होम Geology Diverse biomineralizing animals in the terminal Ediacaran Period herald the Cambrian explosion

Diverse biomineralizing animals in the terminal Ediacaran Period herald the Cambrian explosion

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english
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Geology
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10.1130/G45949.1
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April, 2019
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https://doi.org/10.1130/G45949.1
Manuscript received 20 December 2018
Revised manuscript received 8 February 2019
Manuscript accepted 9 February 2019
© 2019 Geological Society of America. For permission to copy, contact editing@geosociety.org.

Published online 28 February 2019

Diverse biomineralizing animals in the terminal Ediacaran
Period herald the Cambrian explosion
Yaoping Cai1*, Shuhai Xiao2*, Guoxiang Li 3, and Hong Hua1
State Key Laboratory of Continental Dynamics, Shaanxi Key Laboratory of Early Life and Environments, Department of Geology,
Northwest University, Xi’an 710069, China
2
Department of Geosciences, Virginia Tech, Blacksburg, Virginia 24061, USA
3
State Key Laboratory of Palaeobiology and Stratigraphy, Nanjing Institute of Geology and Palaeontology, Chinese Academy of
Sciences, Nanjing 210008, China
1

ABSTRACT
The origin and radiation of biomineralizing metazoans represents an important evolutionary innovation in the history of life. The earliest known skeletal metazoans are dominated by
four genera in the terminal Ediacaran Period (ca. 550–539 Ma), followed by the diversification of new and diverse shelly fossils in the early Cambrian Period (ca. 539–510 Ma). Thus,
terminal Ediacaran skeletal fossils and early Cambrian shelly fossils are commonly regarded
as two distinct assemblages, with little overlap in stratigraphic distribution and taxonomic
composition, implying a possible extinction event and a subsequent radiation event at the
Ediacaran-Cambrian boundary. However, it has been shown recently that some Ediacaran
skeletal taxa may have extended into the early Cambrian, indicating evolutionary continuity
between these two assemblages. Here we document an assemblage of diverse skeletal fossils from the terminal Ediacaran Dengying Formation in South China. This assemblage is
dominated by terminal Ediacaran taxa such as Cloudina and Sinotubulites, but also contains
rare elements that morphologically resemble early Cambrian shelly fossils. This finding
suggests that terminal Ediacara; n skeletal animals are more diverse than previously thought
and further reinforces the evolutionary continuity of biomineralizing animals across the
Ediacaran-Cambrian transition.

METHODS
Samples were collected from a 2 m interval (70.8–72.8 m in Fig. 1A) in the terminal
Ediacaran Beiwan Member, upper Dengying
Formation, Lijiagou section (Fig. 1; Figs. DR1
and DR2 in the GSA Data Repository1; also see
the Data Repository for section location, sample horizon, and age constraints). Phosphatized
microfossils were extracted from clastic dolo­
stone of the Beiwan Member using a maceration technique with 5%–8% acetic acid. Fossils were handpicked from maceration residues
and mounted on aluminum stubs for observation
on a FEI Quanta 600 field-emission scanning
electron microscope. Fossils illustrated in this
paper are reposited in the Department of Geology, Northwest University (Xi’an, China).

INTRODUCTION
The origin of biomineralizing animals in the
terminal Ediacaran (ca. 550–539 Ma) is a transformative evolutionary event with global impact on
the Earth system (Wood et al., 2017). This event
is represented by the appearance of at least four
genera of weakly biomineralized forms, including Cloudina, Sinotubulites, Namacalathus, and
Namapoikia (Germs, 1972; Chen et al., 1981;
Grotzinger et al., 1995, 2000; Wood et al., 2002;
Penny et al., 2014). Immediately above the Ediacaran-Cambrian boundary (ECB), a more diverse
assemblage of new shelly fossils appears in the
basal Cambrian (Zhuravlev and Wood, 2018). It
was thought that the terminal Ediacaran skeletal
animals may have gone extinct near the ECB
(Amthor et al., 2003). Recent reports, however,
have shown that cloudinids may extend above

