The cloudinids, an early metazoan family containing the genera
Cloudina and Conotubus, lived in the late Ediacaran
period and became extinct at the base of the Cambrian. They formed
millimetre-scale conical fossils consisting of calcareous cones nested
within one another; the appearance of the organism itself remains
unknown. The name
Cloudina honors the 20th-century geologist and
paleontologist Preston Cloud.
Cloudinids comprise two genera:
Cloudina itself is mineralized,
whereas Conotubus is at best weakly mineralized, whilst sharing the
same "funnel-in-funnel" construction.
Cloudinids had a wide geographic range, reflected in the present
distribution of localities in which their fossils are found, and are
an abundant component of some deposits. They never appear in the same
layers as soft-bodied
Ediacaran biota, but the fact that some
sequences contain cloudinids and
Ediacaran biota in alternating layers
suggests that these groups had different environmental preferences. It
has been suggested that cloudinids lived embedded in microbial mats,
growing new cones to avoid being buried by silt. However no specimens
have been found embedded in mats, and their mode of life is still an
The classification of the cloudinids has proved difficult: they were
initially regarded as polychaete worms, and then as coral-like
cnidarians on the basis of what look like buds on some specimens.
Current scientific opinion is divided between classifying them as
polychaetes and regarding it as unsafe to classify them as members of
any broader grouping.
Cloudinids are important in the history of animal evolution for two
reasons. They are among the earliest and most abundant of the small
shelly fossils with mineralized skeletons, and therefore feature in
the debate about why such skeletons first appeared in the Late
Ediacaran. The most widely supported answer is that their shells are a
defense against predators, as some
Cloudina specimens from
the marks of multiple attacks, which suggests they survived at least a
few of them. The holes made by predators are approximately
proportional to the size of the
Cloudina specimens, and Sinotubulites
fossils, which are often found in the same beds, have so far shown no
such holes. These two points suggest that predators attacked in a
selective manner, and the evolutionary arms race which this indicates
is commonly cited as a cause of the
Cambrian explosion of animal
diversity and complexity.
5 Paleontological importance
6 See also
7 References and footnotes
Cutaway diagram of
Cloudina showing "living space" within the shell.
Cloudina varies in size from a diameter of 0.3 to 6.5 mm, and 8
to 150 mm in length. Fossils consist of a series of stacked
vase-like calcite tubes, whose original mineral composition is
unknown, but inferred to be high-magnesium calcite. Each cone
traps a significant pore space beneath it, and stacks eccentrically in
the one below. This results in a ridged external appearance. The
overall tube is curved or sinuous, and occasionally branches. The tube
walls are 8 to 50 micrometers thick, usually lying in the range 10 to
25 μm. Although it used to be thought that the tubes had test-tube
like bases, detailed three-dimensional reconstruction has shown
that the tubes had an open base. There is evidence that the tube
Cloudina was originally classified in 1972 as a member of the
Cribricyathea, a class known from the Early Cambrian. Glaessner
(1976) accepted this classification and also proposed that Cloudina
was similar to the annelid worms, particularly serpulid
polychaetes. However, Hahn & Pflug (1985) and Conway Morris et
al.. (1990) doubted both Germs' and Glaessner's suggested
relationships, and were unwilling to classify it to anything more than
its own family, Cloudinidae. Some specimens of Cloudina
hartmannae display budding, which implies asexual reproduction.
On this basis Grant (1990) classified
Cloudina as a coral-like
cnidarian. Since the tubes had an open base, creating a single
living space rather than a series of separate chambers,
more likely to be a stem group polychaete worm, in other words an
evolutionary "aunt" or "cousin" of more recent polychaetes. This
interpretation is reinforced by the even distribution of bore-holes
made by predators. However, as with so many
forms, there is great debate surrounding its position in the tree of
life, and classification between the kingdom and family level may be
Cloudina is usually found in association with microbial stromatolites,
which are limited to shallow water; their isotopic composition
suggests that water temperatures were relatively cool. They have also
been found in normal sea-floor sediments, suggesting that they were
not only restricted to dwelling on microbial mounds. On the other
Cloudina has never been found in the same layers as the
soft-bodied Ediacara biota, but
Ediacara biota have been
found in alternating layers. This suggests that the two groups of
organisms had different environmental preferences.
Cloudina specimens the ridges formed by the cones are of
varying width, which suggests the organisms grew at a variable rate.
Adolf Seilacher suggests that they adhered to microbial mats, and that
the growth phases represented the organism keeping pace with
sedimentation—growing through new material deposited on it that
would otherwise bury it. Kinks in the developing tube are easily
explained by the mat falling slightly from the horizontal. Because
of its small size,
Cloudina would be expected to be found in situ in
the microbial mat, especially if, as Seilacher suggests, sedimentation
built up around it during its lifetime. But all the many specimens
discovered to date have only been found having been washed out of
their places of growth. A further argument against Seilacher's
hypothesis is that the predatory borings found in many specimens are
not concentrated at what would be the top end, as one would expect if
the animal was mainly buried. An alternative is that the organism
dwelt on seaweeds, but until a specimen unquestionably in situ is
discovered, its mode of life remains open to debate.
The tubes often appear to form colonies, although they are sometimes
found in more isolated situations. The frequent appearance of large
and sometimes single-species colonies has been attributed to the lack
of significant predation. On the other hand, in some locations up
to 20% of
Cloudina fossils contain predatory borings ranging from 15
to 400 µm in diameter. The boreholes are rather evenly
distributed along the tube length, and some tubes had been bored
multiple times—hence the organism could survive attacks, since
predators do not attack empty shells. This may indicate that the
animal could vary its position in the tube in response to predation,
or that it occupied the full length—but not the full width—of the
tube. The even distribution is perhaps difficult to reconcile with an
infaunal lifestyle, mainly buried in a microbial mat, and adds weight
to Miller's suggestion that the animal lived on seaweeds or in a reef
environment. If modern-day molluscs are a suitable analogy, the size
distribution of the borings suggests that the predator was similar in
size to Cloudina.
Fossil findings in the Nama Group, Namibia, suggest that
one of the first reef-building animals.
Cloudina occurred in calcium carbonate rich areas of stromatolite
reefs. It is found in association with Namacalathus, which like
Cloudina was "weakly skeletal" and solitary, and Namapoikia, which was
"robustly skeletal" and formed sheets on open surfaces.
First found in the
Nama Group in Namibia,
Cloudina has also been
reported in Oman, China's Dengying Formation, Canada,
Uruguay, Argentina, Antarctica, Brazil,
Nevada, central Spain, northwest
Mexico and California, in west
and south Siberia. The
Cloudina fossils found in association with late
Cambrian anabaritids SSF and tubular agglutinated
Platysolenites and Spirosolenites in Siberia.
Although not the first small shelly fossil to be found,
one of the earliest and most abundant. The evolution of external
shells in the Late
Ediacaran is thought to be a defence against
predators, marking the start of an evolutionary arms race.
While predatory borings are common in
Cloudina specimens, no such
borings have been found in Sinotubulites, a similar shelly fossil
sometimes found in the same beds. In addition, the diameters of
Cloudina are proportional to the sizes of specimens, which
suggests that predators were selective about the size of their prey.
These two indications that predators attacked selectively suggest the
possibility of speciation in response to predation, which is often
postulated as a potential cause of the rapid diversification of
animals in the Early Cambrian.
References and footnotes
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