads’ fossil (Horo-
dyskia) in the mid-Proterozoic of Tasmania: Precambrian Research, v. 180, 
p. 18–25, doi: 10 .1016 /j .precamres .2010 .02 .005.
Cavalier-Smith, T., 2002, The neomuran origin of archaebacteria, the negibacte-
rial root of the universal tree and bacterial megaclassification: International 
Journal of Systematic and Evolutionary Microbiology, v. 52, p. 7–76, doi: 
10.1099 /00207713 -52 -1 -7.
Dong, L., Xiao, S., Shen, B., and Zhou, C., 2008, Silicified Horodyskia and Palaeo-
pascichnus from upper Ediacaran cherts in south China: Tentative phylogenetic 
interpretation and implications for evolutionary stasis: Journal of the Geologi-
cal Society [London], v. 165, p. 367–378, doi: 10 .1144 /0016 -76492007 -074.
Du, R., Tian, L., and Li, H., 1986, Discovery of megafossils in the Gaoyuzhuang 
Formation of the Changchengian System, Jixian: Acta Geologica Sinica, v. 60, 
p. 115–120.
Evans, K.V., and Fischer, L.B., 1986, U-Pb geochronology of two augen gneiss 
terranes, Idaho—New data and tectonic implications: Canadian Journal of 
Earth Sciences, v. 23, p. 1919–1927, doi: 10 .1139 /e86 -179.
Evans, K.V., Aleinikoff, J.N., Obradovich, J.D., and Fanning, C.M., 2000, SHRIMP 
U-Pb geochronology of volcanic rocks, Belt Supergroup, western Montana: 
Evidence for rapid deposition of sedimentary strata: Canadian Journal of Earth 
Sciences, v. 37, p. 1287–1300, doi: 10 .1139 /e00 -036.
Fedonkin, M.A., and Yochelson, E.L., 2002, Middle Proterozoic (1.5 Ga) Horo-
dyskia moniliformis Yochelson and Fedonkin, the oldest known tissue-grade 
colonial eucaryote: Smithsonian Contributions to Paleobiology, v. 94, p. 1–29, 
doi: 10 .5479 /si .00810266 .94 .1.
Grey, K., Yochelson, E.L., Fedonkin, M.A., and Martin, D.M., 2010, Horodyskia 
williamsii new species, a Mesoproterozoic macrofossil from Western Australia: 
Precambrian Research, v. 180, p. 1–7, doi: 10 .1016 /j .precamres .2010 .02 .006.
Han, T.M., and Runnegar, B., 1992, Megascopic eukaryotic algae from the 2.1-bil-
lion-year-old Negaunee Iron-Formation, Michigan: Science, v. 257, p. 232–
235, doi: 10 .1126 /science .1631544.
Hofmann, H.J., 1999, Global distribution of the Proterozoic sphaeromorph acri-
tarch Valeria lophostriata (Jankauskas): Acta Micropalaeontologica Sinica, 
v. 16, p. 215–224.
Hofmann, H.J., and Jackson, G., 1994, Shale-facies microfossils from the Pro-
terozoic Bylot Supergroup, Baffin Island, Canada: Paleontological Society 
Memoir 37, 39 p.
Horodyski, R.J., 1980, Middle Proterozoic shale-facies microbiota from the lower 
Belt Supergroup, Little Belt Mountains, Montana: Journal of Paleontology, 
v. 54, p. 649–663.
Horodyski, R.J., 1993, Paleontology of Proterozoic shales and mudstones: Ex-
amples from the Belt Supergroup, Chuar Group and Pahrump Group, west-
ern USA: Precambrian Research, v. 61, p. 241–278, doi: 10 .1016 /0301 -9268 
(93) 90116-J.
Javaux, E.J., and Knoll, A.H., 2017, Micropaleontology of the lower Mesoprotero-
zoic Roper Group, Australia, and implications for early eukaryotic evolution: 
Journal of Paleontology, doi: 10 .1017 /jpa .2016 .124.
Javaux, E.J., Knoll, A.H., and Walter, M.R., 2001, Morphological and ecological 
complexity in early eukaryotic ecosystems: Nature, v. 412, p. 66–69, doi: 10 
.1038 /35083562.
Lyons, T.W., Luepke, J.J., Schreiber, M.E., and Zieg, G.A., 2000, Sulfur geochemi-
cal constraints on Mesoproterozoic restricted marine deposition: Lower Belt 
Supergroup, northwestern United States: Geochimica et Cosmochimica Acta, 
v. 64, p. 427–437, doi: 10 .1016 /S0016 -7037 (99)00323 -3.
Mueller, P., Mogk, D., Wooden, J., and Spake, D., 2016, U-Pb ages of zircons from 
the Lower Belt Supergroup and proximal crystalline basement: Implications 
for the early evolution of the Belt Basin, in MacLean, J.S., and Sears, J.W., 
eds., Belt Basin: Window to Mesoproterozoic Earth: Geological Society of 
America Special Paper 522, p. 283–303, doi: 10 .1130 /2016 .2522 (11).