RESULTS
Approximately 5220 kg of dolostone samples were collected from a 2 m interval that is
5–7 m below the top of the Beiwan Member
and ~1.25 m below a δ13C feature identified
as the basal Cambrian negative carbon isotope
excursion (BACE in Fig. 1A; Zhu et al., 2007).
A total of 8782 fossils (Cloudina = 6029; Sino­
tubulites = 1964; others = 789; Table DR1) were
extracted from these samples using the acetic
acid maceration technique. Common elements
include previously recognized tubular fossils such as Cloudina hartmannae (Fig. 2A),
C. ningqiangensis (Fig. 2B), Sinotubulites
baimatuoensis (Fig. 2C), S. triangularis (Fig.
2D), Protolagena limbata (Fig. 2E), and Multi­
conotubus chinensis (Fig. 2F). Cloudina and
Sinotubulites are the most abundant genera,
accounting for 68.65% and 22.36%, respectively, of the recovered specimens (Table DR1).

*E-mails: yaopingcai@​nwu​.edu​.cn, xiao@​vt​.edu

the ECB (Yang et al., 2016; Han et al., 2017; Zhu
et al., 2017). This evolutionary continuity hints
that elements of early Cambrian shelly fossils
may extend below the ECB (Zhu et al., 2017),
but this has not been thoroughly tested against the
fossil record. Here, we document a diverse assemblage of biomineralizing tubular fossils from the
terminal Ediacaran D
­ engying Formation in South
China. Although this assemblage is dominated
by classical terminal Ediacaran skeletal fossils
such as Cloudina and Sinotubulites, several rare
elements show similarities to early Cambrian
shelly fossils. This finding indicates that terminal Ediacaran skeletal fossils are more diverse
than previously thought. Further, the new data
show that, although there is a taxonomic turnover across the ECB, the evolutionary connection
between terminal Ediacaran and early Cambrian
biomineralizing animal assemblages is stronger
than previously thought.

1
GSA Data Repository item 2019128, section location, sample horizon, age constrains, and taxonomic identification, is available online at http://​www​.geosociety​
.org​/datarepository​/2019/, or on request from editing@​geosociety​.org.

CITATION: Cai, Y., Xiao, S., Li, G., and Hua, H., 2019, Diverse biomineralizing animals in the terminal Ediacaran Period herald the Cambrian explosion: Geology,
v. 47, p. 380–384, https://​doi​.org​/10​.1130​/G45949.1

380

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B

Shizhonggou
155 m

145 m

80°E

120 m
115 m
110 m

P P P

105 m

P P P

100 m

55 m

P P P

P P
P P
P P
P P
P P

Unconformity
Shaanxilithes

30 m
25 m
20 m
15 m

Chert
P P P
P
P
P P

Phosphatic sandy
limestone
Sandy limestone

10 m

35 m

North China
B
AC

360 m
340 m
320 m

30 m
25 m

Shale

20 m

Silty mudstone

15 m

Siltstone

380 m

45 m

300 m
280 m
260 m
240 m

10 m

220 m

5m

200 m

0m

180 m

Sandstone

ia

30°N

Ca
th
a

Yangtze

40°N

400 m

50 m

Limestone

Tarim

420 m

55 m

40 m

120°E

440 m

60 m

Silty dolostone

100°E

460 m

65 m

0m

Dolostone

480 m

70 m

Clastic dolostone

580 m

500 m

75 m

5m

620 m

520 m

80 m

35 m

640 m

540 m

85 m

40 m

660 m

560 m

90 m

45 m

680 m

600 m

95 m

50 m

Conglomerate
20°N

160 m
0m

A. t.-P. a.=Anabarites trisulcatus-Protohertzina anabarica assemblage zone

0 500 1000 km

100°E

700 m

125 m

Late Ediacaran positive carbon isotope plateau

Protolagena limbata

Multiconotubus chinensis

Sinotubulites hexagonus

Sinotubulites pentacarinalis

Sinotubulites triangularis

Sinotubulites baimatuoensis

Cloudina xuanjiangpingensis

60 m

20°N

80°E

720 m

130 m

65 m

30°N

740 m

135 m

70 m

ys

40°N

Cloudina ningqiangensis

BACE

DST

AD

Ediacaran

Dengying Formation
Beiwan Mb.
Gaojiashan Mb.

