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Skeletogenesis and asexual reproduction in the earliest biomineralizing animal Cloudina



The tubular fossil Cloudina is emerging as an important Ediacaran index fossil. However, its morphology, skeletogenesis, reproduction, and phylogenetic affinity have not been fully resolved. New material from the Dengying Formation of south China confirms that Cloudina tubes consist of eccentrically and sometimes deeply nested funnels and that the tubes lack transverse cross-walls, inconsistent with the traditional cone-in-cone morphological reconstruction. Tube walls are composed of micrometer-sized, more or less equant crystals. A number of Cloudina tubes branch dichotomously, in which daughter funnels split within parent ones. The Cloudina animal is interpreted to have been able to initiate biomineralization of new funnels within old ones. Its skeleton was probably secreted as calcite crystals suspended in organic matrix; the crystals do not appear to have nucleated and grown on a sheeted substrate. It was clearly capable of asexual reproduction, through budding within parent funnels rather than at the apertural end. The morphology, skeletogenesis, and asexual reproduction of Cloudina are broadly similar to modern serpulid annelids, indicating possible phylogenetic relationships or morphological convergence.
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Geology; April 2005; v. 33; no. 4; p. 277–280; doi: 10.1130/G21198.1; 2 figures. 277
Skeletogenesis and asexual reproduction in the earliest
biomineralizing animal
Hong Hua Department of Geology and Laboratory of Continental Dynamics, Northwest University, Xi’an 710069, China
Zhe Chen
Xunlai Yuan
Nanjing Institute of Geology and Paleontology, Nanjing 210008, China
Luyi Zhang Xi’an Institute of Geology and Mineral Resources, Xi’an 710054, China
Shuhai Xiao Department of Geosciences, Virginia Polytechnic Institute, Blacksburg, Virginia 24061, USA
The tubular fossil Cloudina is emerging as an important Ediacaran index fossil. How-
ever, its morphology, skeletogenesis, reproduction, and phylogenetic affinity have notbeen
fully resolved. New material from the Dengying Formation of south China confirms that
Cloudina tubes consist of eccentrically and sometimes deeply nested funnels and that the
tubes lack transverse cross-walls, inconsistent with the traditional cone-in-cone morpho-
logical reconstruction. Tube walls are composed of micrometer-sized, more or less equant
crystals. A number of Cloudina tubes branch dichotomously, in which daughter funnels
split within parent ones. The Cloudina animal is interpreted to have been able to initiate
biomineralization of new funnels within old ones. Its skeleton was probably secreted as
calcite crystals suspended in organic matrix; the crystals do not appear to have nucleated
and grown on a sheeted substrate. It was clearly capable of asexual reproduction,through
budding within parent funnels rather than at the apertural end. The morphology, skele-
togenesis, and asexual reproduction of Cloudina are broadly similar to modern serpulid
annelids, indicating possible phylogenetic relationships or morphological convergence.
Keywords: Cloudina, skeletogenesis, asexual reproduction, Ediacaran System, Dengying For-
mation, south China.
The millimeter-sized tubular fossil Cloudi-
na is one of the earliest biomineralizing ani-
mals that occurs widely in deposits of the ter-
minal Neoproterozoic Ediacaran Period
(Grant, 1990; Hofmann and Mountjoy, 2001).
Radiometric dates from Namibia and Oman
constrain its stratigraphic occurrences to the
late Ediacaran, between 549 61 Ma and 542
61 Ma (Grotzinger et al., 1995; Amthor et
al., 2003). Its wide geographic distribution
and geochronologically calibrated stratigraph-
ic occurrences make Cloudina an excellent in-
dex fossil for the biostratigraphic subdivision
of the Ediacaran System.
However, the morphological detail of Clou-
dina has not been fully resolved, and little is
known about its skeletogenesis, mode of re-
production, and phylogenetic affinity. The
poor understanding of this important fossil is
largely because most Cloudina populations
are preserved as calcareous tubes in carbon-
ates (Germs, 1972) or as bedding-plane im-
pressions in siliciclastic rocks (Hagadorn and
Waggoner, 2000). As a result, it is difficult to
accurately reconstruct the three-dimensional
morphology of Cloudina based on the inter-
pretation of thin sections or external molds. In
this regard, phosphatized Cloudina from do-
lostone of the Dengying Formation in south-
ern Shaanxi Province, south China (Conway
Morris et al., 1990; Bengtson and Yue, 1992;
Hua et al., 2003), offers an opportunity to
learn more about its morphology, because the
Dengying material can be isolated from the
rock matrix. In this paper we report new ob-
servations and interpretations of the skeleto-
genesis and reproduction of Cloudina.
The Dengying Formation in southern
Shaanxi Province is underlain by the Dou-
shantuo Formation (siltstone) and overlain by
the Kuanchuanpu Formation. The latter con-
sists of black chert and phosphatic dolostone
with basal Cambrian acritarchs (Yin, 1987)
and small shelly fossils (Xing et al., 1984).
The Dengying Formation comprises two
members—in stratigraphic order, the Algal
Dolostone Member (.10 m thick; light gray,
thick-bedded dolostone) and the Gaojiashan
Member (72 m thick; gray, thin-bedded cal-
careous mudstone and siltstone in the lower
part and thick-bedded dolostone in the upper
part). The Gaojiashan Member yields the
wormlike fossil Shaanxilithes ningqiangensis,
the tubular fossils Sinotubulites and Cloudina,
and many other metazoan fossils (Hua et al.,
2000). The Dengying Formation in the Yang-
tze Gorges area, ;400 km southeast of south-
ern Shaanxi Province, yields the tubular fossil
Sinotubulites baimatuoensis (Chen et al.,
1981) and frondose Ediacaran fossils that have
affinity with the Nama assemblage (Waggoner,
2003). Biostratigraphic and chemostratigraph-
ic correlations with Neoproterozoic succes-
sions in Namibia and Oman suggest that the
Dengying Formation in south China is prob-
ably ca. 550–542 Ma (Amthor et al., 2003;
Waggoner, 2003).
The Cloudina fossils described in this paper
were collected from dolostone of the upper
Gaojiashan Member at Lijiagou, southern
Shaanxi Province. They came from the same
horizon where previously reported Cloudina
was collected in this region. A stratigraphic
column can be found in Bengtson and Yue
(1992) and Hua et al. (2003). Samples were
digested in 5%–10% acetic acid for three
days, and Cloudina fossils were handpicked
from digestion residues. Isolated specimens
were studied by using a JSM 6300 scanning
electron microscope.
