Cloudinid
Encyclopedia
The Cloudinids, an early metazoan family
containing the genus
Cloudina, lived in the late Ediacaran
period and became extinct at the base of the Cambrian
. They formed millimetre-scale conical fossil
s 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 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 mat
s, 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 unresolved question.
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
skeleton
s, 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 China bear 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.
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. 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 was flexible.
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 polychaete
s. 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, Cloudina is 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 Ediacaran
life forms, there is great debate surrounding its position in the tree of life, and classification between the kingdom
and family level may be unwise.
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 hand Cloudina has never been found in the same layers as the soft-bodied Ediacara biota
, but Cloudina and Ediacara biota have been found in alternating layers. This suggests that the two groups of organisms had different environmental preferences.
In many 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 mat
s, 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 be 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.
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 Formation 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 Precambrian-Early Cambrian anabaritids
SSF and tubular agglutinated skeletal fossils Platysolenites
and Spirosolenites in Siberia.
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 borings in 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
.
Family (biology)
In biological classification, family is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, class, order, genus, and species, with family fitting between order and genus. As for the other well-known ranks, there is the option of an immediately lower rank, indicated by the...
containing the genus
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
Cloudina, lived in the late Ediacaran
Ediacaran
The Ediacaran Period , named after the Ediacara Hills of South Australia, is the last geological period of the Neoproterozoic Era and of the Proterozoic Eon, immediately preceding the Cambrian Period, the first period of the Paleozoic Era and of the Phanerozoic Eon...
period and became extinct at the base of the Cambrian
Cambrian
The Cambrian is the first geological period of the Paleozoic Era, lasting from Mya ; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux. The period was established by Adam Sedgwick, who named it after Cambria, the Latin name for Wales, where Britain's...
. They formed millimetre-scale conical fossil
Fossil
Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...
s consisting of calcareous
Calcareous
Calcareous is an adjective meaning mostly or partly composed of calcium carbonate, in other words, containing lime or being chalky. The term is used in a wide variety of scientific disciplines.-In zoology:...
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
Preston Cloud
Preston Ercelle Cloud, Jr. was an American paleontologist, geographer, and professor. He was best-known for his work on the geologic time scale and the origin of life on Earth.-Early life:...
.
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 mat
Microbial mat
A microbial mat is a multi-layered sheet of micro-organisms, mainly bacteria and archaea. Microbial mats grow at interfaces between different types of material, mostly on submerged or moist surfaces but a few survive in deserts. They colonize environments ranging in temperature from –40°C to +120°C...
s, 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 unresolved question.
The classification
Taxonomy
Taxonomy is the science of identifying and naming species, and arranging them into a classification. The field of taxonomy, sometimes referred to as "biological taxonomy", revolves around the description and use of taxonomic units, known as taxa...
of the Cloudinids has proved difficult: they were initially regarded as polychaete
Polychaete
The Polychaeta or polychaetes are a class of annelid worms, generally marine. Each body segment has a pair of fleshy protrusions called parapodia that bear many bristles, called chaetae, which are made of chitin. Indeed, polychaetes are sometimes referred to as bristle worms. More than 10,000...
worms, and then as coral-like cnidarians on the basis of what look like buds
Budding
Budding is a form of asexual reproduction in which a new organism grows on another one. The new organism remains attached as it grows, separating from the parent organism only when it is mature. Since the reproduction is asexual, the newly created organism is a clone and is genetically identical...
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
Mineralization (biology)
In biology, mineralization refers to the process where an organic substance is converted to an inorganic substance.This may also be a normal biological process which takes place during the life of an organism such as the formation of bone tissue or egg shells, largely with calcium.This term may...
skeleton
Skeleton
The skeleton is the body part that forms the supporting structure of an organism. There are two different skeletal types: the exoskeleton, which is the stable outer shell of an organism, and the endoskeleton, which forms the support structure inside the body.In a figurative sense, skeleton can...
s, 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 China bear 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
Sinotubulites
Sinotubulites is a genus of small, tube-shaped shelly fossils from the Ediacaran period. It is often found in association with Cloudina.Its tube has a "tube-in-tube" structure composed of several thin layers...
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
Evolutionary arms race
In evolutionary biology, an evolutionary arms race is an evolutionary struggle between competing sets of co-evolving genes that develop adaptations and counter-adaptations against each other, resembling an arms race, which are also examples of positive feedback...
which this indicates is commonly cited as a cause of the Cambrian explosion
Cambrian explosion
The Cambrian explosion or Cambrian radiation was the relatively rapid appearance, around , of most major phyla, as demonstrated in the fossil record, accompanied by major diversification of other organisms, including animals, phytoplankton, and calcimicrobes...
of animal diversity
Biodiversity
Biodiversity is the degree of variation of life forms within a given ecosystem, biome, or an entire planet. Biodiversity is a measure of the health of ecosystems. Biodiversity is in part a function of climate. In terrestrial habitats, tropical regions are typically rich whereas polar regions...
and complexity.
Morphology
Calcite
Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate . The other polymorphs are the minerals aragonite and vaterite. Aragonite will change to calcite at 380-470°C, and vaterite is even less stable.-Properties:...
tubes, whose original mineral composition is unknown. 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 was flexible.
Classification
proposed that Cloudina was similar to the annelid
Annelid
The annelids , formally called Annelida , are a large phylum of segmented worms, with over 17,000 modern species including ragworms, earthworms and leeches...
worms, particularly serpulid polychaete
Polychaete
The Polychaeta or polychaetes are a class of annelid worms, generally marine. Each body segment has a pair of fleshy protrusions called parapodia that bear many bristles, called chaetae, which are made of chitin. Indeed, polychaetes are sometimes referred to as bristle worms. More than 10,000...
s. 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
Family (biology)
In biological classification, family is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, class, order, genus, and species, with family fitting between order and genus. As for the other well-known ranks, there is the option of an immediately lower rank, indicated by the...