75 m

Anabarites sp.

80 m

Unnamed taxa

13

δ C (‰VPDB)
–2 0
2 4

Cloudina hartmannae

90 m
85 m

760 m

140 m

95 m

BACE

780 m

A. t.-P. a.
Olivooides
Protohertzina
Anabarites
Conotheca
Cambrotubulus

100 m

Gaojiashan
800 m

150 m

AHC

105 m

13

δ C (‰VPDB)
–2 0 2 4 6

Protolagena
Conotubus
Gaojiashania
Cloudina

P P P
P P P
P P P
P P P
P P P
P P P

Anabarites
Conotheca
Olivooides
A. t.-P. a.

KCP

Cambrian

110 m

C

Shaanxilithes

Lijiagou
115 m

Shaanxilithes

GJB

A

120°E

AHC=Asteridium-Heliosphaeridium-Comasphaeridium assemblage zone

Figure 1. Litho-, bio-, and chemo-stratigraphic data from Lijiagou (A), Shizhonggou (B), and Gaojiashan (C) sections in the southern Shaanxi
Province, South China (see Fig. DR1 [see footnote 1] for section location). Terminal Ediacaran non-biomineralized (open vertical bars) and
weakly biomineralized (filled bars, new taxa in red) fossils are present in the Beiwan Member of the Dengying Formation at Lijiagou (A; this
study), as well as in the Gaojiashan Member at Shizhonggou and Gaojiashan (B–C; Cai et al., 2010). Basal Cambrian shelly fossil Anabarites
trisulcatus–Protohertzina anabarica assemblage zone (A.t.-P.a.) (Steiner et al., 2007) and acritarch Asteridium-Heliosphaeridium-Comasphaeridium assemblage zone (AHC) (Yao et al., 2005; Yin, 1987) have been reported from the Kuanchuanpu Formation at Lijiagou and Shizhonggou.
Basal Cambrian negative carbon isotope excursion (BACE) has been reported from the uppermost Beiwan Member at Lijiagou (Zhu et al.,
2007) and lower Kuanchuanpu Formation at Gaojiashan (Cui et al., 2016). Lines linking sections represent lithostratigraphic correlation,
and are not necessarily exact time lines because of lateral facies changes. Inset map shows approximate location of sections (triangles).
DST—Doushantuo Formation; AD—Algal Dolomite Member; Mb.—Member; KCP—Kuanchuanpu Formation; GJB—Guojiaba Formation.

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381

500 μm

μm
0

100 μm

50

1 mm

1 mm

200 μm

500 μ
m

1 mm

500 μm

500 μm

1 mm

Figure 2. Typical terminal Ediacaran skeletal fossils (A–F) and newly discovered Cambrianstyle skeletal fossils (G–K) from the Beiwan Member of the Dengying Formation at Lijiagou,
South China. A: Cloudina hartmannae. B: C. ningqiangensis. C: Sinotubulites baimatuoensis.
D: S. triangularis. E: Protolagena limbata. F: Multiconotubus chinensis. G–K: Internal molds of
Anabarites sp. Arrows indicate longitudinal sulci, which initiate apically (J–K) or subapically
(I). K is an enlargement of J. See Table DR2 (see footnote 1) for museum catalog numbers.