Most Cloudina tubes from the Dengying
Formation are incomplete and show evidence
of brittle breakage. They are preserved in
three dimensions, with none as a compressed
tube. Their basic morphology is broadly con-
sistent with previous descriptions (Germs,
1972; Glaessner, 1976; Conway Morris et al.,
1990; Grant, 1990; Bengtson and Yue, 1992;
Hagadorn and Fedo, 2000; Hua et al., 2003).
Tubes are typically 250–450 mm, but can
reach 2–3 mm, in diameter. They are as long
as 15 mm still-incomplete specimens. The
Dengying population can be referred to as
Cloudina riemkeae—the smaller of the two
recognized Cloudina species (Germs, 1972;
Grant, 1990). However, as the two species
may be synonymous (Grant, 1990), we follow
previous workers (Bengtson and Yue, 1992;
Hua et al., 2003) to refer the Dengying pop-
ulation to the type species, Cloudina
The tube of Cloudina hartmannae is often
curved, with a closed base and an aperture
(Figs. 1A, 1B, 1F). There are no attachment
scars or anchoring structures on the hemi-
spherical basal end. The tube is made of a
series of funnel-shaped units, each 100–1000
mm in length. Except for the basal unit, the
funnels do not have a base; thus the tube has
no transverse cross-walls. Funnels are eccen-
trically stacked on each other so that neigh-
boring funnels may be closely juxtaposed on
278 GEOLOGY, April 2005
Figure 1.
Cloudina hartmannae
from Dengying Formation (A–P) and modern serpulid
macina dysteri
from Qingdao, North China (Q–S). A–B: Hemispherical or bulbous basalend.
C: Deeply nested funnels. D–E: Eccentrically nested funnels with external annulations or
corrugations (E). F: Tube with funnel walls peeled off.No transverse cross-walls arepresent
within tube. G: Magnified view (arrow in F) showing stacked funnels. H: Magnified view
(arrow in G) showing granular crystals in funnel walls. Exterior to left. I: Dichotomous
branching (at upper end) within parent funnel. J–K: Magnified views of lower (J) and upper
(K) ends of specimen illustrated in I. L: Transverse cross section of two daughter funnels
within parent funnel. M–P: Daughter funnels diverge after reaching parent aperture. N is
basal view of specimen illustrated in M, showing medial split of daughter funnels. Parent
funnel in O is incompletely preserved, exposing daughter funnels. Q:
showing asexual reproduction through dichotomous branching. Note annulations and cor-
rugations on external surface of tube wall. R: Dichotomous branching with daughter funnel
nested within parent funnel. S: Cross-section view of tube wall showing calcite crystals.
Internal surface (upper side) of tube wall is smooth. All are scanning electron microscope
images. Illustrated fossils are reposited in Nanjing Institute of Geology and Paleontology
(NIGPAS) and Early Life Institute (ELI) of Northwest University.Scale bar represents 400 mm
for A (NIGPAS 132516), F (ELI 20040014), I (ELI 20040015), M (ELI 20040016), O (ELI
20040018); 100 mm for B (ELI 20040011), G; 1000 mm for C (ELI 4370); 200 mm for D (ELI
20040012), E (ELI 20040013), J–L (NIGPAS 132529), N (ELI 20040017), P (NIGPAS 132528),
Q–R; 4 mm for H, S.
one side but widely separated on the other side
of the tube (Figs. 1D, 1E). The space between
neighboring funnels is variable (Figs. 1C–1E).
The funnels are sometimes deeply nested (Fig.
1C) and typically flare to form flanges (Figs.
1E–1G). Flanges may bend adaperturally to
join neighboring flanges, forming an outer
wall (Fig. 1C). The outer surface of the fun-
nels often has closely spaced annulations or
corrugations (Fig. 1E), but the inner surface is
typically smooth. In cross section, the funnels
are more or less circular with a 3–10-mm-thick
wall composed of more or less equant apatite
crystals ;0.5–2 mm in size (Fig. 1H).
Approximately 20 specimens in our collec-
tion of several hundred branch dichotomously
(Figs. 1I–1P). Branching begins with an al-
most equal split of a cylindrical funnel into
two half-cylinders (Figs. 1L–1N). The split
occurs within the parent funnel, typically
100–1000 mm below the aperture (Figs. 1I–
1K, 1O). The two daughter funnels gradually
become cylindrical but remain confluent with-
in the parent funnel. As soon as they reach the
parent aperture, the two daughters diverge at
an angle of ;458and their diameter increases
significantly (Figs. 1M–1P).
Germs (1972) reconstructed Cloudina as a
tubular fossil made of truncated cones stack-
ing on each other. On the basis of thin-section
interpretation, Grant (1990) suggested that
each stacking unit had a closed base, seen as
a transverse cross-wall in thin section. Grant
(1990) thus reconstructed Cloudina as a cone-
in-cone structure with multiple cross-walls, al-
though the majority of his material shows no
evidence of cross-walls. Hagadorn and Fedo
(2000) showed, through microscopic comput-
ed tomographic analysis, that Cloudina tubes
have no cross-walls.
No specimen isolated from the Dengying
Formation shows any evidence of transverse
cross-walls. It is unlikely that the cross-walls
were present but not preserved in our material
because their preservability would be similar
to the sidewalls. Instead, Cloudina tubes like-
ly did not have cross-walls (Hagadorn and
Fedo, 2000; J. Hagadorn, 2004, personal com-
mun.). The apparent cross-walls in the few
thin sections (Germs, 1972, Plate 1, Fig. 1
therein; Glaessner, 1976, Plate 2, Fig. 2 there-
in; Grant, 1990, Figs. 4B, 7A therein) are
probably artifacts resulting from oblique sec-
tions. As the stacking funnels are eccentric
and the tubes are curved, an oblique section
slanted toward the side where the funnels are
widely separated would produce apparent
cross-walls (Fig. 2). Such cross-walls are ac-
tually sections of sidewalls of nested funnels.
