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The cloudinids, an early metazoan family containing the genus Acuticocloudina, Cloudina and Conotubus, lived in the late Ediacaran period about 550 million years ago and became extinct at the base of the Cambrian. They formed millimetre-scale conical 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 , 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 on the basis of what look like Budding 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 in Nevada showed the presence of type guts, the oldest on record, supporting the 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 , 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 Biodiversity and complexity.
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 , 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 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 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.
First found in the Nama Group in Namibia, Cloudina has also been reported in Oman, China's Dengying Formation, Canada, Uruguay, Argentina,by Yochelson and Herrera, 1974; they could have mistaken them for Salterella. See Grant 1990 for reference and discussion. 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 Anabarites SSF and tubular agglutinated skeletal fossils Platysolenites and Spirosolenites in Siberia.
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