A variety of new but rare tubular fossils were
also recovered (Figs. 2G–2K and 3), and they
share morphological similarities with some
Cambrian shelly fossils. Importantly, these
fossils include forms that can be assigned to
Anabarites (Figs. 2G–2K)—a genus characterized by three longitudinal sulci and that occurs
widely in the basal Cambrian (Kouchinsky
et al., 2009)—as well as a number of unnamed
and morphologically simple tubular fossils that
resemble some Cambrian taxa (Fig. 3).
DISCUSSION
The ECB marks the decline of the Ediacara
biota characterized by soft-bodied macrofossils such as rangeomorphs and erniettomorphs
(Xiao and Laflamme, 2009) and the initial diversification of bioturbating and biomineralizing
animals (Droser et al., 2017; Chen et al., 2018;
Darroch et al., 2018). Whether this evolutionary transition was triggered by environmental or
ecological factors is a matter of current debate
(Laflamme et al., 2013; Smith et al., 2016; Darroch et al., 2018; Tarhan et al., 2018). To resolve

382

the competing hypotheses about the triggers,
it is critical to have a more nuanced picture of
the evolutionary pattern across the ECB. In
this regard, the recent reports of cloudinids and
possible rangeomorphs in early Cambrian rocks
(Yang et al., 2016; Han et al., 2017; Zhu et al.,
2017; Hoyal Cuthill and Han, 2018) indicate that
the extent of the taxonomic turnover across the
ECB may have been overestimated. The new
discovery of tubular fossils from the terminal
Ediacaran Beiwan Member that morphologically resemble some early Cambrian shelly fossils further reinforces the evolutionary continuity
across the ECB. Together, these fossils suggest
not only that cloudinid skeletal fossils, which
were previously regarded as exclusively terminal
Ediacaran taxa, may extend into the basal Cambrian, but also that a small number of lineages
of basal Cambrian shelly taxa may have had
their origins in the terminal Ediacaran Period
(e.g., Anabarites and Cambrotubulus; see also
Zhu et al., 2017). We emphasize that, despite the
evolutionary continuity, there is also evolutionary turnover across the ECB in both soft-bodied

and skeletal animals. Currently available data
indicate that most erniettomorphs and rangeomorphs likely went extinct at the ECB, and a
large number of Fortunian (earliest Cambrian
Period) shelly fossils have no counterparts in
the terminal Ediacaran Period. Even for those
skeletal fossils that do cross the ECB, their relative abundance is drastically different on either
side of the boundary. For example, Cloudina
is the dominant genus in terminal Ediacaran
strata (e.g., accounting for 68.65% of all fossil specimens in our collection) but is relatively
rare in basal Cambrian rocks. Similarly, tubular
microfossils resembling basal Cambrian taxa are
numerically rare in terminal Ediacaran strata (no
more than 8.23% of all Beiwan fossils recovered
in this study; Table DR1). This further highlights
the need to process large amounts of material
(~5220 kg in our case) in order to recover the
rare elements of Cambrian-style tubular fossils
from terminal Ediacaran rocks.
The data presented in this paper show that
although the terminal Ediacaran and earliest
Cambrian faunas remain distinct, emerging
data—including those presented here—suggest that the evolutionary connection between
these two faunas may be stronger than previously thought. At one level, such a connection
would be expected unless life started anew after
the ECB (Xiao and Laflamme, 2009). What is
important, however, is that paleontologists can
now distinguish animal taxa that survived the
ECB from those that failed. This development
is key to demonstrating the survival and extinction selectivity across the ECB and to testing the
various environmental and ecological processes
behind the Ediacaran-Cambrian turnover.
Among all tubular microfossils described
in this paper, the anabaritids with triradial sulci
(Figs. 2G–2K) stand out in their remarkable
resemblance to Anabarites trisulcatus, which is
present in abundance in basal Cambrian strata
(Kouchinsky et al., 2009). Rare specimens of
A. trisulcatus have been previously reported
from strata of possible terminal Ediacaran age
(Zhuravlev et al., 2012; Rogov et al., 2015;
Zhu et al., 2017; Zhuravlev and Wood, 2018),
and the new data enhance the possibility that
anabaritids first appeared in the terminal Ediacaran Period. With representatives in the terminal Ediacaran Period, it is possible to chart the
morphological and evolutionary trends of anabaritids across the ECB. In this regard, we note
that, despite the common presence of triradial
sulci, the Beiwan anabaritids are subtly different from Cambrian specimens of A. trisulcatus
(Kouchinsky et al., 2009). The three longitudinal sulci of Cambrian A. trisulcatus initiate a
short distance from the apex (Kouchinsky et al.,
2009; Shao et al., 2015), whereas the sulci start
apically in some Beiwan anabaritids (Fig. 2K)
but subapically in others (Fig. 2I). These fossils
cast doubt on the proposition that anabaritids

