The cloudinids, an early metazoan family containing the

genera Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial nomenclat ...

''Acuticocloudina'', ''Cloudina'' and ''Conotubus'', lived in the late

Ediacaran The Ediacaran Period ( ) is a geological period that spans 96 million years from the end of the Cryogenian Period 635 million years ago (Mya), to the beginning of the Cambrian Period 538.8 Mya. It marks the end of the Proterozoic Eon, and th ...

period about 550 million years ago. and became extinct at the base of the

Cambrian The Cambrian Period ( ; sometimes symbolized C with bar, Ꞓ) was the first geological period of the Paleozoic Era, and of the Phanerozoic Eon. The Cambrian lasted 53.4 million years from the end of the preceding Ediacaran Period 538.8 million ...

. 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 The Ediacaran (; formerly Vendian) biota is a Taxonomy (biology), taxonomic period classification that consists of all life forms that were present on Earth during the Ediacaran Period (). These were composed of enigmatic tubular and frond-sh ...

, 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 unresolved question. The

classification Classification is a process related to categorization, the process in which ideas and objects are recognized, differentiated and understood. Classification is the grouping of related facts into classes. It may also refer to: Business, organizat ...

of the cloudinids has proved difficult: they were initially regarded as polychaete worms, and then as coral-like

cnidarian Cnidaria () is a phylum under kingdom Animalia containing over 11,000 species of aquatic animals found both in freshwater and marine environments, predominantly the latter. Their distinguishing feature is cnidocytes, specialized cells that th ...

s on the basis of what look like

buds In botany, a bud is an undeveloped or embryonic shoot and normally occurs in the axil of a leaf or at the tip of a stem. Once formed, a bud may remain for some time in a dormant condition, or it may form a shoot immediately. Buds may be specia ...

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. In 2020, a new study of pyritized specimens from the

Wood Canyon Formation The Wood Canyon Formation is a geologic formation in the northern Mojave Desert of Inyo County, California and Nye County and Clark County, Nevada.Nephrozoan type guts, the oldest on record, supporting the

bilateria The Bilateria or bilaterians are animals with bilateral symmetry as an embryo, i.e. having a left and a right side that are mirror images of each other. This also means they have a head and a tail (anterior-posterior axis) as well as a belly and ...

n interpretation. 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 A skeleton is the structural frame that supports the body of an animal. There are several types of skeletons, including the exoskeleton, which is the stable outer shell of an organism, the endoskeleton, which forms the support structure inside ...

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 The Cambrian explosion, Cambrian radiation, Cambrian diversification, or the Biological Big Bang refers to an interval of time approximately in the Cambrian Period when practically all major animal phyla started appearing in the fossil recor ...

of animal diversity and complexity.

Morphology

''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 Calcite is a Carbonate minerals, carbonate mineral and the most stable Polymorphism (materials science), polymorph of calcium carbonate (CaCO3). It is a very common mineral, particularly as a component of limestone. Calcite defines hardness 3 on ...

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 was flexible.

Classification

''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 The annelids (Annelida , from Latin ', "little ring"), also known as the segmented worms, are a large phylum, with over 22,000 extant species including ragworms, earthworms, and leeches. The species exist in and have adapted to various ecol ...

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

. 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

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.

Ecology

''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 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 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

infauna Fauna is all of the animal life present in a particular region or time. The corresponding term for plants is ''flora'', and for fungi, it is '' funga''. Flora, fauna, funga and other forms of life are collectively referred to as '' biota''. Zoo ...

l 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 ''Cloudina'' was one of the first reef-building animals, but machine-learning facilitated 3D tomography indicates that the 'reef-forming' fossils are in fact simply aggregations of solitary individuals.

Fossil locations

''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 China, officially the People's Republic of China (PRC), is a country in East Asia. It is the world's most populous country, with a population exceeding 1.4 billion, slightly ahead of India. China spans the equivalent of five time zones and ...

Dengying Formation The Dengying Formation is an upper Ediacaran (551-541 Ma) fossiliferous geologic formation found in South China. It was deposited on a shallow marine carbonate platform. Members Listed by ascending age: * The Hamajing Member is characterized by l ...

, 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 Platysolenites is a genus of agglutinated foraminifera known from Ediacaran and lower Cambrian The Cambrian Period ( ; sometimes symbolized Ꞓ) was the first geological period of the Paleozoic Era, and of the Phanerozoic Eon. The Cambrian ...

'' 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

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 Speciation is the evolutionary process by which populations evolve to become distinct species. The biologist Orator F. Cook coined the term in 1906 for cladogenesis, the splitting of lineages, as opposed to anagenesis, phyletic evolution within ...

in response to predation, which is often postulated as a potential cause of the rapid diversification of animals in the Early Cambrian.

References

{{good article Prehistoric marine animals Ediacaran life Monogeneric animal families