As most Cloudina tubes are curved, the like-
lihood of oblique sections is high. Apparent
cross-walls are expected to concentrate where
the tube curves out of the thin-section plane,
GEOLOGY, April 2005 279
Figure 2. A: Schematic diagram showing
oblique section (gray) of
tube. Ap-
parent transverse cross-walls are actually
sections of funnel sidewalls. B–E: Trans-
verse cross sections at different levels (ar-
rows) of
tube. Dashed lines repre-
sent approximate location of oblique
section. Numbers 1–4 indicate reference
points on tube and transverse cross
and the number of cross-walls in an oblique
section is expected to be fewer than that of
flanges. These appear to be true in sections
with apparent cross-walls (Germs, 1972;
Glaessner, 1976; Grant, 1990).
Cloudina hartmannae is thus reconstructed
as a tubular fossil with a closed base, an ap-
erture, but no transverse cross-walls (see also
Hagadorn and Fedo, 2000). The tube grows
through addition of funnel-shaped units that
are eccentrically stacked on each other. The
funnels have flared flanges that may join to-
gether to form an outer wall. The tube branch-
es dichotomously.
The stacking pattern of the funnels suggests
that the Cloudina organism secreted new cal-
careous funnels within old ones. No algae are
known to grow by constructing new skeletons
within old ones. Protistan skeletons (e.g., for-
aminifers) tend to have restricted, rather than
flaring, apertures. The style of Cloudina skel-
etal growth is most consistent with animal
biomineralization (see following for modern
The stacking pattern indicates that Cloudina
skeletal growth was episodic. The deeply nest-
ed funnels require that each episode of ske-
letogenesis was initiated within older funnels.
It is possible that the Cloudina animal had a
cylindrical mantle that was capable of build-
ing a new funnel instantaneously. Alternative-
ly, the Cloudina animal was able to retract it-
self to allow its specialized skeleton-secreting
organs to incrementally construct a new fun-
nel within an old one.
The funnel walls are made of micrometer-
sized apatite crystals (Hua et al., 2003). The
calcareous tubes from Namibia and Oman also
appear to be micritic (Conway Morris et al.,
1990; Grant, 1990). Diagenetic recrystalliza-
tion and replacement must have occurred, but
the fine grain size in the phosphatic and cal-
careous walls suggests that the original walls
were also made of micrometer-sized, probably
high-magnesium, calcite crystals (Grant,
1990). Because Cloudina tubes do not show
radial fibrous microstructure or palisade ori-
entation of crystals, crystal nucleation and
growth on a sheeted organic or inorganic sub-
strate did not play a significant role in Clou-
dina skeletogenesis. Rather, crystals were
probably precipitated by skeleton-secreting or-
gans and mixed with an organic matrix. The
mixture was then molded by the epidermis of
the Cloudina animal and cured to make cal-
careous walls. Such a process can explain why
the Cloudina skeletons show evidence of flex-
ibility (e.g., corrugation and folding on outer
surface) at skeletogenesis but brittleness at
Possible budding in Cloudina hartmannae
was reported by Germs (1972), and the Deng-
ying population includes well-preserved di-
chotomously branched Cloudina tubes, which
can be interpreted as evidence for asexual re-
production. The dichotomous split occurs
deep within the parent funnel, suggesting that
new buds were produced within the tube rath-
er than at the aperture. Thus, asexual budding
or splitting must have occurred at a subaper-
tural position of the Cloudina animal, or after
the parent animal retracted itself within the
parent funnel. Either way, the daughter ani-
mals, more or less simultaneously and shortly
after budding, began secretion of new funnels,
perhaps partially using the parent funnel wall
for economy of material (Figs. 1D, 1L). As no
polytomous branching has been known, the
Cloudina animal seems to be able to asexually
produce only two individuals at a time.
Cloudina has been compared with serpulid
annelids and fossil cribricyathans, the latter
interpreted by some paleontologists as serpu-
lid annelids (Germs, 1972; Glaessner, 1976).
The Dengying fossils shed new light on the
comparison between Cloudina, serpulids, and
other living animals.
Serpulids are filter-feeding polychaete an-
nelids that make submillimeter- to millimeter-
sized calcareous tubes with small amount of
phosphorus (Vovelle et al., 1991). Like Clou-
dina, their tubes have a closed base, an aper-
ture, but no transverse cross-walls. Serpulid
tubes grow as new material—a mixture of cal-
cium carbonate and mucopolysaccharide—is
secreted by a pair of glands and molded onto
the apertural end by the collar (Hanson, 1948;
Hedley, 1958; Nott and Parker, 1975). Also
like Cloudina, the outer surface of serpulid
tubes has annulations or corrugations, whereas
the inner surface is smooth (Figs. 1Q, 1S).
Some serpulid tubes (e.g., Serpula) have reg-
ularly spaced, flaring flanges. Microstructure
of serpulid tubes is diverse. Many have a char-
acteristic chevron-like structure (Weedon,
1994). Neovermilia falcigera tube consists of
two layers—an inner layer of micrometer-
sized granular crystals and an outer layer of
micrometer-sized, radially oriented, acicular
crystals (Zibrowius and ten Hove, 1987). The
serpulid tube of Ficopomatus enigmaticus also
contains acicular calcite crystals, although not
radially arranged (Aliani et al., 1995). Our
own observation of Salmacina dysteri shows
that its tube is made of micrometer-sized gran-
ular calcite crystals (Fig. 1S), similar to that
of Cloudina (Fig. 1H).
Serpulids (e.g., Salmacina and Filograna)
can reproduce asexually, and they do so by
posterior budding in which a daughter bud is
formed at the basal (posterior) end of the an-
imal (Hanson, 1948; Pernet, 2001). When
asexual reproduction occurs, a biologically
programmed opening, temporarily covered by
a loosely secured calcareous disc known as an
escape hatch (Pernet, 2001), is created near
the aperture. After the daughter bud reaches
the opening, it either breaks away from the old
tube or secretes a new tube on the old one
(Hanson, 1948). Thus, serpulids can form di-
chotomous branches broadly similar to those
of Cloudina (cf. Figs. 1P, 1Q).
There are some significant differences be-
tween Cloudina and modern serpulid tubes,
however. Serpulid tube walls are thicker and
solid, and they do not consist of nested fun-
nels. In addition, the dichotomous branches of
serpulids are not as deeply nested within the
parent tube (Figs. 1Q, 1R). The solid and
thicker tubes of modern serpulids are likely an
adaptive response to stronger predation pres-
sure in the Phanerozoic. As the tubes become
thicker, the degree of funnel-in-funnel nesting
may be reduced because of economy of ma-
terial. Even Cloudina funnels partially utilize
parent walls when they are abutted against
each other (Figs. 1D, 1L).