Nagovitsin, K., 2009, Tappania-bearing association of the Siberian platform: Bio-
diversity, stratigraphic position and geochronological constraints: Precambrian 
Research, v. 173, p. 137–145, doi: 10 .1016 /j .precamres .2009 .02 .005.
Pang, K., Tang, Q., Schiffbauer, J., Yao, J., Yuan, X., Wan, B., Chen, L., Ou, Z., and 
Xiao, S., 2013, The nature and origin of nucleus-like intracellular inclusions 
in Paleoproterozoic eukaryote microfossils: Geobiology, v. 11, p. 499–510, 
doi: 10 .1111 /gbi .12053.
Porter, S.M., and Riedman, L.A., 2016, Systematics of organic-walled microfossils 
from the ca. 780–740 Ma Chuar Group, Grand Canyon, Arizona: Journal of 
Paleontology, v. 90, p. 815–853, doi: 10 .1017 /jpa .2016 .57.
Prasad, B., Uniyal, S., and Asher, R., 2005, Organic-walled microfossils from 
the Proterozoic Vindhyan Supergroup of Son Valley, Madhya Pradesh, India: 
Palaeobotanist, v. 54, p. 13–60.
Sears, J.W., and Price, R.A., 2003, Tightening the Siberian connection to western 
Laurentia: Geological Society of America Bulletin, v. 115, p. 943–953, doi: 
10 .1130 /B25229 .1.
Sears, J.W, Chamberlain, K., and Buckley, S., 1998, Structural and U-Pb geochro-
nological evidence for 1.47 Ga rifting in the Belt basin, western Montana: 
Canadian Journal of Earth Sciences, v. 35, p. 467–475, doi: 10 .1139 /e97 -121.
Sharma, M., and Shukla, Y., 2009, Taxonomy and affinity of early Mesoproterozoic 
megascopic helically coiled and related fossils from the Rohtas Formation, 
the Vindhyan Supergroup, India: Precambrian Research, v. 173, p. 105–122, 
doi: 10 .1016 /j .precamres .2009 .05 .002.
Singh, V.K., and Sharma, M., 2014, Morphologically complex organic-walled micro-
fossils (owm) from the late Palaeoproterozoic–early Mesoproterozoic Chitrakut 
Formation, Vindhyan Supergroup, central India and their implications on the an-
tiquity of eukaryotes: Palaeontological Society of India Journal, v. 59, p. 89–102.
Slotznick, S.P., Winston, D., Webb, S.M., Kirschvink, J.L., and Fischer, W.W., 
2016, Iron mineralogy and redox conditions during deposition of the mid-
Proterozoic Appekunny Formation, Belt Supergroup, Glacier National Park, in 
MacLean, J.S., and Sears, J.W., eds., Belt Basin: Window to Mesoproterozoic 
Earth: Geological Society of America Special Paper 522, p. 221–242, doi: 
10.1130 /2016 .2522 (09).
Vorob’eva, N.G., Sergeev, V.N., and Petrov, P.Y., 2015, Kotuikan Formation as-
semblage: A diverse organic-walled microbiota in the Mesoproterozoic Anabar 
succession, northern Siberia: Precambrian Research, v. 256, p. 201–222, doi: 
10 .1016 /j .precamres .2014 .11 .011.
Walter, M., Oehler, J.H., and Oehler, D.Z., 1976, Megascopic algae 1300 million 
years old from the Belt Supergroup, Montana: A reinterpretation of Walcott’s 
Helminthoidichnites: Journal of Paleontology, v. 50, p. 872–881.
Willison, J.H., and Brown, R.M., 1978, Cell wall structure and deposition in Glau-
cocystis: Journal of Cell Biology, v. 77, p. 103–119, doi: 10 .1083 /jcb .77 .1 .103.
Winston, D., 1993, Lacustrine cycles in the Helena and Wallace Formations, Middle 
Proterozoic Belt Supergroup, western Montana, in Proceedings of the Belt Sym-
posium III: Montana Bureau of Mines and Geology Special Publication 112, 
p. 70–86.
Winston, D., and Link, P., 1993, Middle Proterozoic rocks of Montana, Idaho 
and eastern Washington: The Belt Supergroup, in Reed, J.C., Jr., et al., eds., 
Precambrian: Conterminous U.S.: Boulder, Colorado, Geological Society of 
America, Geology of North America, v. C-2, p. 487–517.
Yin, L.-M., 1997, Acanthomorphic acritarchs from Meso-Neoproterozoic shales of 
the Ruyang Group, Shanxi, China: Review of Palaeobotany and Palynology, 
v. 98, p. 15–25, doi: 10 .1016 /S0034 -6667 (97)00022 -5.
Yin, L.-M., Xunlai, Y., Fanwei, M., and Jie, H., 2005, Protists of the upper Meso-
proterozoic Ruyang Group in Shanxi Province, China: Precambrian Research, 
v. 141, p. 49–66, doi: 10 .1016 /j .precamres .2005 .08 .001.
Manuscript received 31 October 2016 
Revised manuscript received 23 December 2016 
Manuscript accepted 28 December 2016
Printed in USA