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1 mm

1 mm

500 μm

1 mm

1 mm

250 μm

500 μm

at the ECB, a number of skeletal taxa may
have persisted through this geological divide
and later diversified into the Cambrian shelly
fauna. These terminal Ediacaran skeletal animals represent a prelude to the Cambrian explosion of shelly animals, echoing recent views that
an early phase of the Cambrian explosion may
have actually started in the terminal Ediacaran
Period (­Darroch et al., 2018).

1

0μ

m

500 μm

m

m

25

μm

m
500 μ

250 μm

50

0μ

m

250 μm

500 μ

m

500

Figure 3. New skeletal fossils from the Beiwan Member of the Dengying Formation at Lijiagou, South China. A–C: Conical tube with round apex and subapical constriction (arrows).
D–E: Incompletely preserved cylindrical tubes. Note triangular cross section (lower end of E).
F–G: Incompletely preserved cylindrical tubular fossils with poorly defined transverse annulations or undulations. H–I: Slightly tapering tubular fossils with round apex but no constrictions.
J–K: Conotubular and cylindrical fossils with thin and faintly preserved transverse annulations.
L: Incompletely preserved tube. M–N: Cylindrical or slightly conotubular tube with broad and
round apex. O: Incompletely preserved tube with transverse annulations and ridges (arrows).
P: Incompletely preserved cylindrical tube with poorly defined transverse annulations. See
Table DR2 (see footnote 1) for museum catalog numbers.

originated from a cylindrical tubular ancestor
and progressively developed their triradial sulci
from the aperture to the apex (Chen and Peng,
2005; Shao et al., 2015).
The Beiwan anabaritids join a number
of other terminal Ediacaran tubular fossils
(e.g., S. triangularis, S. pentacarinalis, and S.
hexagonus [Cai et al., 2015]) in exhibiting triradial, pentaradial, and hexaradial symmetry

characteristic of some Cambrian fossils (e.g.,
Emeiconularia amplicanalis [Liu et al., 2005],
Eopriapulites sphinx [Liu et al., 2014], and A.
sexalox [Bengtson et al., 1990]). Although the
phylogenetic links among these fossils remain
to be established, the broad similarities in body
symmetry further echo the possible evolutionary continuity across the ECB. Thus, although
iconic elements of the Ediacara biota disappear

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CONCLUSIONS
Skeletal fossils in the terminal Ediacaran
Beiwan Member are numerically dominated by
Cloudina and Sinotubulites, but also include previously undescribed forms that are morphologically similar to early Cambrian tubular shelly
fossils. The new fossils increase the known
diversity of skeletal animals in the terminal
Ediacaran. They also suggest that although a
macroevolutionary turnover occurred at the
ECB, a small number of skeletal animal lineages
did cross this geological boundary. These terminal Ediacaran fossils thus herald the Cambrian
explosion. The recognition of such evolutionary
dynamics opens new avenues to characterizing
survival and extinction selectivity and to constraining the roles of environmental changes or
ecological interactions as triggers for the evolutionary turnover at the ECB.
ACKNOWLEDGMENTS
This work was supported by National Natural Science
Foundation of China (grants 41572012, 41672025,
41621003, 41890840, 41890844), State Key Lab­
oratory of Continental Dynamics Research Project
(grant 201210128), Shaanxi Young Scientist Fund
(grant 2015KJXX-26), and U.S. National Science
Foundation (grant EAR-1528553). We thank Stefan
Bengtson, Simon Darroch, Artem Kouchinsky, and an
anonymous reviewer for their constructive comments.
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