Other animal tubes depart from Cloudina
tubes, in morphology and mineralogy, much
farther than serpulid tubes. Hydrozoans and
anthozoans, for example, make tubular calcar-
280 GEOLOGY, April 2005
eous skeletons. But hydrozoan skeletons have
distinct canals and pores, and anthozoan tubes
typically have vertical septa and transverse ta-
bulae. Anthozoan skeletons are secreted by a
calicoblastic layer that forms cup-like skeletal
units with transverse cross-walls. Further-
more, anthozoan skeletons are made of calcite
or aragonite needles that are organized into
fascicular sclerodermites, which nucleate and
grow on centers of calcification (Hill, 1981;
Cohen and McConnaughey, 2003). This struc-
tural organization is different from the gran-
ular microstructures seen in Cloudina tubes.
In addition, anthozoan asexual reproduction is
through longitudinal, lateral, or transverse fis-
sion (Ezaki and Yasuhara, 2004); it is impos-
sible for this style of asexual reproduction to
generate dichotomous branches that are deeply
nested in parent funnels. Other animals—such
as scyphozoans (e.g., Stephanoscyphus)
(Chapman and Werner, 1972), pogonophorans
(Ivanov, 1963), and pterobranchs—also pro-
duce dichotomous tubes. However, these are
all organic tubes, and pogonophorans are not
known to be able to reproduce asexually (Iva-
nov, 1963).
Several Neoproterozoic and Cambrian tu-
bular fossils, including Anabarites (Kouchin-
sky and Bengtson, 2002), Sphenothallus (van
Iten et al., 1992), and Sinocyclocyclicus (Xiao
et al., 2000), may also be compared with
Cloudina. However, these are distinguished
from Cloudina by their biomineralogy (Sphen-
othallus), transverse cross walls (Sinocyclo-
cyclicus), and triradial symmetry (Anabari-
tes). More significantly, the phylogenetic
affinities of most Neoproterozoic–Cambrian
tubular fossils are uncertain. Thus, compari-
son with these phylogenetically unresolved
fossils does not provide phylogenetic insight
for Cloudina.
The best modern analogs to Cloudina are
probably serpulid annelids. The morphology,
skeletogenesis, and asexual reproduction of
Cloudina are similar to serpulid annelids but
inconsistent with other tube-building animals.
Although such similarities can be convergent,
the traditional interpretation that Cloudina
was a close phylogenetic relative of annelids
(Germs, 1972; Glaessner, 1976) remains a
plausible hypothesis. Definitive tests of the
nature of this and other phylogenetic hypoth-
eses depend on knowledge about the soft-
tissue anatomy of the Cloudina animal. Con-
fident resolution of the phylogenetic position
of Cloudina is an important step toward a bet-
ter understanding of the early evolution of
biomineralizing animals.
This work was supported by the National Natural
Science Foundation of China, the Chinese Ministry
of Sciences and Technology, the Chinese Ministry
of Education, and the National Science Foundation.
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Manuscript received 13 September 2004
Revised manuscript received 16 December 2004
Manuscript accepted 17 December 2004
Printed in USA
... Carbonate precipitation in Proterozoic oceans was either inorganically, i.e., direct precipitation from supersaturated solution, or biologically-induced, i.e., supersaturation in micro-environment driven by microbial metabolic processes (Bartley and Kah, 2004;Dupraz et al., 2009;Kah and Riding, 2007;Mackenzie and Morse, 1992). With the evolution of biomineralization in the latest Ediacaran to early Cambrian (Conway Morris et al., 1990;Grant, 1990;Grotzinger et al., 2000;Hua et al., 2005), Phanerozoic carbonate was mainly biologically controlled (Tucker and Wright, 1990). Unlike biomineralization that does not require a supersaturation of Ca-carbonate, either abiotic or biologicallyinduced precipitations requires a (near)-supersaturation condition (Dupraz et al., 2009;Higgins et al., 2009). ...
Cap carbonate is the most unique component of a Snowball Earth glaciation, i.e., two Cryogenian global glaciations. In the canonical model, cap carbonate precipitation requires a deglacial extremely high atmospheric pCO2 level, which is unique for a Snowball Earth glaciation but did not occur in other non-Snowball Earth glaciations. However, some Ediacaran glacial successions also have a carbonate unit directly overlying the glacial deposits, although the Ediacaran glaciation is widely accepted to be a non-Snowball Earth ice age. Therefore, occurrence of Ediacaran glacial cap carbonate is counterintuitive, challenging the traditional model of cap carbonate precipitation. Here, we investigate two Ediacaran glacial cap carbonates, i.e. the Zhoujieshan cap carbonate above the Hongtiegou glacial deposits in the Chaidam Block, and the Hankalchough cap carbonate overlying the Hankalchough glacial deposits in the Tarim Block. The Zhoujieshan cap carbonate shows a transitional contact with the underlying glacial deposits, and has a nearly invariant carbonate carbon isotope value (δ13Ccarb) of ~+1‰. The Hankalchough cap carbonate from the Luobupo section has a persistent δ13Ccarb value of ~-5‰, but display a ~15‰ spatial isotopic gradient in the Quruqtagh area. Combining with other occurrences, the Ediacaran glacial cap carbonate differs from the Snowball Earth cap carbonate in paleogeographic distributions, sedimentary structures, and mineralogical and geochemical compositions. We noticed that the Ediacaran glacial cap carbonates were restricted at ~30-50° N/S, which may reflect a rapid transition from glaciation to carbonate deposition in subtropics. Thus, we speculate that the Ediacaran glaciation might have extended to latitude low enough, while the subsequent deglaciation and global warming may associate with an expansion of carbonate precipitation toward higher latitudes. Thus, the Ediacaran glacial cap carbonate precipitation occurred when carbonate precipitation and glaciation spatially overlapped in subtropics. Alternatively, considering paleomagnetic evidence for a ~90˚ reorientation of all continents due to the Ediacaran true polar wander (TPW) event, some Ediacaran glacial cap carbonates could also be driven by glaciated continents moving into lower latitudes, where glacier melt and carbonate precipitated. Either way, a typical Snowball Earth glaciation may lead to global cap carbonate depositions, while most of Phanerozoic glaciations are characterized by glacial deposits in mid- to high-latitude and carbonate precipitation in tropics, i.e., showing no spatial overlapping of glaciation and carbonate precipitation. Thus, the enigmatic cap carbonate precipitation after the Ediacaran glaciations imply a unique climatic condition that differs from either a Snowball Earth glaciation or a Phanerozoic ice age.
