Full text of DOI:10.1007/BF02900561

NOTES
1
Doushantuo phosphorites and phosphatized fossils
New observations on spicule-
like structures from Dou-
shantuo phosphorites at
Sandwiched between the subjacent Nantuo tillite and
superjacent Dengying dolomite, the Doushantuo Forma-
tion can be easily recognized and correlated across the
Yangtze Platform. The Doushantuo Formation contains a
unique acritarch biota featuring large (ca. 100ü1000 mi-
crometers in diameter) acanthomorphs. Similar acritarchs
have been found in Neoproterozoic rocks deposited after
the Marinoan or equivalent tillites but before typical
Ediacaran animals (see Knoll and Xiao[12] for a summary).
On the basis of bio- and chemo-stratigraphic correlations
with other well-dated Neoproterozoic sequences, Knoll
and Xiao[12] propose that the depositional age of the Dou-
shantuo Formation is between 600 and 550 Ma. This es-
timate is further supported by more recent U-Pb dating of
Ediacaran fossil-bearing successions in the White Sea area
Weng’an, Guizhou Province
YIN Leiming1, XIAO Shuhai2 & YUAN Xunlai1
1. Nanjing Institute of Geology and Paleontology, Chinese Academy of
Sciences, Nanjing 210008, China;
2. Department of Geology, Tulane University, New Orleans, LA70118,
USA
Correspondence should be addressed to Yin Leiming (e-mail: leimingyin
@yahoo.com.cn)
Abstract
Doushantuo phosphorites at Weng’an, Guizhou
in Russia, which gives an age of (555.3 0.3)Ma[13]
The Doushantuo Formation contains silicified[14]
.
Province, provide a unique opportunity for the study of early
evolution of multicellular organisms, including metazoans.
Our light microscope and SEM observations, however, do
not substantiate a sponge spicule interpretation of spicular
structures. No convincing axial canals have been seen in the
observed spicule population. Instead, the coexistence of some
monaxonal spicules with clearly diagenetic crystal fascicles
and dumbbells suggest that these monaxons may also be
diagenetic in origin. Our preliminary EDS analyses detect no
significant silicon in spicule-containing intraclasts (including
abiotic clasts and fragments of sphaeromorphic acritarchs
and algal thalli), that contain abundant monaxonal spicular
structures. We, therefore, believe that the evidence for a
sponge spicule interpretation of the Doushantuo spicular
structures are at best ambiguous at present; these alleged
sponge spicules are indistinguishable from coexistent diagen-
etic crystals.
,
phosphatized[15], pyritized[16], and carbonaceous fossils[17]
that in combination provide a fuller picture of late Neo-
proterozoic biodiversity. In particular, Doushantuo phos-
phorites at Weng’an provide important information about
the Neoproterozoic evolution of multicellularity because
of their strikingly exquisite preservation. Exceptionally
preserved multicellular eukaryotes, including fossil red
ü
algae and animal embryos[14,18 22], as well as alleged
sponge larvae and spicules[11], have been described from
the upper Doushantuo phosphorites deposited above a
flooding surface in the middle of the Doushantuo Forma-
tion. Li et al.[11] reported sponge spicules and larvae, in-
cluding demospongian parenchymellae and calcarean
amphiblastulae, from organic-rich phosphorites in the
upper Doushantuo Formation at Weng’an. More recently,
Chen et al.[23] followed up with more putative animal em-
bryos and larvae, including cnidarian and bilaterian gas-
trulae. Potentially, these recently discoveries are powerful
paleontological evidence to test hypotheses about early
animal evolution based on molecular and developmental
Keywords: Doushantuo Formation, Neoproterozoic, Guizhou,
phosphorite, sponge.
Phylogenetic analyses based on morphology and
biomolecules both suggest that sponges are a sister group
of eumetazoans[1,2]. It is therefore not surprising to see
fossil sponges among the earliest metazoan records.
Sponge biomarkers have been detected from Vendian and
late Riphean bitumens[3]. Sponge spicules have been re-
covered from the late Neoproterozoic Doushantuo and
ü
biology[24 27]. However, despite the extraordinary ta-
phonomic detail in Doushantuo phosphorites, authigenetic
and diagenetic overprinting still plays an important role in
preservation and alteration of Doushantuo multicellular
organisms[15]. Therefore, diagenesis should be carefully
considered when interpreting the biology of the Doushan-
tuo fossils. For example, the interpretation of certain
Doushantuo multicellular remains as sponge larvae by Li
et al.[11] has already been questioned[28]. On the basis of
careful observation, here we evaluate the evidence for and
against a sponge interpretation of spicule-like structures in
Doushantuo phosphorites at Weng’an.