, 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, Cloudina is 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 Ediacaran
Ediacaran
The Ediacaran Period , named after the Ediacara Hills of South Australia, is the last geological period of the Neoproterozoic Era and of the Proterozoic Eon, immediately preceding the Cambrian Period, the first period of the Paleozoic Era and of the Phanerozoic Eon...
life forms, there is great debate surrounding its position in the tree of life, and classification between the kingdom
Kingdom (biology)
In biology, kingdom is a taxonomic rank, which is either the highest rank or in the more recent three-domain system, the rank below domain. Kingdoms are divided into smaller groups called phyla or divisions in botany...
and family level may be unwise.
Ecology
Cloudina is usually found in association with microbial stromatolites, which are limited to shallow water; their isotopic compositionIsotope analysis
Isotope analysis is the identification of isotopic signature, the distribution of certain stable isotopes and chemical elements within chemical compounds. This can be applied to a food web to make it possible to draw direct inferences regarding diet, trophic level, and subsistence...
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 hand Cloudina has never been found in the same layers as the soft-bodied Ediacara biota
Ediacara biota
The Ediacara biota consisted of enigmatic tubular and frond-shaped, mostly sessile organisms which lived during the Ediacaran Period . Trace fossils of these organisms have been found worldwide, and represent the earliest known complex multicellular organisms.Simple multicellular organisms such as...
, but Cloudina and Ediacara biota have been found in alternating layers. This suggests that the two groups of organisms had different environmental preferences.
In many Cloudina specimens the ridges formed by the cones are of varying width, which suggests the organisms grew at a variable rate. Adolf Seilacher
Adolf Seilacher
Adolf "Dolf" Seilacher is a German palaeontologist who has made major contributions to evolutionary and ecological palaeobiology in a career stretching over 60 years. He won the Crafoord Prize in 1992, the Paleontological Society Medal in 1994 and the Palaeontological Association's Lapworth Medal...
suggests that they adhered to microbial mat
Microbial mat
A microbial mat is a multi-layered sheet of micro-organisms, mainly bacteria and archaea. Microbial mats grow at interfaces between different types of material, mostly on submerged or moist surfaces but a few survive in deserts. They colonize environments ranging in temperature from –40°C to +120°C...
s, 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 be 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 locations
Cloudina occurred in calcium carbonateCalcium carbonate
Calcium carbonate is a chemical compound with the formula CaCO3. It is a common substance found in rocks in all parts of the world, and is the main component of shells of marine organisms, snails, coal balls, pearls, and eggshells. Calcium carbonate is the active ingredient in agricultural lime,...
rich areas of stromatolite
Stromatolite
Stromatolites or stromatoliths are layered accretionary structures formed in shallow water by the trapping, binding and cementation of sedimentary grains by biofilms of microorganisms, especially cyanobacteria ....
reefs. It is found in association with Namacalathus
Namacalathus
NamacalathusNama, from the geological group where it was described, + Greek κάλαθος, kalathos, meaning "basket in the shape of a lily", or "wine goblet" is a problematic metazoan fossil occurring in the latest Ediacaran. The single species N...
, which like Cloudina was "weakly skeletal" and solitary, and Namapoikia
Namapoikia
Namapoikia rietoogensis is among the earliest known animals to produce a calcareous skeleton.Known from the Ediacaran period, before the Cambrian explosion of calcifying animals, the organism grew up to a metre in diameter and resembles a colonial sponge or cnidarian...
, which was "robustly skeletal" and formed sheets on open surfaces.
First found in the Nama Formation 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 Precambrian-Early Cambrian anabaritids
Anabarites
Anabarites is a problematic lower Cambrian genus, and is one of the small shelly fossils. It was abundant in the early Tommotian and is also found in the Nemakit-Daldynian....
SSF and tubular agglutinated skeletal fossils Platysolenites
Platysolenites
Platysolenites is an agglutinated foramanifera known from Ediacaran and lower Cambrian assemblages.-External links:*...
and Spirosolenites in Siberia.
Paleontological importance
Although not the first small shelly fossil to be found, Cloudina is one of the earliest and most abundant. The evolution of external shells in the Late EdiacaranEdiacaran
The Ediacaran Period , named after the Ediacara Hills of South Australia, is the last geological period of the Neoproterozoic Era and of the Proterozoic Eon, immediately preceding the Cambrian Period, the first period of the Paleozoic Era and of the Phanerozoic Eon...
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
Sinotubulites
Sinotubulites is a genus of small, tube-shaped shelly fossils from the Ediacaran period. It is often found in association with Cloudina.Its tube has a "tube-in-tube" structure composed of several thin layers...
, a similar shelly fossil sometimes found in the same beds. In addition, the diameters of borings in 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
Speciation
Speciation is the evolutionary process by which new biological species arise. The biologist Orator F. Cook seems to have been the first to coin the term 'speciation' for the splitting of lineages or 'cladogenesis,' as opposed to 'anagenesis' or 'phyletic evolution' occurring within lineages...
in response to predation, which is often postulated as a potential cause of the rapid diversification of animals in the Early Cambrian
Cambrian explosion
The Cambrian explosion or Cambrian radiation was the relatively rapid appearance, around , of most major phyla, as demonstrated in the fossil record, accompanied by major diversification of other organisms, including animals, phytoplankton, and calcimicrobes...
.