... The same evolutionary window also witnessed the diversifications of macroscopic algae [17,18] and acanthomorphic acritarchs [19], followed by the subsequent Cambrian Explosion of the earliest macroscopic biomineralization (e.g., Cloudina and associated small shelly fossils) [20,21] and bilaterians [22][23][24]. Finally, unlike the pervasive depositions of cap carbonate sharply overlying Sturtian and Marinoan glacial deposits with characteristic negative (or close to the mantle value of -5‰) carbonate carbon isotopes (δ 13 C carb ) [3,25,26] Debate over the extent of Ediacaran glaciation is largely attributable to the absence of precise geochronological and biostratigraphic constraints on most Ediacaran glacial deposits. ...
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The emergence of the Ediacara biota soon after the Gaskiers glaciation ca. 580 million years ago (Ma) implies a possible glacial fuse for the evolution of animals. However, the timing of Ediacaran glaciation remains controversial because of poor age constraints on the ∼30 Ediacaran glacial deposits known worldwide. In addition, paleomagnetic constraints and a lack of convincing Snowball-like cap carbonates indicate that Ediacaran glaciations likely did not occur at low latitudes. Thus, reconciling the global occurrences without global glaciation remains a paradox. Here, we report that the large amplitude, globally synchronous ca. 571–562 Ma Shuram carbon isotope excursion occurs below the Ediacaran Hankalchough glacial deposit in Tarim, confirming a post-Shuram glaciation. Leveraging paleomagnetic evidence for a ∼90˚ reorientation of all continents due to true polar wander (TPW), and a non-Snowball condition ruling out low-latitude glaciations, we use paleogeographic reconstructions to further constrain glacial ages. Our results depict a “Great Ediacaran Glaciation (GEG)” occurring diachronously but continuously from ca. 580–560 Ma as different continents migrated through polar–temperate latitudes. The succession of radiation, turnover, and extinction of the Ediacara biota strongly reflects glacial-deglacial dynamics.
... 550-541 Ma; Condon et al., 2005) witnessed spectacular changes in geological evolution, including the breakup of Rodina and reorganization of Gondwana (e.g., Wang and Li, 2003;Wu et al., 2018a, with pulsing oxygenation of the global ocean (e.g., Canfield, 1998;Och and Shields-Zhou, 2012;Wallace et al., 2017), dramatic biological innovation (e.g., Wood, 2011;Wood et al., 2019), a dynamic redox-stratification marine landscape (e.g., Wood et al., 2015Wood et al., , 2018Bowyer et al., 2017), and variation in seawater Mg/Ca ratios (e.g., Wood et al., 2017;Hood and Wallace, 2018;. Numerous studies have contributed to our understanding of (bio)geochemical cycling (e.g., Wei et al., 2020;Zhang et al., 2021) and unique terminal Ediacaran ecosystems (e.g., Hua et al., 2005;Chen et al., 2018;Cai et al., 2019). Moreover, the transition of seawater chemistry has been highlighted as promoting the development of the Ediacaran petroleum system (e.g., Zhu et al., 2021Zhu et al., , 2022Gu et al., 2021;Zhao et al., 2022a). ...
The terminal Ediacaran was a critical period in Earth history due to the coincidence of climate change, seawater chemistry variability, biological innovation, and tectonic activity. The Wushi–Aksu area in the NW Tarim Block contains widely distributed terminal Ediacaran outcrops, providing important sedimentological and paleo-environmental information. A detailed investigation of the Ediacaran Xiaoerbulake–Xigou (XG) and Shayilike (SA) outcrop sections in the NW Tarim Block identified 14 types of lithofacies: mudstone (F1), sandstone (F2), conglomerate (F3), mixed siliciclastic–dolomicrite (F4), mixed siliciclastic–dolograinstone (F5), mixed siliciclastic–dolo-oolite (F6), dolomicrite (F7), dolograinstone (F8), dolo-oolite (F9), planar–wavy stromatolitic dolomite (F10), domal stromatolitic dolomite (F11), stratiform-like thrombolitic dolomite (F12), domal thrombolitic dolomite (F13), and foamy microbial dolomite (F14). A model of the depositional evolution of the Ediacaran Qigebrak Formation in the study area was established, which indicates a shift in the depositional system from an early mixed carbonate–siliciclastic tidal-flat environment to a late carbonate platform, and the Tabei massif was confirmed as the provenance of terrigenous detrital input in early mixed tidal flats. The Ce anomalies of the carbonate fraction ranged from negligible (Ce/Ce* = 1.18 ± 0.18) in the lower mixed carbonate–siliciclastic system to clearly negative (Ce/Ce* = 0.74 ± 0.10) in the upper carbonate-platform facies, indicating the progressive oxidation of shallow seawaters. The coupled evolution of depositional systems and paleo-redox conditions may explain the positive anomaly of ∼ 7‰ in the lower part of δ¹³Ccarb profiles (i.e., in the mixed carbonate–siliciclastic facies). In the early mixed carbonate–siliciclastic system, terrigenous input provided nutrients, enhancing primary productivity and increasing the burial of organic carbon in the suboxic environment, resulting in an increase in ¹³C levels in seawater and a positive δ¹³Ccarb anomaly. This study highlights the contribution of regional sedimentary environments to the positive δ¹³Ccarb anomaly, indicating that caution is necessary in its application in global Ediacaran correlations.
... Because of the lack of soft tissue, these fossils have long been controversial in affinity [38,39], thus, to find exceptional specimens having soft body structures it is necessary to address the classification position of these fossils. (2) To trace the evolution of the Ediacaran relicts in the Cambrian: the latest Ediacaran Gaojiashan Biota in southern Shaanxi is represented by various tubular fossils, such as Cloudina [40]. Fortunately, we found a single specimen of Feiyanella from the Kuanchuanpu Biota that is highly comparable to Cloudina [41], therefore, the identification of more specimens of Feiyanella is helpful in understanding the fate and evolution of these problematical animals. ...