ü
6]
Dengying formations in South China[4
and terminal
Proterozoic rocks in northern Iran[7] and Mongolia[8] (but
see Zhou et al.[9] for an alternative interpretation). Prob-
able sponge body fossils have also been reported from the
Ediacaran deposits in Australia[10]. The interpretation of
some micrometer-sized acicular structures from Dou-
shantuo phosphorites at Weng’an, Guizhou Province, as
sponge spicules[11] is, however, not straightforward. Here,
we present our results from new SEM observations on and
EDS analyses of these Doushantuo acicular objects, and
argue that the evidence for sponge spicules is ambiguous
and at least some of these spicule-like structures are of
authigenic/diagenetic origins.
2
Preservation of spicular structures
The spicule-like structures are found most abun-
dantly along with algal thalli in a horizon of black, or-
ganic-rich phosphorites, but they also occur in an organic-
poor horizon of intraclastic phosphodolomite. Often, they
1828
Chinese Science Bulletin Vol. 46 No. 21 November 2001

occur exclusively in submillimetric phosphatic intraclasts
(including abiotic clasts and fragments of poorly pre-
served sphaeromorphic acritarchs and algal thalli) (fig.
1(a), (b)) that Li et al. interpreted as sponge body fos-
sils[11], but not in the matrix. Nor are they found in any
cellularly preserved, demonstrably algal fossils (similarly
observed result by Yin and Gao[29]). It therefore appears
that the spatial distribution of the spicule-like structures is
at least consistent (although not conclusive) evidence for a
sponge interpretation. An alternative, however, would be
that the spicule-like structures are diagenetic products that
preferentially occur in poorly preserved (and thus no cel-
lular preservation) fossils or fossil fragments. To resolve
this issue, one needs to exam the morphological and geo-
chemical aspects of the spicules themselves.
These spicules are about 1ü2 Pm in diameter and
several ten Pm in length (fig. 1(c), (d)). Although their
small size in and of itself does not rule out the possibility
of sponge spicules (sponge microscleres can be only a
couple of Pm thick), careful observations on a large
population of spicule-like objects suggest that they at least
some such structures are diagenetic minerals.
(ν) SEM studies. The alleged sponge spicules are
dominated by monaxons[11]. We have examined, under a
transmitted light microscope, some intraclasts (=sponge
body fossils of Li et al.[11]) that contain monaxonal struc-
Fig.1. (a) and (b) Light photomicrographs of phosphatic intraclasts that Li et al.[11] interpreted as sponge body fossils. Notice the abun-
dance of micrometer-sized acicular structures in the intraclasts. (a) Plain light; b, cross-nicols. (c)ü(f), SEM photomicrographs of
monaxonal spicular structures. Notice multiple longitudinal striae in (d), a single longitudinal groove in (e), and a central rod enveloped
with unidentifiable minerals in (f). Scale bar (in (e)): 12 Pm for (a) and (b); 3.5 Pm for (c); 1.5 Pm for (d); 7.5 Pm for (e); 2 Pm for (f).
Chinese Science Bulletin Vol. 46 No. 21 November 2001
1829

NOTES
tures. In some populations, there are, however, a small
percentage of spicules that aggregate to form termi-
nally-spreading bundles (figs. 2(a), (b), 3(a)). We then
treat some of these intraclasts with 2%ü5% HCl and
examine them under a scanning electronic microscope.
We have observed some bundles that are composed of a
bunch of subcrystals (figs. 2(c), (d); 3(b), (c)). The coex-
istence of the bundles and the monaxons indicates that
they are of similar origin. Clearly, the bundles are not cel-
lularly controlled biomineralization as demonstrated in
sponge spicule formation sponge spicules[30]. Instead,
some of the bundles are actually thin dumbbells that may
be authigenic minerals precipitated during organic degra-
dation; similar dumbbells, though much fatter, are seen in
bacterially induced calcium carbonate precipitates[31,32]. It
is therefore possible that the co-existing monaxons are
diagenetic minerals.
Li et al.[33], in their response to Zhang et al.[28]
,
showed a monaxon with a longitudinal groove that seem-
ingly resembles an axial canal. An axial canal would be
definitive evidence for sponge spicules. Unfortunately, in
all our SEM examination, no convincing axial canals have
been found. In a few specimens, a single (fig. 1(e)) or
multiple (fig. 1(d)) longitudinal striae or grooves can be
Fig. 2. (a) and (b) Light photomicrographs of fascicles of acicular structures. (c) and (d) SEM photomicrographs of mineral aggregates
(c) and dumbbells (d) that coexist with acicular structures similar to the alleged sponge spicules[11] ((e)). Clearly, structures shown in (a)
ü(d) are unlikely to be sponge spicules. (e): light photomicrograph showing acicular structures that are interpreted as sponge spicules[11]
Note that SEM photomicrographs (c) and (d) were taken from this same spicule-bearing intraclast. (f): Light photomicrograph of spicular
.
structures that are coated with clear phosphate. Scale bar (in (a)): 35 Pm for (a); 40 Pm for (b); 3.5 Pm for (c); 2 Pm for (d); 20 Pm for (e)
and (f).
1830
Chinese Science Bulletin Vol. 46 No. 21 November 2001

Fig. 3. (a) Light photomicrographs of fascicles of acicular structures. (b) and (c) SEM photomicrographs of acicular structures and spi-
cular aggregates.
Table 1 Results of EDS analyses of phosphatized multicellular algae and spicule-bearing intraclasts.
Phophatized multicellular algae,
no coating
spicule-bearing intraclast,
no coating
Spicules (fig. 1(c)),
carbon coating
Spicules (2d),
carbon coating
Element
wt.%
12.37
26.77
2.13
error (%)
0.485
at.%
wt.%
11.59
error (%)
0.804
at.%
23.40
31.26
1.19
wt.%
error (%)
at.%
wt.%
24.05
error (%)
at.%
C
O
23.29
0.285
37.83 20.62
0.469
24.73
1.20
0.340
0.053
42.68
1.75
0.429
42.21
F
0.063
2.53
0.20
0.93
0.074
Na
Al
Si
P
0.21
0.012
0.15
1.00
0.015
0.035
0.158
0.033
0.311
0.13
0.87
0.17
0.62
0.013
0.024
0.112
0.109
0.275
0.076
0.18
0.62
0.97
16.66
0.98
0.020
0.085
0.022
0.162
0.78
12.16
0.69
1.33
0.027
0.110
1.30
18.14
0.71
14.46
0.53
20.63
18.39
12.80
11.27
49.78
1.31
11.60
9.87
S
Ca
Fe
39.92
22.52
46.54
28.16
52.11
0.218
35.88
34.87
0.66
seen, but, because these grooves are not centrally located,
they may not be axial canals. One rare specimen (fig. 1(f))
appears to have a centrally located rod (ca. 0.5 ȝm thick)
that is partially enveloped with a 0.5-ȝm-thick layer of
unknown mineral. It is possible that this rod might be the
cast of an axial canal. Alternatively, the rod is originally
solid but coated with diagenetic minerals. Such coating is
partially dissolved in HCl maceration (fig. 1(f)) and
therefore is not siliceous. Phosphatic coating on spi-
cule-like structures is commonly seen in thin sections (fig.
2(f)). Therefore, evidence for axial canal is ambiguous.
(ξ) EDS analysis. Because these spicule-like
structures are so small and so closely associated with or-
ganic matter, their exact chemical and mineralogical
compositions are difficult to determine, and their bire-
fringence pattern is hardly recognizable. Li et al.[11] re-
ported that these spicules survived 4 mol/L HCl digestion
for 20 min, as consistent (but, again, not compelling) evi-
dence for their silica composition. Our own experiment,
however, demonstrates that some spicules dissolve in 2%
ü5% HCl, although some can survive HCl digestion for a
short time (< 1 min), allowing SEM observation of their
morphology. Thus, at least some spicular objects are not
silica in composition. Multiple EDS analyses, performed
on Sigmae EDS attached to a JEOL LMSM-6300 SEM-
machine, give major calcium and phosphorus peaks, with
only a trace amount of silicon (table 1). Admittedly, the
X-ray beam used in EDS is 10ü15 ȝm in diameter and is
thus greater than the spicules. Therefore, the EDS results
reflect a mixture of both spicule and matrix compositions.
However, EDS measurement of (spicule + matrix) dose
not show significantly more silicon than, for example,
phosphatized multicellular algae (table 1), suggesting that
the spicules do not contain a significant amount of silicon.