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Various microfossils from the early Cambrian provide crucial clues for understanding the Cambrian explosion and the origin of animal phyla. However, specimens with important anatomical structures are extremely rare and the efficiency of retrieving such fossils by traditional manual selection under a microscope is quite low. Such a contradiction has hindered breakthroughs in micropaleontology for a long time. Here, we propose a solution for identifying specific taxa of Cambrian microfossils using only a few available specimens by transferring a model pre-trained on natural image datasets to the field of paleontological artificial intelligence. The method employs a 34-layer deep residual neural network as the underlying framework, migrates the ImageNet pre-trained model, freezes the low-layer network parameters and retrains the high-layer parameters to build a microfossil image recognition model. We built training sets with randomly selected images of varied number for each taxon. Our experiments show that the average recognition accuracy for specific taxa of Cambrian microfossils (50 images for each taxon) is higher than 0.97 and it can reach 0.85 with only three training samples per taxon. Comparative analyses indicate that our results are much better than those of various prevalent methods, such as the transpose convolutional neural network (TCNN). This demonstrates the feasibility of using natural images (ImageNet) for the training of microfossil recognition models and provides a promising tool for the discovery of rare fossils.
... The Dengying Formation in this area is conformably underlain by the Doushantuo Formation and unconformably overlain by the basal Cambrian Kuanchuanpu Formation (Bureau of Geology and Mineral Resources of Shaanxi Province, 1989). At the studied section, the Dengying Formation (551-538 Ma) consists of three lithographic units: the lower Algal Dolostone Member, composed of thick-bedded dolostone, the middle Gaojiashan Member, consisting of thin-bedded calcareous mudstones and siltstones, and the upper Beiwan Member, dominated by thick-bedded intraclastic dolostone [35,36] (Fig. 1). The five fossil specimens studied here were extracted from a total 500 kg of rock sample. ...
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Background In recent years, Precambrian lifeforms have generated an ever-increasing interest because they revealed a rich eukaryotic diversity prior to the Cambrian explosion of modern animals. Among them, macroalgae are known to be a conspicuous component of Neoproterozoic ecosystems, and chlorophytes in particular are already documented in the Tonian, when they were so far expected to originate. However, like for other major eukaryotic lineages, and despite predictions of molecular clock analyses placing roots of these lineages well into the Neoproterozoic, a taxonomic constraint on Precambrian green algae has remained difficult. Results Here, we present an exceptionally preserved spherical, coenocytic unicellular alga from the latest Ediacaran Dengying Formation of South China (> ca. 541 Ma), known from both external and internal morphology, fully tridimensional and in great detail. Tomographic X-ray and electronic microscopy revealed a characteristic medulla made of intertwined siphons and tightly packed peripheral utricles, suggesting these fossils belong to the Bryopsidales genus Codium. However, its distinctly smaller size compared to extant species leads us to create Protocodium sinense gen. et sp. nov. and a phylomorphospace investigation points to a possible stem group affinity. Conclusions Our finding has several important implications. First, Protocodium allows for a more precise calibration of Archaeplastida and directly confirms that a group as derived as Ulvophyceae was already well diversified in various ecosystems prior to the Cambrian explosion. Details of tridimensional morphology also invite a reassessment of the identification of other Ediacaran algae, such as Chuaria, to better discriminate mono-versus multicellularity, and suggest unicellular Codium-like morphotypes could be much older and widespread. More broadly, Protocodium provides insights into the early diversification of the plant kingdom, the composition of Precambrian ecosystems, and the extreme longevity of certain eukaryotic plans of organization.
... (Beurlen and Sommer, 1957) (Beurlen and Sommer, 1957)影 组 高 家 山 段 -碑 湾 段 层 位 相 当 , 都 归 属 于 Bengtson & Yue, 1992;张录易等, 1992;Hua et al., 2005)和碑湾段 (Hua et al., 2007;Cai et al., 2017Cai et al., , 2019 Guazu 组 (Warren et al., 2011(Warren et al., , 2017; 纳米比亚南部 Nama 群 (Germs, 1972;Grant, 1990); 美国内华达州 埃迪卡拉系 Deep Spring 组 (Mount et al., 1983;Signor et al., 1987); 西班牙中部埃迪卡拉系 Ibor 群 (Cortijo et al., 2015); 加拿大上新元古界 Miette 群 (Hofmann & Mountjoy, 2001); 阿曼 Salt 盆地埃迪 卡拉系 Ara 群(Conway Morris et al., 1990); 西伯利 (Kontorovich et al., 2008), Gornaya Shoriya 地区 West Siberian 和 Belka 组 (Terleev et al., 2011),亚 东 南 地 区 埃 迪 卡 拉 系 -寒 武 系 过 渡 地 层 Ust'-Yudoma 组(Zhu et al., 2017)、 Tomsk 地区 Raiga 地 层 学 杂 志 44 卷 组下段以及 Kuznetsk Alatau 地区 Tarzhul 组(Terleev et al., ...
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The terminal Ediacaran Period, spanning from 565-538 million years ago, was a time of significant ecological restructuring, linked to the emergence of diverse multicellular marine organisms, and the first appearance of bilaterian animals in the fossil record. Among important evolutionary novelties the Ediacaran-Cambrian Transition was marked by the appearance of skeletonized benthic taxa and the increase in abundance and disparity (denoting further ecological and behavioural complexity) of bioturbation structures. Within this context, the Tagatiya Guazú Formation (TGF) (Itapucumi Group) in northeastern Paraguay contains abundant fossil skeletal remains and an undescribed ichnofossil assemblage that date back to ~536 Ma ago. Thus, the main objectives of this thesis are to 1) characterize the taxonomic diversity of the fossil assemblage, 2) stratigraphically position the fossil associations, 3) describe the bioturbation structures, and 4) discuss their paleoecological and biostratigraphic implications. The taxonomic assessment of body fossils indicates the presence of C. hartmannae, C. cf. carinata, Cloudina sp., cf. Zuunia chimidtsereni, Sinotubulites sp., Corumbella werneri, cf. Namacalathus hermanastes, abundant thalli of vendotaeniaceans, as well as macroalgal remains of uncertain affinity. The fossils are predominantly distributed within microbialitic lagoonal facies (from shallow sub- to inter- and supratidal settings) and were preserved through events of rapid sedimentation and early carbonate cementation. Both processes are indicated as key for the high-quality preservation of both delicate, and predominantly organic carapaces, as well as for the soft tissues of macroalga as carbonaceous compressions. Towards the top of the stratigraphic interval a decrease in the abundance and diversity of body fossils is observed, along with an abrupt shift in the patterns of bioturbation. While the lower and middle parts of the succession display only morphologically simple and inconsistent structures, the uppermost interval contains abundant ichnofossils (Bedding Plane Bioturbation Indexes of ~1-9%), with considerable ichnodisparity. These include ichnotaxa such as Bergaueria hemisphaerica, Skolithos isp., Planolites isp., cf. Torrowangea isp., Treptichnus pedum, Phycodes palmatus, abundant treptichniids, potential helicoidal structures (Streptichnus-like), besides rare hypichnial bilobed rusophyciform burrows. The diversity of architectural designs suggests a bioturbation pattern more typical of Terraneuvian ecosystems than those reported for the terminal Ediacaran. Thus, given the 1) absence of stratigraphical hiatuses and sharp faciological shifts along the succession, 2) the recent geochronological data providing minimum age of ~536 Ma for the unit, and 3) the vertical association between trace fossil-bearing intervals and abundant typically Ediacaran skeletal metazoans, results reinforce the prominent gradational character of the major faunal turnovers that markedthe Ediacaran-Cambrian Transition. Keywords: Ediacaran-Cambrian transition, bioestratigraphy, taphonomy, ichnology, trepitchnids, cloudiniids
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The generation of artifacts during sample preparation must be considered in paleobiological studies, particularly during the Ediacaran and Cambrian, since such artifacts can assume forms similar to those of cloudinids and other problematic taxa commonly described in samples from these systems. Chemical reactions between hydrogen peroxide and sulfides from the samples can lead to the formation of tubular and vase-shaped structures. The visual description alone does not allow a conclusion about whether their origin is organic or inorganic. In these cases, chemical composition and ultrastructure analysis are tools that help to distinguish artifacts from bona fide fossils. Scanning electron microscopy can be successfully employed to characterize and differentiate fossils from artifacts. The presence or absence of these structures in thin sections is also an essential piece of information to discuss their biogenicity. Furthermore, not using hydrogen peroxide avoids the risk of formation of the artifacts described here.
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Phosphate rocks, an important ore resource in Guizhou Province, China, are mainly hosted within the Sinian Doushantuo Formation and the Cambrian Meishucun Formation. In addition, the phosphate rocks of the Cambrian Meishucun Formation are rich in biological fossils. Although numerous studies investigating the genesis of phosphate deposits have been performed, the relationship between biological activity and the formation of phosphate deposits in the lower Cambrian Meishucun Formation has not been convincingly explained. This study focuses on the biological fossil assemblage, the characteristics of phosphorus, and the relationship between biological and phosphorus enrichment of the lower Cambrian phosphorites. The primary objectives of our study are to analyze the role of organisms in the formation of phosphorites, restore the phosphorus-formation environment of the Cambrian Meishucun Formation, and construct a sedimentary model of the phosphorites in the Meishucun Formation. The results indicate that there is a significant positive correlation between biological activity and the deposition of phosphorites, that is, the higher the degree of biological enrichment and differentiation, the stronger the deposition. The geochemical analysis of several profiles in the Zhijin phosphorite block shows that the phosphorite block was deposited in an oxygen-rich environment and was affected by a high-temperature hydrothermal fluid. Upwelling ocean currents supplied abundant phosphorus and other nutrients, which provided the conditions for small shells and algae to flourish. Biochemical activity was a crucial factor in the deposition of the phosphorite.
Abundant micro- and macrofossilsChinese Academy of Geological Sciences have been found from the Ediacaran (Sinian) deposits in South China. These fossils offer diverse information for understanding the evolution of early lives before the Cambrian Explosion, and also provide fundamental evidences for the chronological division and correlation of Ediacaran System. Until now all the Ediacaran microfossils were found from the middle and lower part of Doushantuo Formation in South China with two assemblages established, i.e., the lower Tianzhushania spinosa assemblage in the Dou-2 Member of Doushantuo Formation and the upper Hocosphaeridium anozos-H. scaberfacium assemblage in Dou-3 Member. Tianzhushania spinosa assemblage can be correlated with the microfossil assemblage in the Infrakrol Formation in Lesser Himalaya of India, and the Hocosphaeridium anozos-H. scaberfacium assemblage can be correlated with the ECAP (Ediacaran complex acritarch palynoflora) assemblage in South Australia. In other aspects, several exceptionally preserved Ediacaran macrofossil biotas have been reported from South China, including the Lantian biota from the lower part of Doushantuo Formation, Wenghui and Miaohe biotas from the upper part of Doushantuo Formation, as well as Xilingxia (Shibantan), Gaojiashan, Wulingshan and Jiangchuan biotas from the middle and upper part of the Dengying Formation. Among these macrofossil biotas, Miaohe and Wenghui biotas can be correlated with the White Sea biota in Russia and the well-known Ediacara assemblage from the western Flinders Ranges, South Australia; Xilingxia (Shibantan), Gaojiashan, Wulingshan and Jiangchuan biotas can be correlated with the Nama biota in Namibia. According to the biostratigraphy and carbon isotope stratigraphy, the Ediacaran system in South China is suggested to be divided into two series and six stages.