Precise elemental measurements of the spicular objects
are critical in their interpretation. It is possible that ion
probing could have the resolution to determine the spi-
cules’ elemental composition. But our initial attempt to
examine the spicules under an ion probe failed to show
any significant silicon. So far, there is no convincing evi-
Chinese Science Bulletin Vol. 46 No. 21 November 2001
1831

NOTES
621.
dence to prove that these spicular objects are silica in
composition. This, however, does not necessarily rule out
the possibility that some spicular objects may be sponges
spicules, given that these alleged sponge spicules may
have undergone some degrees of mineral replacement
during diagenesis.
8. Brasier, M., Green, O. Shields, G., Ediacarian sponge spicule
clusters from southwestern Mongolia and the origins of the Cam-
brian fauna, Geology (Boulder), 1997, 25: 303.
9. Zhou, C., Yuan, X. Xue, Y., Sponge spicule-like pseudofossils frm
the Neoproterozoic Doushantuo Formation in Weng'an, Guizhou,
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3
Conclusions
Doushantuo phosphorites at Weng’an contain rich
and extraordinarily preserved microfossils, including both
algae and animals. However, the current evidence for
sponge spicules in Doushantuo phosphorites is tenuous.
Our light microscope and SEM observations and EDS
analyses do not provide substantial evidence for sponge
spicules. The monaxonal spicular objects are best candi-
dates for sponge spicules, but no axial canals have been
seen even in these spicular objects and no convincing
evidence has been shown to verify their silica composition.
Coexistent with the monaxons are fascicles and dumbbells
of mineral aggregates, which are certainly not sponge
spicules; instead, they are diagenetic products, indicating
that the monaxons may have a similar origin. The exact
chemical and mineralogical composition of these spicular
objects is difficult to determine, but our initial EDS
analyses do not provide unequivocal evidence that they
contain significant amount of silicon, which is inconsis-
tent with a demosponge spicule interpretation unless one
can demonstrate that these spicules have been replaced
during secondary mineralization. We, therefore conclude
that interpretation of the spicular structures in Doushantuo
phosphorites at Weng’an is based on ambiguous evidence
and further investigation should be carried out to verify or
reject such an interpretation. We emphasize that, despite
our initial questioning the proposed interpretation of
Doushantuo spicular structures as demosponge micro-
scleres, we do not deny that sponges spicules do exist in
Doushantuo cherts[7].
12. Knoll, A. H. Xiao, S., On the age of the Doushantuo Formation,
Acta Micropalaeontologica Sinica, 1999, 16: 225.
13. Martin, M. W., Grazhdankin, D. V., Bowring, S. A. et al., Age of
Neoproterozoic bilaterian body and trace fossils, White Sea, Rus-
sia: Implications for metazoan evolution, Science, 2000, 288: 841.
14. Zhang, Y., Yin, L., Xiao, S. et al., Permineralized fossils from the
terminal Proterozoic Doushantuo Formation, South China, The
Paleontological Society, Memoir, 1998, 50: 1.
15. Xiao, S. Knoll, A. H., Fossil preservation in the Neoproterozoic
Doushantuo phosphorite Lagerstatte, South China, Lethaia, 1999,
32: 219.
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the Miaohe biota, South China, Journal of Paleontology, 2001.
18. Xiao, S., Zhang, Y. Knoll, A. H., Three-dimensional preservation
of algae and animal embryos in a Neoproterozoic phosphorite,
Nature, 1998, 391: 553.
19. Xiao, S. Knoll, A. H., Phosphatized animal embryos from the
Neoproterozoic Doushantuo Formation at Weng'an, Guizhou,
South China, Journal of Paleontology, 2000, 74: 767.
20. Zhang, Y., Multicellular thallophytes with differentiated tissues
from late Proterozoic phosphate rocks of South China, Lethaia,
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21. Zhang, Y. Yuan, X., New data on multicellular thallophytes and
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22. Yuan, X. Hofmann, H. J., New microfossils from the Neoprotero-
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southwestern China, Alcheringa, 1998, 22: 189.
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24. Peterson, K. J. Davidson, E. H., Regulatory evolution and the ori-
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Acknowledgements We thank Profs. A. H. Knoll and Xue Yaosong for
valuable discussions. This work was supported by the National Natural
Science Foundation of China (Grant No. 95-Z-02-01), the Ministry of
Science and Technology of China (Grant No. G2000077700) and Tulane
University.
25. Peterson, K. J., Cameron, R. A. Davidson, E. H., Bilaterian origins:
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