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Summary Abundant and well-preserved microfossils have been discovered from the black cherts in the phosphatic rocks of the lowest Kuanchuanpu Member in Ningqiang of southern Shaanxi. In this area, the Shizhonggou section has been well-known as a stratotype for the uppermost Precambrian-lowermost Cambrian sequence. In this section, the Kuanchuanpu rocks which contain a lot of small Shelly faunas, conformably underlie the lowermost Cambrian Guojiaba Formation and conformably overlie the uppermost Proterozoic Gaojiashan Member. There occur two principle categories of mic-rofossils: 1. microplanktonic spheromorphids and acanthomorphids and 2. filamentous and coccoid microorganisms. They are identified as Micrhy- szridium regulare sp. nov., Myxococcoides szaphylidion Lo, 1980, Tetraphycus? mirus sp. nov. and Leiosphaeridia spp., with the exception of some poorly preserved tubular filaments. In the sense of biologic evolution and in consideration of eustatic sea level, the appearance of numerous Micrhystridium together with abundant small Shelly faunas indicates a reasonable boundary between Precambrian and Cambrian. In accordance with the data available so far, similar cases can be seen in Guizhou, Yunnan, Sichuan and Hubei of China, and in some other places over the world. Description of new species Micrhystridium regulare sp. nov. (P1. II, figs. 1, 2, 4, 8, 10) Description: Coccoid acritarch represented by unitary vesicle or irregularly congregated vesicles. Vesicle spherical in from, 6-12 μm (mostly 8-l0μm) in diameter; surface with long, straight spiny processes (30-50 in number) tapering gradually toward the simple, unfurcated end and communicating with the interior of the vesicle, 4--12 μm in length and 1.0-1.5 }.m in basal diameter; rest part of the surface and the surface of the processes nearly smooth. Comparison: The present species is certainly conspecific with Micrhystridium sp. A described by the writer from the Upper Dongying Formation of W. Hubei (Yin, 1985). It differs from M. Ian- ceolazum Yin 1985 from the Doushantou Formation of W. Hubei and other known species of this genus in having longer and denser or unfurcated processes, and from Aranidium izhoricum Jankauskas 1975 from the Lower Cambrian of the Baltic region in having a regularly circular outline. Here Aranidium is considered to be a synonym of Micrhystridium by the writer. Tetraphycus? mirus sp. nov. (P1. I, figs. I-3, 5, 6; PI. II, figs. 6, 9) Description: Cell nearly spherical in form, generally with 2-8 cells grouped in a small, regular colony. Colonies varied in number arrayed densely or loosely in irregular congregations. Colony commonly present as a diad or triad. Unicell semi-spherical (in diad) or elongately spherical toirregularly spherical (in triad or colony composed of more than three cells) in form. No lamellar sheath or homogeneous organic mucilage enclosing the unicell or colony. Unicells 4-7μm in diameter with smooth and unornamented surface. Comparison: Judging from the cell outline and the colony generally composed of 2-8 cells, the present species is somewhat comparable with Tetraphycus Oehler 1978; however, the writer prefers to identify it under this genus with reserve because planar tetrads or cross-tetrads, if any, are indistinct in our specimens. Based on the features mentioned above, the present species differs from other known species under Tetraphycus, e. g. those from the Early-Middle Proterozoic (Oehler, 1978; Liu, 1982) or those from the Late Proterozoic (Lo, 1980; Zhang, 1984).
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Ediacaran fossils from the southwestern Great Basin may help constrain regional Vendian-Cambrian biostratigraphy and provide biogeographic links between facies in this region and elsewhere. Locally, trace fossils suggest the Vendian-Cambrian boundary occurs within or below the upper third of the lower member of the Wood Canyon Formation. Ediacaran soft-bodied and tubular fossils, including the frondlike fossil Swartpuntia and tubular, mineralized or agglutinated fossils similar to Archaeichnium, Cloudina, Corumbella, and Onuphionella occur in the lowermost Wood Canyon Formation. Discoidal forms referred to Nimbia occur in both the lowermost Wood Canyon Formation and the underlying strata of the Stirling Quartzite. These fossils occur directly below Lower Cambrian trace fossils, including Treptichnus pedum, and confirm the persistence of the Ediacaran biota to near the base of the Cambrian. These faunas may also help strengthen previously proposed correlation schemes between the two main facies belts of the southwestern Great Basin (the Death Valley and White-Inyo facies), because a nearly identical Vendian-lowest Cambrian succession of faunas occurs in both regions. Lastly, lack of cosmopolitan Ediacaran faunas in these strata suggests a paleobiogeographic link between the southwestern U.S. and southern Africa in Vendian time.
"Ordered' and "unordered chevron structures' in serpulid tubes comprise minute calcite lath-like crystals. In ordered chevron structure the crystals parallel each other within each chevron layer, whilst between layers the alignment direction alternates. The laths have no alignment in unordered chevron structure. In "homogeneous chevron structure' (found in Jurassic pomatocerids) the layers comprise a granular or homogeneous fabric. This structure possibly represents a diagenetic replacement of lath-like crystals. Serpulid chevron structures are quite dissimilar from any shell microstructures described in molluscs or lophophorates. The microstructure of Recent spirorbids is quite dissimilar to that of Palaeozoic fossils (microconchids) previously assigned to the genus Spirorbis. -from Author
The uppermost part of the Miette Group (Windermere Supergroup) in eastern British Columbia has yielded shelly macrofossils in cliff-forming biostromal carbonates (Byng Formation). The biostromes are made up of two principal elements: intergrowths of complex sinuous, plate-like stromatolites (cf. Platella), and intervening planar to curviplanar pockets filled with packstone and wackestone crowded with Namacalathus and Cloudina, presumed calcified metazoans of uncertain biological affinities. Preservation is best in limestone, and the shells are mostly obliterated where the carbonate is dolomitized. This assemblage was previously known only from the Nama sequence in Namibia. The new find in the antipodal Miette Group in the Canadian Rocky Mountains greatly extends its geographic range, and suggests a more widespread distribution in similar facies in intermediate areas. Both assemblages constitute the earliest occurrences of shelly fossils in their respective regions.
Predation as an important driver of evolutionary change long has been assumed, despite difficulties to substantiate it with specific examples of predatory interaction, especially for the early Paleozoic diversification of animal life. This study corroborates the existence of shell-drilling predation in the uppermost Neoproterozoic of China. Nearly one-fifth of almost one hundred tubular shells of one of the earliest mineralized animals, Cloudina, are perforated by undoubted predatory borings 15-85 μm wide. By contrast, no specimens of co-occurring shells belonging to Sinotubulites were affected. The identity of the predator remains elusive, but variation in size of the borings suggests a predatory lifestyle throughout its growth, after it reached a minimum size. The relatively uniform distance of the borings from the shell apertures points to either control by the life orientation of the shells, such as the position of the sediment surface, or, more likely, an avoidance response by the predator to protective measures located near the aperture. Assuming Sinotubulites had similar life habits and was potential prey, the absence of borings in this taxon is evidence that these tubes may have been protected by organic material or toxins that fended off shell-drilling predators. Hence, this earliest example of predation in the fossil record already shows prey selectivity and site-specific behavior, pointing to a level of Precambrian predator-prey interaction that approaches the complexity seen in younger Paleozoic benthic animal communities. This is consistent with the suggestion that predation was indeed an active contributor to the Cambrian radiations.